I am new to c++ and i was trying to do folloing two things without help of std::vector( This was done earlier)
Define an array of integer and the size of array is not known to me.
Pass this array into another function and output all the values stored in array.
int _tmain()
{
int* a = NULL;
int n;
std::cin >> n;
a = new int[n];
for (int i=0; i<n; i++) {
a[i] = 0;
}
testFunction(a,n);
delete [] a;
a = NULL;
}
void testFunction( int x[], int n)
{
for(int i =0;i<n;++n)
{
std::cout<<x[i];
}
}
But i can see that its not allocating memory of 10 bytes and all the time a single memory is filled up with 0.
Can anyone please help me if i am lacking something ? Or is there any alternative way for this apart from vector.
Thanks in Advance
I modified with one thing as i realized that i put ++n instead of i
int _tmain()
{
int* a = NULL;
int n;
std::cin >> n;
a = new int[n];
for (int i=0; i<n; i++) {
a[i] = i;
}
testFunction(a,n);
delete [] a;
a = NULL;
}
void testFunction( int x[], int n)
{
for(int i =0;i<n;++i)
{
std::cout<<x[i];
}
}
I’m not sure I understand all yours problems but the typo
for(int i =0;i<n;++n) in testFunction led to a very long loop.
Write:
for(int i =0;i<n;++i)
this print yours n "0"
Related
#include <iostream>
using namespace std;
struct stud
{
char name[10];
int id;
};
int input(stud a[], int size)
{
for(int i=1; i<=size; i++)
{
cout<<"name = ";
cin>>a[i].name;
cout<<"id = ";
cin>>a[i].id;
}
cout<<endl;
return 0;
}
int output(stud a[], int size)
{
for(int i=1; i<=size; i++)
{
cout<<"name = "<<a[i].name<<" ";
cout<<"id = "<<a[i].id<<" ";
}
cout<<endl;
return 0;
}
int copy(stud a[], stud x[], int size)
{
for(int i=1; i<=size; i++)
{
x[i].name=a[i].name;
x[i].id=a[i].id;
}
output(x,size);
cout<<endl;
return 0;
}
int main()
{
struct stud s[3], x[3];
input(s,3);
output(s,3);
copy(s,x,3);
return 0;
}
In this program the statement in function copy x[i].name =a[i].name; is not copying contents from 1 structure object to another. I have tried to put this statement in for loop for(int j=1;j<=10;j++) x[i].name[j] =a[i].name[j]; but still not working.
please suggest what should be changed or some alternatives for this.
i'll be very thankful to you for this.
regards,
umar
Either using a loop to copy each character in the name field or using thestrcpy function from <cstring> header works.
int copy(stud a[], stud x[], int size) {
for(int i = 1; i <= size; i++) {
// for(unsigned j = 0; j < 10; j++) {
// x[i].name[j] = a[i].name[j];
// }
strcpy(x[i].name, a[i].name);
x[i].id = a[i].id;
}
output(x, size);
cout << endl;
return 0;
}
But since you tagged this as c++, consider using std::string instead of a char array, unless you have a particular reason for using a char array. In that case x[i].name = a[i].name would have worked just fine and you could also use the standard algorithm library for copy. Also, using std::array instead of a raw C array for you "array of structures" might be a better option (does not degenerate into a pointer like a regular C array does).
Evrey single one of your loops is wrong, because in C++ arrays start at zero. So not
for(int i=1; i<=size; i++)
instead
for(int i=0; i<size; i++)
You cannot copy arrays by writing a = b;. Since your arrays are really strings there's a built in function strcpy to copy strings.
strcpy(x[i].name, a[i].name);
If you use = to copy struct, the char array inside that struct will be copied. You don't need to do anything more.
#include <iostream>
using namespace std;
typedef struct{
char name[10];
} type_t;
int main() {
type_t a = {"hihi"};
type_t b;
b = a;
a.name[0] = 'a';
cout<<a.name<<endl;
cout<<b.name<<endl;
return 0;
}
output:
aihi
hihi
ideone: https://ideone.com/Zk5YFd
This question already has answers here:
Closed 10 years ago.
The following code is printing garbage values. I am passing an array to a function which adds 5 to every element, but when it returns that array's pointer, the main is showing garbage.
I have tried both indexing and pointers there in main but still same results. How can I fix this?
# include <conio.h>
# include <iostream>
using namespace std;
int * add5ToEveryElement(int arr[], int size)
{
int theArray[5];
for(int i=0; i<size; i++)
{
theArray[i] = arr[i] + 5;
cout<<theArray[i]<<endl;
}
return theArray;
}
void main()
{
const int size = 5;
int noArr[size];
for(int i=0; i<size; i++)
{
noArr[i] = i;
}
int *arr = add5ToEveryElement(noArr, size);
cout<<endl;cout<<endl;
for(int i=0; i<size; i++)
{
cout<<arr[i]<<endl;
}
cout<<endl;cout<<endl;cout<<endl;cout<<endl;
for(int i=0; i<size; i++)
{
cout<<*arr<<endl;
*arr++;
}
getch();
}
theArray is a local array in the function add5ToEveryElement() which you are returning to main(). This is undefined behaviour.
Minimally you can change this line:
int theArray[5];
to:
int *theArray = new int[5];
It'll work fine. Don't forget to delete it later in main(). SInce you modify the original pointer, save it:
int *arr = add5ToEveryElement(noArr, size);
int *org = arr;
// Rest of the code
//Finally
delete[] org;
Returning an array from a function is generally considered bad.
Unless you MUST have a "new" array, I would suggest the typical case in C and C++ is to modify the input array. If the CALLING function wants to have two separate arrays, then it can do so by making it's own copy. Alternatively, you could write your code to have two arrays passed into your function, e.g.
void add5ToEveryElement(int arr[], int arr2[], int size)
{
for(int i=0; i<size; i++)
{
arr2[i] = arr[i] + 5;
cout<<theArray[i]<<endl;
}
}
then your main would call with two array arguments, and if you wish to use the same as input and output it will do that too.
Sure, this isn't exactly the answer to your question, but it gives a "better" solution to your problem.
I generally dislike allocation in functions - especially "hidden" allocation (this function says it's adding 5 to every element, not "allocate array with added 5 to each element". Code should never do surprising things, and allocating memory is a little bit of a surprise if you only asked for adding 5 to each element)
this is the perfect code
# include <conio.h>
# include <iostream>
using namespace std;
int * add5ToEveryElement(int arr[], int size)
{
int *theArray = new int[5];
for(int i=0; i<size; i++)
{
theArray[i] = arr[i] + 5;
cout<<theArray[i]<<endl;
}
return theArray;
}
void main()
{
const int size = 5;
int noArr[size];
for(int i=0; i<size; i++)
{
noArr[i] = i;
}
int *arr = add5ToEveryElement(noArr, size);
int *p = arr;
cout<<endl;cout<<endl;
for(int i=0; i<size; i++)
{
cout<<arr[i]<<endl;
}
cout<<endl;cout<<endl;cout<<endl;cout<<endl;
for(int i=0; i<size; i++)
{
cout<<*arr<<endl;
*arr++;
}
getch();
delete[] p;
}
I wrote two functions - one to create two-dimentional double array, and another one to delete it.
double** createMatrix(int n)
{
double **a = new double *[n];
for (int i=0; i < n; i++)
a[i] = new double[n];
return a;
}
void deleteMatrix(double** a, int n)
{
for (int i=0; i < n; i++)
delete [] a[i]; // ERROR HERE
delete []a;
}
Allocated array is working fine. But when I try to free it, I get an error (on a marked line): "project2.exe has triggered a breakpoint.".
I'm using Visual Studio 2012.
edit:
I created a full program:
int main()
{
const int n = 10;
double **m = createMatrix(n);
deleteMatrix(m, n);
return 0;
}
And it's working fine. Also, I found my problem. It was a typo in copyMatrix function.
for (int j=0; j <= n; j++) // should be < instead of <=
a[i][j] = originalMatrix[i][j];
Thanks a lot for your help!
The obvious solution is not to use an array in the first place.
How to create an n x n matrix ?
#include <iostream>
#include <vector>
using Row = std::vector<int>;
using Matrix = std::vector<Row>;
int main() {
size_t const n = 5;
Matrix matrix(Row(n), n);
}
Simple right ? And as a bonus, copy, move and destruction are provided free of charge.
don't know why but i get an error: after this structure i can't index the matrix, so i cant use the "indexing method" over the defined matrix.Can anyone tell me why? or how to fix it?
Header:
const int days=31;
const int exp=6;
struct Array{
int days;
int exp;
int **M;
};
Constuctor:
void constr(Array loc){
//Construct of 31*6 Matrix, were 31 nr. of days and 6 specific types:
//0-HouseKeeping, 1-Food, 2-Transport, 3-Clothing, 4-TelNet, 5-others
loc.days = days;
loc.exp = exp;
loc.M = new int*[loc.days];
for(int i=0; i<loc.days;i++ ){
loc.M[i] = new int[loc.exp];
for (int j = 0; j< loc.exp; j++){
loc.M[i][j] = 0;
}
}
}
Controller.cpp
void add(int cant,int tip, Array M){
//Adds to current day the amount to a specific type
currDay();
M[currentDay][tip] += cant; ////////////error
}
void insert(int zi,int tip,int cant, Array M){
//Adds to current day the amount to a specific type
M[zi][tip] = cant; ///////////error
}
void removeDay(int day, Array M){
for(int i = 0; i<6; i++)
M[day][i] = 0; ///////////error
//zi and tip ~ day type... for easier read.
//i need to manage the expenses of a family in a month doesn't matter which
ERROR: error: no match for 'operator[]'
UI(where constructor is used):
int main(){
Array M;
constr(M);
printMenu();
return 0;
}
You're not accessing the member:
M.M[currentDay][tip]
instead of
M[currentDay][tip]
Or you could define operator [] for your struct:
struct Array{
int days;
int exp;
int **M;
int*& operator[] (int idx) { return M[idx]; }
};
You are trying to call operator[] on the type array. You need to get the pointer member first.
M.M[day][i];
That said: You are not writing C++ but some obscure form of bad C. You might want to have a look at the book list and read one of them before pursuing coding any further.
You have at least two problems:
The first is that you are using the actual structure as the array, which wont work. Use e.g. M.M[day][i].
The second is that when you create the array, you pass the structure by value, this means it will be copied to a local variable in the constr function and the data will not be available in the function calling constr. Pass it as a reference instead, i.e. void constr(Array &loc).
The second problem can also be solved by using a constructor in the structure, instead of a separate function:
const int DAYS=31;
const int EXP=6;
struct Array{
int days;
int exp;
int **M;
// Constructor, called when an instance of structure/class is created
Array(){
days = DAYS;
exp = EXP;
M = new int*[days];
for(int i=0; i<days;i++ ){
M[i] = new int[exp];
for (int j = 0; j< exp; j++){
M[i][j] = 0;
}
}
}
// Destructor, called when structure/class is destroyed
~Array(){
if(M){
for(int i=0;i<days;i++){
if(M[i])
delete [] M[i];
}
delete [] M
}
}
// Copy constructor, called when instance of structure/class is copied
Array(const Array &array){
days = array.days;
exp = array.exp;
M = new int*[days];
for(int i=0; i<days;i++ ){
M[i] = new int[exp];
for (int j = 0; j< exp; j++){
M[i][j] = array.M[i][j];
}
}
}
};
I am solving a quantum-mech problem which requires me to find some eigenvalues by manipulating some matrices. The specifics of this problem is not relevant, I just need help with the c++ problem, I am new to this language and after a couple of hours I figured any more attempts at solving it myself would be futile and so I turn to you for help.
I have this problem where glibc detects an error at the end of my program and I cannot deallocate properly, it is far too big to copypaste here so I will just replicate the part that actually gives the error.
void hamiltonian(int, double **&);
int i,j;
int main()
{
int N = 1000; double **A;
hamiltonian(N, A);
//Physics here
.
.
.
.
.
//Delete
for(i=0; i<N; i++){delete []A[i];}
delete []A;
return 0;
}
void hamiltonian(int N, double **&A)
{
A = new double *[N];
for(i=0; i<N; i++)
{
A[i] = new double[N];
for(j=0; j<N; j++)
{
if(i==j)A[i][j] = 2;
if(i==j+1 || i==j-1){A[i][j] = 1;}
}
}
}
According to my professor I have to deallocate in the same function as I allocate but I didn't even think about deallocation after being nearly done with my project and so I have to rewrite a lot of code, the problem is that I cannot deallocate A inside the hamiltonian function as I need it in other functions (inside //Physics).
Surely there must be a way around this? Might sound a bit ignorant of me but this sounds like a less efficient design if I have to deallocate in the same function as I allocate.
According to my professor I have to deallocate in the same function as I allocate
That is pure silliness. Sometimes (almost always) you need to use the allocated struct outside the function. Definitely false for objects, since constructors and destructors are different functions.
Any way, you can get away without using classes, if you make a Matrix struct and associated newMatrix and deleteMatrix functions :)
#include <cstddef>
#include <iostream>
using namespace std;
struct Matrix
{
int n;
int m;
double** v;
};
Matrix newMatrix (int n, int m)
{
Matrix A;
A.n = n;
A.m = m;
A.v = new double*[n];
for( int i = 0; i < n; i++ ){
A.v[i] = new double[m];
}
return A;
}
Matrix newHamiltonianMatrix (int n, int m)
{
Matrix A = newMatrix(n, m);
for( int i = 0; i < A.n; i++ ){
for( int j = 0; j < A.m; j++ ){
A.v[i][j] = 0.0;
if( i == j ){
A.v[i][j] = 2.0;
}
if( i == j + 1 or i == j - 1 ){
A.v[i][j] = 1.0;
}
}
}
return A;
}
void deleteMatrix (Matrix A)
{
for( int i = 0; i < A.n; i++ ){
delete [] A.v[i];
}
delete [] A.v;
A.v = NULL;
}
int main ()
{
Matrix A = newHamiltonianMatrix(10, 20);
for( int i = 0; i < A.n; i++ ){
for( int j = 0; j < A.m; j++ ){
cout << A.v[i][j] << " ";
}
cout << endl;
}
deleteMatrix(A);
}
delete A;
Needs to be
delete[] A;
If you new[] it, you MUST delete[] it. Also, use a vector- they take care of themselves.
vector<vector<double>> matrix;
There are couple of problems with your code.
(1) You are not allocating memory to the pointer members of A. i.e. A[i] are not allocated with new[]. So accessing them is an undefined behavior.
(2) You must do delete[] for a pointer if it was allocated with new[]. In your other function delete A; is wrong. Use delete[] A;
(3) Using new/new[] is not the only way of allocation. In fact you should use such dynamic allocation when there is no choice left. From your code it seems that you are hard coding N=1000. So it's better to use an 2D array.
const int N = 1000; // globally visible
int main ()
{
double A[N][N];
...
}
void hamiltonian (double (&A)[N][N])
{
...
}