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i start to code on c++, i have good background from java, i have some issue with the syntax of C++.
i stuck on one thing, i create a method "SortCloums" , and i missing something on it. in the for loop its give me a massage of :
expression must have pointer-to-object type
this is the short code of my, yes i still learning about pointers. need some direction of you guys.
#include <iostream>
using namespace std;
const int size = 4;
void SortCloums(int arr[size][size], int sizeOfArray);
int main(){
int arr[size][size] = { { 0, 4, 6, 0 },
{5, 6 , 8, 12},
{50, 8, 12, 24},
{900, 10, 30, 50} };
SortCloums(arr, size);
return 0;
}
void SortCloums(int arr ,int sizeOfArray) {
// now we check if the array is column sorted or not.
bool flag = true;
for (int i = 0; i < sizeOfArray && flag != false; i++){
for (int j = 0; j < sizeOfArray && flag != false; j++){
if (arr[j][i] > arr[j+1][i]){
flag = false;
std::cout << "The array of column unsorted" << endl;
std::cin.get();
}
}
}
if (flag == true){
std::cout << "The array is column sorted" << endl;
std::cin.get();
}
}
I guess the error message points to this line:
if (arr[j][i] > arr[j+1][i]){
This code treats arr as a 2-D array. However arr is declared here in an incorrect way:
void SortCloums(int arr ,int sizeOfArray) {
Because the declaration comes before usage, the compiler thinks that the declaration is correct, and usage is incorrect. So it outputs an error message that is not very clear.
To fix, declare arr correctly:
void SortCloums(int arr[size][size], int sizeOfArray) {
Here arr is conceptually a 2-D array, but for various technical reasons it's formally a pointer to an array. Your error message mentions a "pointer-to-object" type; a "pointer to array" is a special case of this, so this fixes the error.
I have a function which I want to take, as a parameter, a 2D array of variable size.
So far I have this:
void myFunction(double** myArray){
myArray[x][y] = 5;
etc...
}
And I have declared an array elsewhere in my code:
double anArray[10][10];
However, calling myFunction(anArray) gives me an error.
I do not want to copy the array when I pass it in. Any changes made in myFunction should alter the state of anArray. If I understand correctly, I only want to pass in as an argument a pointer to a 2D array. The function needs to accept arrays of different sizes also. So for example, [10][10] and [5][5]. How can I do this?
There are three ways to pass a 2D array to a function:
The parameter is a 2D array
int array[10][10];
void passFunc(int a[][10])
{
// ...
}
passFunc(array);
The parameter is an array containing pointers
int *array[10];
for(int i = 0; i < 10; i++)
array[i] = new int[10];
void passFunc(int *a[10]) //Array containing pointers
{
// ...
}
passFunc(array);
The parameter is a pointer to a pointer
int **array;
array = new int *[10];
for(int i = 0; i <10; i++)
array[i] = new int[10];
void passFunc(int **a)
{
// ...
}
passFunc(array);
Fixed Size
1. Pass by reference
template <size_t rows, size_t cols>
void process_2d_array_template(int (&array)[rows][cols])
{
std::cout << __func__ << std::endl;
for (size_t i = 0; i < rows; ++i)
{
std::cout << i << ": ";
for (size_t j = 0; j < cols; ++j)
std::cout << array[i][j] << '\t';
std::cout << std::endl;
}
}
In C++ passing the array by reference without losing the dimension information is probably the safest, since one needn't worry about the caller passing an incorrect dimension (compiler flags when mismatching). However, this isn't possible with dynamic (freestore) arrays; it works for automatic (usually stack-living) arrays only i.e. the dimensionality should be known at compile time.
2. Pass by pointer
void process_2d_array_pointer(int (*array)[5][10])
{
std::cout << __func__ << std::endl;
for (size_t i = 0; i < 5; ++i)
{
std::cout << i << ": ";
for (size_t j = 0; j < 10; ++j)
std::cout << (*array)[i][j] << '\t';
std::cout << std::endl;
}
}
The C equivalent of the previous method is passing the array by pointer. This should not be confused with passing by the array's decayed pointer type (3), which is the common, popular method, albeit less safe than this one but more flexible. Like (1), use this method when all the dimensions of the array is fixed and known at compile-time. Note that when calling the function the array's address should be passed process_2d_array_pointer(&a) and not the address of the first element by decay process_2d_array_pointer(a).
Variable Size
These are inherited from C but are less safe, the compiler has no way of checking, guaranteeing that the caller is passing the required dimensions. The function only banks on what the caller passes in as the dimension(s). These are more flexible than the above ones since arrays of different lengths can be passed to them invariably.
It is to be remembered that there's no such thing as passing an array directly to a function in C [while in C++ they can be passed as a reference (1)]; (2) is passing a pointer to the array and not the array itself. Always passing an array as-is becomes a pointer-copy operation which is facilitated by array's nature of decaying into a pointer.
3. Pass by (value) a pointer to the decayed type
// int array[][10] is just fancy notation for the same thing
void process_2d_array(int (*array)[10], size_t rows)
{
std::cout << __func__ << std::endl;
for (size_t i = 0; i < rows; ++i)
{
std::cout << i << ": ";
for (size_t j = 0; j < 10; ++j)
std::cout << array[i][j] << '\t';
std::cout << std::endl;
}
}
Although int array[][10] is allowed, I'd not recommend it over the above syntax since the above syntax makes it clear that the identifier array is a single pointer to an array of 10 integers, while this syntax looks like it's a 2D array but is the same pointer to an array of 10 integers. Here we know the number of elements in a single row (i.e. the column size, 10 here) but the number of rows is unknown and hence to be passed as an argument. In this case there's some safety since the compiler can flag when a pointer to an array with second dimension not equal to 10 is passed. The first dimension is the varying part and can be omitted. See here for the rationale on why only the first dimension is allowed to be omitted.
4. Pass by pointer to a pointer
// int *array[10] is just fancy notation for the same thing
void process_pointer_2_pointer(int **array, size_t rows, size_t cols)
{
std::cout << __func__ << std::endl;
for (size_t i = 0; i < rows; ++i)
{
std::cout << i << ": ";
for (size_t j = 0; j < cols; ++j)
std::cout << array[i][j] << '\t';
std::cout << std::endl;
}
}
Again there's an alternative syntax of int *array[10] which is the same as int **array. In this syntax the [10] is ignored as it decays into a pointer thereby becoming int **array. Perhaps it is just a cue to the caller that the passed array should have at least 10 columns, even then row count is required. In any case the compiler doesn't flag for any length/size violations (it only checks if the type passed is a pointer to pointer), hence requiring both row and column counts as parameter makes sense here.
Note: (4) is the least safest option since it hardly has any type check and the most inconvenient. One cannot legitimately pass a 2D array to this function; C-FAQ condemns the usual workaround of doing int x[5][10]; process_pointer_2_pointer((int**)&x[0][0], 5, 10); as it may potentially lead to undefined behaviour due to array flattening. The right way of passing an array in this method brings us to the inconvenient part i.e. we need an additional (surrogate) array of pointers with each of its element pointing to the respective row of the actual, to-be-passed array; this surrogate is then passed to the function (see below); all this for getting the same job done as the above methods which are more safer, cleaner and perhaps faster.
Here's a driver program to test the above functions:
#include <iostream>
// copy above functions here
int main()
{
int a[5][10] = { { } };
process_2d_array_template(a);
process_2d_array_pointer(&a); // <-- notice the unusual usage of addressof (&) operator on an array
process_2d_array(a, 5);
// works since a's first dimension decays into a pointer thereby becoming int (*)[10]
int *b[5]; // surrogate
for (size_t i = 0; i < 5; ++i)
{
b[i] = a[i];
}
// another popular way to define b: here the 2D arrays dims may be non-const, runtime var
// int **b = new int*[5];
// for (size_t i = 0; i < 5; ++i) b[i] = new int[10];
process_pointer_2_pointer(b, 5, 10);
// process_2d_array(b, 5);
// doesn't work since b's first dimension decays into a pointer thereby becoming int**
}
A modification to shengy's first suggestion, you can use templates to make the function accept a multi-dimensional array variable (instead of storing an array of pointers that have to be managed and deleted):
template <size_t size_x, size_t size_y>
void func(double (&arr)[size_x][size_y])
{
printf("%p\n", &arr);
}
int main()
{
double a1[10][10];
double a2[5][5];
printf("%p\n%p\n\n", &a1, &a2);
func(a1);
func(a2);
return 0;
}
The print statements are there to show that the arrays are getting passed by reference (by displaying the variables' addresses)
Surprised that no one mentioned this yet, but you can simply template on anything 2D supporting [][] semantics.
template <typename TwoD>
void myFunction(TwoD& myArray){
myArray[x][y] = 5;
etc...
}
// call with
double anArray[10][10];
myFunction(anArray);
It works with any 2D "array-like" datastructure, such as std::vector<std::vector<T>>, or a user defined type to maximize code reuse.
You can create a function template like this:
template<int R, int C>
void myFunction(double (&myArray)[R][C])
{
myArray[x][y] = 5;
etc...
}
Then you have both dimension sizes via R and C. A different function will be created for each array size, so if your function is large and you call it with a variety of different array sizes, this may be costly. You could use it as a wrapper over a function like this though:
void myFunction(double * arr, int R, int C)
{
arr[x * C + y] = 5;
etc...
}
It treats the array as one dimensional, and uses arithmetic to figure out the offsets of the indexes. In this case, you would define the template like this:
template<int C, int R>
void myFunction(double (&myArray)[R][C])
{
myFunction(*myArray, R, C);
}
anArray[10][10] is not a pointer to a pointer, it is a contiguous chunk of memory suitable for storing 100 values of type double, which compiler knows how to address because you specified the dimensions. You need to pass it to a function as an array. You can omit the size of the initial dimension, as follows:
void f(double p[][10]) {
}
However, this will not let you pass arrays with the last dimension other than ten.
The best solution in C++ is to use std::vector<std::vector<double> >: it is nearly as efficient, and significantly more convenient.
Here is a vector of vectors matrix example
#include <iostream>
#include <vector>
using namespace std;
typedef vector< vector<int> > Matrix;
void print(Matrix& m)
{
int M=m.size();
int N=m[0].size();
for(int i=0; i<M; i++) {
for(int j=0; j<N; j++)
cout << m[i][j] << " ";
cout << endl;
}
cout << endl;
}
int main()
{
Matrix m = { {1,2,3,4},
{5,6,7,8},
{9,1,2,3} };
print(m);
//To initialize a 3 x 4 matrix with 0:
Matrix n( 3,vector<int>(4,0));
print(n);
return 0;
}
output:
1 2 3 4
5 6 7 8
9 1 2 3
0 0 0 0
0 0 0 0
0 0 0 0
Single dimensional array decays to a pointer pointer pointing to the first element in the array. While a 2D array decays to a pointer pointing to first row. So, the function prototype should be -
void myFunction(double (*myArray) [10]);
I would prefer std::vector over raw arrays.
We can use several ways to pass a 2D array to a function:
Using single pointer we have to typecast the 2D array.
#include<bits/stdc++.h>
using namespace std;
void func(int *arr, int m, int n)
{
for (int i=0; i<m; i++)
{
for (int j=0; j<n; j++)
{
cout<<*((arr+i*n) + j)<<" ";
}
cout<<endl;
}
}
int main()
{
int m = 3, n = 3;
int arr[m][n] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
func((int *)arr, m, n);
return 0;
}
Using double pointer In this way, we also typecast the 2d array
#include<bits/stdc++.h>
using namespace std;
void func(int **arr, int row, int col)
{
for (int i=0; i<row; i++)
{
for(int j=0 ; j<col; j++)
{
cout<<arr[i][j]<<" ";
}
printf("\n");
}
}
int main()
{
int row, colum;
cin>>row>>colum;
int** arr = new int*[row];
for(int i=0; i<row; i++)
{
arr[i] = new int[colum];
}
for(int i=0; i<row; i++)
{
for(int j=0; j<colum; j++)
{
cin>>arr[i][j];
}
}
func(arr, row, colum);
return 0;
}
You can do something like this...
#include<iostream>
using namespace std;
//for changing values in 2D array
void myFunc(double *a,int rows,int cols){
for(int i=0;i<rows;i++){
for(int j=0;j<cols;j++){
*(a+ i*rows + j)+=10.0;
}
}
}
//for printing 2D array,similar to myFunc
void printArray(double *a,int rows,int cols){
cout<<"Printing your array...\n";
for(int i=0;i<rows;i++){
for(int j=0;j<cols;j++){
cout<<*(a+ i*rows + j)<<" ";
}
cout<<"\n";
}
}
int main(){
//declare and initialize your array
double a[2][2]={{1.5 , 2.5},{3.5 , 4.5}};
//the 1st argument is the address of the first row i.e
//the first 1D array
//the 2nd argument is the no of rows of your array
//the 3rd argument is the no of columns of your array
myFunc(a[0],2,2);
//same way as myFunc
printArray(a[0],2,2);
return 0;
}
Your output will be as follows...
11.5 12.5
13.5 14.5
One important thing for passing multidimensional arrays is:
First array dimension need not be specified.
Second(any any further)dimension must be specified.
1.When only second dimension is available globally (either as a macro or as a global constant)
const int N = 3;
void print(int arr[][N], int m)
{
int i, j;
for (i = 0; i < m; i++)
for (j = 0; j < N; j++)
printf("%d ", arr[i][j]);
}
int main()
{
int arr[][3] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
print(arr, 3);
return 0;
}
2.Using a single pointer:
In this method,we must typecast the 2D array when passing to function.
void print(int *arr, int m, int n)
{
int i, j;
for (i = 0; i < m; i++)
for (j = 0; j < n; j++)
printf("%d ", *((arr+i*n) + j));
}
int main()
{
int arr[][3] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
int m = 3, n = 3;
// We can also use "print(&arr[0][0], m, n);"
print((int *)arr, m, n);
return 0;
}
#include <iostream>
/**
* Prints out the elements of a 2D array row by row.
*
* #param arr The 2D array whose elements will be printed.
*/
template <typename T, size_t rows, size_t cols>
void Print2DArray(T (&arr)[rows][cols]) {
std::cout << '\n';
for (size_t row = 0; row < rows; row++) {
for (size_t col = 0; col < cols; col++) {
std::cout << arr[row][col] << ' ';
}
std::cout << '\n';
}
}
int main()
{
int i[2][5] = { {0, 1, 2, 3, 4},
{5, 6, 7, 8, 9} };
char c[3][9] = { {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I'},
{'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R'},
{'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', '&'} };
std::string s[4][4] = { {"Amelia", "Edward", "Israel", "Maddox"},
{"Brandi", "Fabian", "Jordan", "Norman"},
{"Carmen", "George", "Kelvin", "Oliver"},
{"Deanna", "Harvey", "Ludwig", "Philip"} };
Print2DArray(i);
Print2DArray(c);
Print2DArray(s);
std::cout <<'\n';
}
In the case you want to pass a dynamic sized 2-d array to a function, using some pointers could work for you.
void func1(int *arr, int n, int m){
...
int i_j_the_element = arr[i * m + j]; // use the idiom of i * m + j for arr[i][j]
...
}
void func2(){
...
int arr[n][m];
...
func1(&(arr[0][0]), n, m);
}
You can use template facility in C++ to do this. I did something like this :
template<typename T, size_t col>
T process(T a[][col], size_t row) {
...
}
the problem with this approach is that for every value of col which you provide, the a new function definition is instantiated using the template.
so,
int some_mat[3][3], another_mat[4,5];
process(some_mat, 3);
process(another_mat, 4);
instantiates the template twice to produce 2 function definitions (one where col = 3 and one where col = 5).
If you want to pass int a[2][3] to void func(int** pp) you need auxiliary steps as follows.
int a[2][3];
int* p[2] = {a[0],a[1]};
int** pp = p;
func(pp);
As the first [2] can be implicitly specified, it can be simplified further as.
int a[][3];
int* p[] = {a[0],a[1]};
int** pp = p;
func(pp);
You are allowed to omit the leftmost dimension and so you end up with two options:
void f1(double a[][2][3]) { ... }
void f2(double (*a)[2][3]) { ... }
double a[1][2][3];
f1(a); // ok
f2(a); // ok
This is the same with pointers:
// compilation error: cannot convert ‘double (*)[2][3]’ to ‘double***’
// double ***p1 = a;
// compilation error: cannot convert ‘double (*)[2][3]’ to ‘double (**)[3]’
// double (**p2)[3] = a;
double (*p3)[2][3] = a; // ok
// compilation error: array of pointers != pointer to array
// double *p4[2][3] = a;
double (*p5)[3] = a[0]; // ok
double *p6 = a[0][1]; // ok
The decay of an N dimensional array to a pointer to N-1 dimensional array is allowed by C++ standard, since you can lose the leftmost dimension and still being able to correctly access array elements with N-1 dimension information.
Details in here
Though, arrays and pointers are not the same: an array can decay into a pointer, but a pointer doesn't carry state about the size/configuration of the data to which it points.
A char ** is a pointer to a memory block containing character pointers, which themselves point to memory blocks of characters. A char [][] is a single memory block which contains characters. This has an impact on how the compiler translate the code and how the final performance will be.
Source
Despite appearances, the data structure implied by double** is fundamentally incompatible with that of a fixed c-array (double[][]).
The problem is that both are popular (although) misguided ways to deal with arrays in C (or C++).
See https://www.fftw.org/fftw3_doc/Dynamic-Arrays-in-C_002dThe-Wrong-Way.html
If you can't control either part of the code you need a translation layer (called adapt here), as explained here: https://c-faq.com/aryptr/dynmuldimary.html
You need to generate an auxiliary array of pointers, pointing to each row of the c-array.
#include<algorithm>
#include<cassert>
#include<vector>
void myFunction(double** myArray) {
myArray[2][3] = 5;
}
template<std::size_t N, std::size_t M>
auto adapt(double(&Carr2D)[N][M]) {
std::array<double*, N> ret;
std::transform(
std::begin(Carr2D), std::end(Carr2D),
ret.begin(),
[](auto&& row) { return &row[0];}
);
return ret;
}
int main() {
double anArray[10][10];
myFunction( adapt(anArray).data() );
assert(anArray[2][3] == 5);
}
(see working code here: https://godbolt.org/z/7M7KPzbWY)
If it looks like a recipe for disaster is because it is, as I said the two data structures are fundamentally incompatible.
If you can control both ends of the code, these days, you are better off using a modern (or semimodern) array library, like Boost.MultiArray, Boost.uBLAS, Eigen or Multi.
If the arrays are going to be small, you have "tiny" arrays libraries, for example inside Eigen or if you can't afford any dependency you might try simply with std::array<std::array<double, N>, M>.
With Multi, you can simply do this:
#include<multi/array.hpp>
#include<cassert>
namespace multi = boost::multi;
template<class Array2D>
void myFunction(Array2D&& myArray) {
myArray[2][3] = 5;
}
int main() {
multi::array<double, 2> anArray({10, 10});
myFunction(anArray);
assert(anArray[2][3] == 5);
}
(working code: https://godbolt.org/z/7M7KPzbWY)
You could take arrays of an arbitrary number of dimensions by reference and peel off one layer at a time recursively.
Here's an example of a print function for demonstrational purposes:
#include <cstddef>
#include <iostream>
#include <iterator>
#include <string>
#include <type_traits>
template <class T, std::size_t N>
void print(const T (&arr)[N], unsigned indent = 0) {
if constexpr (std::rank_v<T> == 0) {
// inner layer - print the values:
std::cout << std::string(indent, ' ') << '{';
auto it = std::begin(arr);
std::cout << *it;
for (++it; it != std::end(arr); ++it) {
std::cout << ", " << *it;
}
std::cout << '}';
} else {
// still more layers to peel off:
std::cout << std::string(indent, ' ') << "{\n";
auto it = std::begin(arr);
print(*it, indent + 1);
for (++it; it != std::end(arr); ++it) {
std::cout << ",\n";
print(*it, indent + 1);
}
std::cout << '\n' << std::string(indent, ' ') << '}';
}
}
Here's a usage example with a 3 dimensional array:
int main() {
int array[2][3][5]
{
{
{1, 2, 9, -5, 3},
{6, 7, 8, -45, -7},
{11, 12, 13, 14, 25}
},
{
{4, 5, 0, 33, 34},
{8, 9, 99, 54, 44},
{14, 15, 16, 19, 20}
}
};
print(array);
}
... which will produce this output:
{
{
{1, 2, 9, -5, 3},
{6, 7, 8, -45, -7},
{11, 12, 13, 14, 25}
},
{
{4, 5, 0, 33, 34},
{8, 9, 99, 54, 44},
{14, 15, 16, 19, 20}
}
}
I have a function which I want to take, as a parameter, a 2D array of variable size.
So far I have this:
void myFunction(double** myArray){
myArray[x][y] = 5;
etc...
}
And I have declared an array elsewhere in my code:
double anArray[10][10];
However, calling myFunction(anArray) gives me an error.
I do not want to copy the array when I pass it in. Any changes made in myFunction should alter the state of anArray. If I understand correctly, I only want to pass in as an argument a pointer to a 2D array. The function needs to accept arrays of different sizes also. So for example, [10][10] and [5][5]. How can I do this?
There are three ways to pass a 2D array to a function:
The parameter is a 2D array
int array[10][10];
void passFunc(int a[][10])
{
// ...
}
passFunc(array);
The parameter is an array containing pointers
int *array[10];
for(int i = 0; i < 10; i++)
array[i] = new int[10];
void passFunc(int *a[10]) //Array containing pointers
{
// ...
}
passFunc(array);
The parameter is a pointer to a pointer
int **array;
array = new int *[10];
for(int i = 0; i <10; i++)
array[i] = new int[10];
void passFunc(int **a)
{
// ...
}
passFunc(array);
Fixed Size
1. Pass by reference
template <size_t rows, size_t cols>
void process_2d_array_template(int (&array)[rows][cols])
{
std::cout << __func__ << std::endl;
for (size_t i = 0; i < rows; ++i)
{
std::cout << i << ": ";
for (size_t j = 0; j < cols; ++j)
std::cout << array[i][j] << '\t';
std::cout << std::endl;
}
}
In C++ passing the array by reference without losing the dimension information is probably the safest, since one needn't worry about the caller passing an incorrect dimension (compiler flags when mismatching). However, this isn't possible with dynamic (freestore) arrays; it works for automatic (usually stack-living) arrays only i.e. the dimensionality should be known at compile time.
2. Pass by pointer
void process_2d_array_pointer(int (*array)[5][10])
{
std::cout << __func__ << std::endl;
for (size_t i = 0; i < 5; ++i)
{
std::cout << i << ": ";
for (size_t j = 0; j < 10; ++j)
std::cout << (*array)[i][j] << '\t';
std::cout << std::endl;
}
}
The C equivalent of the previous method is passing the array by pointer. This should not be confused with passing by the array's decayed pointer type (3), which is the common, popular method, albeit less safe than this one but more flexible. Like (1), use this method when all the dimensions of the array is fixed and known at compile-time. Note that when calling the function the array's address should be passed process_2d_array_pointer(&a) and not the address of the first element by decay process_2d_array_pointer(a).
Variable Size
These are inherited from C but are less safe, the compiler has no way of checking, guaranteeing that the caller is passing the required dimensions. The function only banks on what the caller passes in as the dimension(s). These are more flexible than the above ones since arrays of different lengths can be passed to them invariably.
It is to be remembered that there's no such thing as passing an array directly to a function in C [while in C++ they can be passed as a reference (1)]; (2) is passing a pointer to the array and not the array itself. Always passing an array as-is becomes a pointer-copy operation which is facilitated by array's nature of decaying into a pointer.
3. Pass by (value) a pointer to the decayed type
// int array[][10] is just fancy notation for the same thing
void process_2d_array(int (*array)[10], size_t rows)
{
std::cout << __func__ << std::endl;
for (size_t i = 0; i < rows; ++i)
{
std::cout << i << ": ";
for (size_t j = 0; j < 10; ++j)
std::cout << array[i][j] << '\t';
std::cout << std::endl;
}
}
Although int array[][10] is allowed, I'd not recommend it over the above syntax since the above syntax makes it clear that the identifier array is a single pointer to an array of 10 integers, while this syntax looks like it's a 2D array but is the same pointer to an array of 10 integers. Here we know the number of elements in a single row (i.e. the column size, 10 here) but the number of rows is unknown and hence to be passed as an argument. In this case there's some safety since the compiler can flag when a pointer to an array with second dimension not equal to 10 is passed. The first dimension is the varying part and can be omitted. See here for the rationale on why only the first dimension is allowed to be omitted.
4. Pass by pointer to a pointer
// int *array[10] is just fancy notation for the same thing
void process_pointer_2_pointer(int **array, size_t rows, size_t cols)
{
std::cout << __func__ << std::endl;
for (size_t i = 0; i < rows; ++i)
{
std::cout << i << ": ";
for (size_t j = 0; j < cols; ++j)
std::cout << array[i][j] << '\t';
std::cout << std::endl;
}
}
Again there's an alternative syntax of int *array[10] which is the same as int **array. In this syntax the [10] is ignored as it decays into a pointer thereby becoming int **array. Perhaps it is just a cue to the caller that the passed array should have at least 10 columns, even then row count is required. In any case the compiler doesn't flag for any length/size violations (it only checks if the type passed is a pointer to pointer), hence requiring both row and column counts as parameter makes sense here.
Note: (4) is the least safest option since it hardly has any type check and the most inconvenient. One cannot legitimately pass a 2D array to this function; C-FAQ condemns the usual workaround of doing int x[5][10]; process_pointer_2_pointer((int**)&x[0][0], 5, 10); as it may potentially lead to undefined behaviour due to array flattening. The right way of passing an array in this method brings us to the inconvenient part i.e. we need an additional (surrogate) array of pointers with each of its element pointing to the respective row of the actual, to-be-passed array; this surrogate is then passed to the function (see below); all this for getting the same job done as the above methods which are more safer, cleaner and perhaps faster.
Here's a driver program to test the above functions:
#include <iostream>
// copy above functions here
int main()
{
int a[5][10] = { { } };
process_2d_array_template(a);
process_2d_array_pointer(&a); // <-- notice the unusual usage of addressof (&) operator on an array
process_2d_array(a, 5);
// works since a's first dimension decays into a pointer thereby becoming int (*)[10]
int *b[5]; // surrogate
for (size_t i = 0; i < 5; ++i)
{
b[i] = a[i];
}
// another popular way to define b: here the 2D arrays dims may be non-const, runtime var
// int **b = new int*[5];
// for (size_t i = 0; i < 5; ++i) b[i] = new int[10];
process_pointer_2_pointer(b, 5, 10);
// process_2d_array(b, 5);
// doesn't work since b's first dimension decays into a pointer thereby becoming int**
}
A modification to shengy's first suggestion, you can use templates to make the function accept a multi-dimensional array variable (instead of storing an array of pointers that have to be managed and deleted):
template <size_t size_x, size_t size_y>
void func(double (&arr)[size_x][size_y])
{
printf("%p\n", &arr);
}
int main()
{
double a1[10][10];
double a2[5][5];
printf("%p\n%p\n\n", &a1, &a2);
func(a1);
func(a2);
return 0;
}
The print statements are there to show that the arrays are getting passed by reference (by displaying the variables' addresses)
Surprised that no one mentioned this yet, but you can simply template on anything 2D supporting [][] semantics.
template <typename TwoD>
void myFunction(TwoD& myArray){
myArray[x][y] = 5;
etc...
}
// call with
double anArray[10][10];
myFunction(anArray);
It works with any 2D "array-like" datastructure, such as std::vector<std::vector<T>>, or a user defined type to maximize code reuse.
You can create a function template like this:
template<int R, int C>
void myFunction(double (&myArray)[R][C])
{
myArray[x][y] = 5;
etc...
}
Then you have both dimension sizes via R and C. A different function will be created for each array size, so if your function is large and you call it with a variety of different array sizes, this may be costly. You could use it as a wrapper over a function like this though:
void myFunction(double * arr, int R, int C)
{
arr[x * C + y] = 5;
etc...
}
It treats the array as one dimensional, and uses arithmetic to figure out the offsets of the indexes. In this case, you would define the template like this:
template<int C, int R>
void myFunction(double (&myArray)[R][C])
{
myFunction(*myArray, R, C);
}
anArray[10][10] is not a pointer to a pointer, it is a contiguous chunk of memory suitable for storing 100 values of type double, which compiler knows how to address because you specified the dimensions. You need to pass it to a function as an array. You can omit the size of the initial dimension, as follows:
void f(double p[][10]) {
}
However, this will not let you pass arrays with the last dimension other than ten.
The best solution in C++ is to use std::vector<std::vector<double> >: it is nearly as efficient, and significantly more convenient.
Here is a vector of vectors matrix example
#include <iostream>
#include <vector>
using namespace std;
typedef vector< vector<int> > Matrix;
void print(Matrix& m)
{
int M=m.size();
int N=m[0].size();
for(int i=0; i<M; i++) {
for(int j=0; j<N; j++)
cout << m[i][j] << " ";
cout << endl;
}
cout << endl;
}
int main()
{
Matrix m = { {1,2,3,4},
{5,6,7,8},
{9,1,2,3} };
print(m);
//To initialize a 3 x 4 matrix with 0:
Matrix n( 3,vector<int>(4,0));
print(n);
return 0;
}
output:
1 2 3 4
5 6 7 8
9 1 2 3
0 0 0 0
0 0 0 0
0 0 0 0
Single dimensional array decays to a pointer pointer pointing to the first element in the array. While a 2D array decays to a pointer pointing to first row. So, the function prototype should be -
void myFunction(double (*myArray) [10]);
I would prefer std::vector over raw arrays.
We can use several ways to pass a 2D array to a function:
Using single pointer we have to typecast the 2D array.
#include<bits/stdc++.h>
using namespace std;
void func(int *arr, int m, int n)
{
for (int i=0; i<m; i++)
{
for (int j=0; j<n; j++)
{
cout<<*((arr+i*n) + j)<<" ";
}
cout<<endl;
}
}
int main()
{
int m = 3, n = 3;
int arr[m][n] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
func((int *)arr, m, n);
return 0;
}
Using double pointer In this way, we also typecast the 2d array
#include<bits/stdc++.h>
using namespace std;
void func(int **arr, int row, int col)
{
for (int i=0; i<row; i++)
{
for(int j=0 ; j<col; j++)
{
cout<<arr[i][j]<<" ";
}
printf("\n");
}
}
int main()
{
int row, colum;
cin>>row>>colum;
int** arr = new int*[row];
for(int i=0; i<row; i++)
{
arr[i] = new int[colum];
}
for(int i=0; i<row; i++)
{
for(int j=0; j<colum; j++)
{
cin>>arr[i][j];
}
}
func(arr, row, colum);
return 0;
}
You can do something like this...
#include<iostream>
using namespace std;
//for changing values in 2D array
void myFunc(double *a,int rows,int cols){
for(int i=0;i<rows;i++){
for(int j=0;j<cols;j++){
*(a+ i*rows + j)+=10.0;
}
}
}
//for printing 2D array,similar to myFunc
void printArray(double *a,int rows,int cols){
cout<<"Printing your array...\n";
for(int i=0;i<rows;i++){
for(int j=0;j<cols;j++){
cout<<*(a+ i*rows + j)<<" ";
}
cout<<"\n";
}
}
int main(){
//declare and initialize your array
double a[2][2]={{1.5 , 2.5},{3.5 , 4.5}};
//the 1st argument is the address of the first row i.e
//the first 1D array
//the 2nd argument is the no of rows of your array
//the 3rd argument is the no of columns of your array
myFunc(a[0],2,2);
//same way as myFunc
printArray(a[0],2,2);
return 0;
}
Your output will be as follows...
11.5 12.5
13.5 14.5
One important thing for passing multidimensional arrays is:
First array dimension need not be specified.
Second(any any further)dimension must be specified.
1.When only second dimension is available globally (either as a macro or as a global constant)
const int N = 3;
void print(int arr[][N], int m)
{
int i, j;
for (i = 0; i < m; i++)
for (j = 0; j < N; j++)
printf("%d ", arr[i][j]);
}
int main()
{
int arr[][3] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
print(arr, 3);
return 0;
}
2.Using a single pointer:
In this method,we must typecast the 2D array when passing to function.
void print(int *arr, int m, int n)
{
int i, j;
for (i = 0; i < m; i++)
for (j = 0; j < n; j++)
printf("%d ", *((arr+i*n) + j));
}
int main()
{
int arr[][3] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
int m = 3, n = 3;
// We can also use "print(&arr[0][0], m, n);"
print((int *)arr, m, n);
return 0;
}
#include <iostream>
/**
* Prints out the elements of a 2D array row by row.
*
* #param arr The 2D array whose elements will be printed.
*/
template <typename T, size_t rows, size_t cols>
void Print2DArray(T (&arr)[rows][cols]) {
std::cout << '\n';
for (size_t row = 0; row < rows; row++) {
for (size_t col = 0; col < cols; col++) {
std::cout << arr[row][col] << ' ';
}
std::cout << '\n';
}
}
int main()
{
int i[2][5] = { {0, 1, 2, 3, 4},
{5, 6, 7, 8, 9} };
char c[3][9] = { {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I'},
{'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R'},
{'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', '&'} };
std::string s[4][4] = { {"Amelia", "Edward", "Israel", "Maddox"},
{"Brandi", "Fabian", "Jordan", "Norman"},
{"Carmen", "George", "Kelvin", "Oliver"},
{"Deanna", "Harvey", "Ludwig", "Philip"} };
Print2DArray(i);
Print2DArray(c);
Print2DArray(s);
std::cout <<'\n';
}
In the case you want to pass a dynamic sized 2-d array to a function, using some pointers could work for you.
void func1(int *arr, int n, int m){
...
int i_j_the_element = arr[i * m + j]; // use the idiom of i * m + j for arr[i][j]
...
}
void func2(){
...
int arr[n][m];
...
func1(&(arr[0][0]), n, m);
}
You can use template facility in C++ to do this. I did something like this :
template<typename T, size_t col>
T process(T a[][col], size_t row) {
...
}
the problem with this approach is that for every value of col which you provide, the a new function definition is instantiated using the template.
so,
int some_mat[3][3], another_mat[4,5];
process(some_mat, 3);
process(another_mat, 4);
instantiates the template twice to produce 2 function definitions (one where col = 3 and one where col = 5).
If you want to pass int a[2][3] to void func(int** pp) you need auxiliary steps as follows.
int a[2][3];
int* p[2] = {a[0],a[1]};
int** pp = p;
func(pp);
As the first [2] can be implicitly specified, it can be simplified further as.
int a[][3];
int* p[] = {a[0],a[1]};
int** pp = p;
func(pp);
You are allowed to omit the leftmost dimension and so you end up with two options:
void f1(double a[][2][3]) { ... }
void f2(double (*a)[2][3]) { ... }
double a[1][2][3];
f1(a); // ok
f2(a); // ok
This is the same with pointers:
// compilation error: cannot convert ‘double (*)[2][3]’ to ‘double***’
// double ***p1 = a;
// compilation error: cannot convert ‘double (*)[2][3]’ to ‘double (**)[3]’
// double (**p2)[3] = a;
double (*p3)[2][3] = a; // ok
// compilation error: array of pointers != pointer to array
// double *p4[2][3] = a;
double (*p5)[3] = a[0]; // ok
double *p6 = a[0][1]; // ok
The decay of an N dimensional array to a pointer to N-1 dimensional array is allowed by C++ standard, since you can lose the leftmost dimension and still being able to correctly access array elements with N-1 dimension information.
Details in here
Though, arrays and pointers are not the same: an array can decay into a pointer, but a pointer doesn't carry state about the size/configuration of the data to which it points.
A char ** is a pointer to a memory block containing character pointers, which themselves point to memory blocks of characters. A char [][] is a single memory block which contains characters. This has an impact on how the compiler translate the code and how the final performance will be.
Source
Despite appearances, the data structure implied by double** is fundamentally incompatible with that of a fixed c-array (double[][]).
The problem is that both are popular (although) misguided ways to deal with arrays in C (or C++).
See https://www.fftw.org/fftw3_doc/Dynamic-Arrays-in-C_002dThe-Wrong-Way.html
If you can't control either part of the code you need a translation layer (called adapt here), as explained here: https://c-faq.com/aryptr/dynmuldimary.html
You need to generate an auxiliary array of pointers, pointing to each row of the c-array.
#include<algorithm>
#include<cassert>
#include<vector>
void myFunction(double** myArray) {
myArray[2][3] = 5;
}
template<std::size_t N, std::size_t M>
auto adapt(double(&Carr2D)[N][M]) {
std::array<double*, N> ret;
std::transform(
std::begin(Carr2D), std::end(Carr2D),
ret.begin(),
[](auto&& row) { return &row[0];}
);
return ret;
}
int main() {
double anArray[10][10];
myFunction( adapt(anArray).data() );
assert(anArray[2][3] == 5);
}
(see working code here: https://godbolt.org/z/7M7KPzbWY)
If it looks like a recipe for disaster is because it is, as I said the two data structures are fundamentally incompatible.
If you can control both ends of the code, these days, you are better off using a modern (or semimodern) array library, like Boost.MultiArray, Boost.uBLAS, Eigen or Multi.
If the arrays are going to be small, you have "tiny" arrays libraries, for example inside Eigen or if you can't afford any dependency you might try simply with std::array<std::array<double, N>, M>.
With Multi, you can simply do this:
#include<multi/array.hpp>
#include<cassert>
namespace multi = boost::multi;
template<class Array2D>
void myFunction(Array2D&& myArray) {
myArray[2][3] = 5;
}
int main() {
multi::array<double, 2> anArray({10, 10});
myFunction(anArray);
assert(anArray[2][3] == 5);
}
(working code: https://godbolt.org/z/7M7KPzbWY)
You could take arrays of an arbitrary number of dimensions by reference and peel off one layer at a time recursively.
Here's an example of a print function for demonstrational purposes:
#include <cstddef>
#include <iostream>
#include <iterator>
#include <string>
#include <type_traits>
template <class T, std::size_t N>
void print(const T (&arr)[N], unsigned indent = 0) {
if constexpr (std::rank_v<T> == 0) {
// inner layer - print the values:
std::cout << std::string(indent, ' ') << '{';
auto it = std::begin(arr);
std::cout << *it;
for (++it; it != std::end(arr); ++it) {
std::cout << ", " << *it;
}
std::cout << '}';
} else {
// still more layers to peel off:
std::cout << std::string(indent, ' ') << "{\n";
auto it = std::begin(arr);
print(*it, indent + 1);
for (++it; it != std::end(arr); ++it) {
std::cout << ",\n";
print(*it, indent + 1);
}
std::cout << '\n' << std::string(indent, ' ') << '}';
}
}
Here's a usage example with a 3 dimensional array:
int main() {
int array[2][3][5]
{
{
{1, 2, 9, -5, 3},
{6, 7, 8, -45, -7},
{11, 12, 13, 14, 25}
},
{
{4, 5, 0, 33, 34},
{8, 9, 99, 54, 44},
{14, 15, 16, 19, 20}
}
};
print(array);
}
... which will produce this output:
{
{
{1, 2, 9, -5, 3},
{6, 7, 8, -45, -7},
{11, 12, 13, 14, 25}
},
{
{4, 5, 0, 33, 34},
{8, 9, 99, 54, 44},
{14, 15, 16, 19, 20}
}
}
My program have to sort an array in another array.
When I run the program it prints 1 2 3 -858993460 5 -858993460 7.
I can not understand where the mistake is in the code.
#include <iostream>
using namespace std;
int main()
{
const int N = 7;
int arr[N] = { 3, 17, 2, 9, 1, 5, 7 };
int max = arr[0];
for (int i = 1; i < N; i++)
{
if (max < arr[i])
max = arr[i];
}
int sort_arr[N];
for (int j = 0; j < N; j++)
{
sort_arr[arr[j] - 1] = arr[j];
}
for (int i = 0; i < N; i++)
{
cout << sort_arr[i] << " ";
}
return 0;
}
Okay lets face the problems in your code.
The "weird" numbers you see there, came from the uninitialzied array sort_arr. What do I mean by uninitialized? Well sort_arr is a little chunck somewhere in your memory. Since a program usually does not clear its memory and rather claims the memory it used as free, the chunk of sort_arr may contain bits and bytes set by another program. The numbers occure since these bytes are interpreted as an integer value. So the first thing to do would be to initialize the array before using it.
sort_arr[N] = { 0, 0, 0, 0, 0, 0, 0 };
Now why did these numbers occure? Well you're probably expecting your algorithm to set all values in sort_arr which would result in an sorted array, right? Well but your algorithm isn't working that well. See this line:
sort_arr[arr[j] - 1] = arr[j];
What happens when j is 1? arr[1] is then evaluated to 17 and 17 - 1 equals 16. So sort_arr[arr[1] - 1] is the same as sort_arr[16] which exceeds the bounds of your array.
If you want to program a sorting algorithm by your self than I would recommend to start with an simple bubble sort algorithm. Otherwise, if you only need to sort the array have a look at the algorithm header. It is fairly simple to use:
#include <iostream>
#include <algorithm>
#include <iterator> // << include this to use begin() and end()
using namespace std;
int main()
{
const int N = 7;
int arr[N] = { 3, 17, 2, 9, 1, 5, 7 };
int sort_arr[N] = { 0, 0, 0, 0, 0, 0, 0 };
copy(begin(arr), end(arr), begin(sort_arr));
sort(begin(sort_arr), end(sort_arr));
for (int i = 0; i < N; i++)
{
cout << sort_arr[i] << " ";
}
cout << endl;
}
By the way. You're looking for the biggest value in your array, right? After you have sorted the array sort_arr[N - 1] is the biggest value contained in your array.
If you want to sort a array into another array then one way is you make a copy of the array and then use the sort function in the standard library to sort the second array.
int arr[10];
int b[10];
for(int i=0;i<10;i++)
{
cin>>arr[i];
b[i]=arr[i];
}
sort(b,b+10);
// this sort function will sort the array elements in ascending order and if you want to change the order then just add a comparison function as third arguement to the sort function.
It seems that you think that sort_arr[arr[j] - 1] = arr[j] will sort arr into sort_arr. It won't.
Sorting is already written for you here: http://en.cppreference.com/w/cpp/algorithm/sort You can use that like this:
copy(cbegin(arr), cend(arr), begin(sort_arr));
sort(begin(sort_arr), end(sort_arr));
Live Example
My guess is this is an attempt to implement a type of counting sort. Note that variable length arrays aren't normally allowed in C++ or some versions of C. You could use _alloca() to allocate off the stack to get the equivalent of a variable length array: int * sort_arr = (int *)_alloca(max * sizeof(int)); .
#include <iostream>
using namespace std;
int main()
{
const int N = 7;
// assuming range of values is 1 to ...
int arr[N] = { 3, 17, 2, 9, 1, 5, 7 };
int max = arr[0];
for (int i = 1; i < N; i++)
{
if (max < arr[i])
max = arr[i];
}
int sort_arr[max];
for (int i = 0; i < max; i++)
{
sort_arr[i] = 0;
}
for (int j = 0; j < N; j++)
{
sort_arr[arr[j] - 1]++;
}
for (int i = 0; i < max; i++)
{
while(sort_arr[i])
{
cout << i+1 << " ";
sort_arr[i]--;
}
}
return 0;
}
How do I find the size of a 2D array in C++? Is there any predefined function like sizeof to determine the size of the array?
Also, can anyone tell me how to detect an error in the getvalue method for arrays while trying to get a value which is not set?
Suppose you were only allowed to use an array, then you could find the size of a 2-D array in the following way.
int ary[][5] = { {1, 2, 3, 4, 5},
{6, 7, 8, 9, 0}
};
int rows = sizeof ary / sizeof ary[0]; // 2 rows
int cols = sizeof ary[0] / sizeof(int); // 5 cols
#include <bits/stdc++.h>
using namespace std;
int main(int argc, char const *argv[])
{
int arr[6][5] = {
{1,2,3,4,5},
{1,2,3,4,5},
{1,2,3,4,5},
{1,2,3,4,5},
{1,2,3,4,5},
{1,2,3,4,5}
};
int rows = sizeof(arr)/sizeof(arr[0]);
int cols = sizeof(arr[0])/sizeof(arr[0][0]);
cout<<rows<<" "<<cols<<endl;
return 0;
}
Output: 6 5
Use an std::vector.
std::vector< std::vector<int> > my_array; /* 2D Array */
my_array.size(); /* size of y */
my_array[0].size(); /* size of x */
Or, if you can only use a good ol' array, you can use sizeof.
sizeof( my_array ); /* y size */
sizeof( my_array[0] ); /* x size */
sizeof(yourObj)/sizeOf(yourObj[0])
should do the trick
int rows = sizeof(arr)/sizeof(arr[0]);
int cols = sizeof(arr[0])/sizeof(arr[0][0]);
Along with the _countof() macro you can refer to the array size using pointer notation, where the array name by itself refers to the row, the indirection operator appended by the array name refers to the column.
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int beans[3][4]{
{ 1, 2, 3, 4 },
{ 5, 6, 7, 8 },
{ 9, 10, 11, 12 }
};
cout << "Row size = " << _countof(beans) // Output row size
<< "\nColumn size = " << _countof(*beans); // Output column size
cout << endl;
// Used in a for loop with a pointer.
int(*pbeans)[4]{ beans };
for (int i{}; i < _countof(beans); ++i) {
cout << endl;
for (int j{}; j < _countof(*beans); ++j) {
cout << setw(4) << pbeans[i][j];
}
};
cout << endl;
}
#include<iostream>
using namespace std ;
int main()
{
int A[3][4] = { {1,2,3,4} , {4,5,7,8} , {9,10,11,12} } ;
for(int rows=0 ; rows<sizeof(A)/sizeof(*A) ; rows++)
{
for(int columns=0 ; columns< sizeof(*A) / sizeof(*A[0]) ; columns++)
{
cout<<A[rows][columns] <<"\t" ;
}
cout<<endl ;
}
}
Here is one possible solution of first part
#include<iostream>
using namespace std;
int main()
{
int marks[][4] = {
10, 20, 30, 50,
40, 50, 60, 60,
10, 20, 10, 70
};
int rows = sizeof(marks)/sizeof(marks[0]);
int cols = sizeof(marks)/(sizeof(int)*rows);
for(int i=0; i<rows; i++)
{
for(int j=0; j<cols; j++)
{
cout<<marks[i][j]<<" ";
}
cout<<endl;
}
return 0;
}
The other answers above have answered your first question.
As for your second question, how to detect an error of getting a value that is not set, I am not sure which of the following situation you mean:
Accessing an array element using an invalid index:
If you use
std::vector, you can use vector::at function instead of [] operator
to get the value, if the index is invalid, an out_of_range exception
will be thrown.
Accessing a valid index, but the element has not been set yet:
As far as I know, there is no direct way of it. However, the following
common practices can probably solve you problem: (1) Initializes all elements to a value that you are certain that is impossible to have. For example, if you are dealing with positive integers, set all elements to -1, so you know the value is not set yet when you
find it being -1. (2). Simply use a bool array of the same size to
indicate whether the element of the same index is set or not, this
applies when all values are "possible".
int arr[5][4];
For the row subscript(4 raise to 2, include cmath to use pow):
sizeof(arr1)/pow(4,2)
Column subscript:
sizeof(*arr1)/4
4 means 4 bytes, size of int.
This can also be tried to get the row and column size of a 2-D array
matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]
int rows = matrix.size();
int cols = matrix[0].size();
cout<<rows<< " "<<cols;
Ouput:
3 4
There is a standard template to determine sizes of the dimensions of arrays: std::extent_v
#include <type_traits>
#include <iostream>
int main()
{
int a[3][4];
std::cout << "rows:" << std::extent_v<decltype(a), 0> << '\n';
std::cout << "cols:" << std::extent_v<decltype(a), 1> << '\n';
std::cout << "size in bytes:" << sizeof(a) << '\n';
}
Output:
rows:3
cols:4
size in bytes:48