c++ Dynamic Allocation of 2D Array Class - c++

Very new beginner here and I'm at the end of my rope with this assignment and would really appreciate any help :). Apologies in advance for the length.
I'm having some trouble with retrieving and setting values for my dynamically allocated 2D array. I have a class, defined below, that should construct a 2D array, at which point I need the option to set a value to a given point in the array and also to retrieve a value at a given point.
I ran the debugger, and got as far as to figure out that I have a segmentation fault when the setValue function runs. Can anyone help me understand what I'm doing wrong? As a beginner, the easier terms the better :). Thank you kindly in advance.
#include <iostream>
using namespace std;
class array2D
{
protected:
int xRes;
int yRes;
float ** xtable;
public:
array2D (int xResolution, int yResolution);
~array2D() {}
void getSize(int &xResolution, int &yResolution);
void setValue(int x,int y,float val);
float getValue(int x,int y);
};
array2D::array2D(int xResolution, int yResolution)
{
xRes=xResolution;
yRes=yResolution;
float ** xtable = new float*[yResolution];
for(int i=0;i < yResolution;i++)
{
xtable[i] = new float[xResolution];
}
for(int i=0;i < yRes;i++)
{
for(int j=0;j < xRes;j++)
{
xtable[i][j]=45;
}
}
}
void array2D::getSize(int &xResolution, int &yResolution)
{
xResolution=xRes;
yResolution=yRes;
cout << "Size of Array: " << xResolution << ", " << yResolution << endl;
}
void array2D::setValue(int x,int y,float val)
{
xtable[x][y] = val;
}
float array2D::getValue(int x,int y)
{
return xtable[x][y];
}
int main()
{
array2D *a = new array2D(320,240);
int xRes, yRes;
a->getSize(xRes,yRes);
for(int i=0;i < yRes;i++)
{
for(int j=0;j < xRes;j++)
{
a->setValue(i,j,100.0);
}
}
for(int i=0;i < yRes;i++)
{
for(int j=0;j < xRes;j++)
{
cout << a->getValue(i,j) << " ";
}
cout << endl;
}
delete[] a;
}

The line
float ** xtable = new float*[yResolution];
creates a function local variable. The member variable of the class still remains uninitialized. That's not what you want. To allocate memory and assign it to the member variable, remove the type specifier from that line. Just use:
xtable = new float*[yResolution];
Also, you need to switch the use of yResolution and xResolution in those lines. Otherwise, getValue and setValue will be using the indices incorrectly.
Swap the use of xResolution and yResolution in the following lines so that you use:
float ** xtable = new float*[xResolution];
for(int i=0;i < xResolution;i++)
{
xtable[i] = new float[yResolution];
}
Swap the use of xRes and yRes in the following lines so that you use:
for(int i=0;i < xRes;i++)
{
for(int j=0;j < yRes;j++)
{
xtable[i][j]=45;
}
}
Since your class acquires resources using dynamic memory allocation, you should read up on The Rule of Three and update your class accordingly.

int r,c;
cin>>r>>c;
int** p=new int*[r];
for(int i=0;i<r;i++) {
p[i]=new int[c];
}
for(int i=0;i<r;i++) {
delete [] p[i];
}
delete [] p;
Using this code you can create a 2D Dynamic Array in C++ which would allocate the memory of this array in the Heap Memory.

Related

Function and Array in C++: Unexpected output

I need some help here please.
I just started learning C++ (coming from Python background).
I'm trying to familiarize myself with arrays and functions. Wrote a bunch of functions to do as stated, above each one.
However, the function which is supposed to sum elements in an array and return their sum, seem to be adding 10 to the result, no matter the argument supplied as input. What am I doing wrong please, as I can't seem to find this out. Any help on general layout of my code also would be appreciated.
// WORKING WITH ARRAYS AND FUNCTIONS
#include<iostream>
using namespace std;
// FUNCTION TO INSTANTIATE ARRAY INT OF LENGTH N.
int* array_creator(int n)
{
static int ary_of_ten[10]; //declare array
for (int i=0; i<n; i++) //use loop to fill it up
{
ary_of_ten[i] = i+1;
}
return ary_of_ten;
}
//FUNCTION TO PRINT ARRAY ELEMENTS
void* array_printer(int arr[], int array_lenght)
{
for (int i=0; i<array_lenght-1; i++)
{
cout << arr[i] << " ";
}
cout << arr[array_lenght-1] << endl;
}
//FUNCTION ACCEPTS INT ARRAYS AND RETURNS ARRAY OF SQUARE OF EACH ELEMENT
int* square_array(int *p, int array_length)
{
const int ary_sz(array_length);
static int sqd_values[10];
for (int i=0; i<ary_sz; i++)
{
*(sqd_values + i) = *(p+i) * *(p+i);
}
return sqd_values;
}
//FUNCTION ACCEPTS INT ARRAYS AND RETURNS SUM OF ITS ELEMENTS
int sum_array(int *arry, int array_length)
{
int summation;
for(int i=0; i<array_length; i++)
{
summation += *(arry + i);
}
return summation;
}
int main()
{
cout << sum_array(array_creator(10), 3) << endl;
array_printer(array_creator(10), 10); //print array of 1-10 elements
array_printer(square_array(array_creator(10), 10), 10); //prt arry of sqrd values
return 0;
}
summation shuld be initialized to 0.
int summation=0;

C++: Error while allocating memory for a two dimensional array using functions

I am trying to understand passing by reference and passing by value. In this program I have a two dimensional array which is declared in main and whose size is allocated in a function. In the allocate2DArrayRef() function, I obtain the two sizes and dynamically allocate and initialize the array Array2D.
Now I am trying to understand how to do the same by pointers. I have written another function allocate2DArrayPtr() in where I pass a pointer to the two dimensional array, get the value of the sizes - sizeX and sizeY and then allocate the memory to the variable secArray2D.
When I run the program it hangs when I try to print out secArray2D. I am assuming this implies that the function allocate2DArrayPtr() has not been successful in dynamically allocating memory to the array secArray2D.
My final goal is to write a program which has a function that dynamically allocated memory and initializes multiple arrays of various dimensions which are read from an input file. I know I can expand on passing by reference function allocate2DArrayRef() to achieve my goal. But I am curious to learn why my function allocate2DArrayPtr() is not working as I want to be clear on how to pass by pointers also. I do know how to change allocate2DArrayPtr() to return a pointer but I would like to pass the array as a parameter.
I am running the program on windows 7 using Codeblocks 13.12 IDE.
#include <iostream>
using namespace std;
void allocate2DArrayRef(int **&, int &, int &);
void allocate2DArrayPtr(int ***, int *, int *);
int main()
{
int sizeX, sizeY;
int **Array2D;
allocate2DArrayRef(Array2D, sizeX, sizeY);
for(int i=0; i< sizeX; i++)
{
for(int j=0; j< sizeY; j++)
{
cout << "Array2D[" << i << "][" << j << "]:" << Array2D[i][j] << endl;
}
}
cout << endl << endl;
int **secArray2D;
allocate2DArrayPtr(&secArray2D, &sizeX, &sizeY);
for(int i=0; i<sizeX; i++)
{
for(int j=0; j<sizeY; j++)
{
cout << "secArray2D[" << i << "][" << j << "]:" << secArray2D[i][j] << endl;
}
}
return 0;
}
void allocate2DArrayRef(int **&locArray, int& indexFirst, int& indexSecond)
{
indexFirst = 4;
indexSecond = 5;
locArray = new int*[indexFirst];
for(int i=0; i<indexFirst ; i++)
{
locArray[i] = new int[indexSecond];
for(int j=0; j<indexSecond; j++)
{
locArray[i][j] = i*j;
}
}
}
void allocate2DArrayPtr(int ***locArray, int *indexFirst, int *indexSecond)
{
*indexFirst = 2;
*indexSecond = 3;
int **temp = *locArray;
temp = new int*[*indexFirst];
for(int i=0; i<(*indexFirst) ; i++)
{
temp[i] = new int[*indexSecond];
for(int j=0; j<(*indexSecond); j++)
{
temp[i][j] = i+j;
}
}
}
The reason allocate2DArrayPtr does not work is you never set locArray to point to the array you create in the function. This means when you return from the function secArray2D still is uninitialized.
Adding
*locArray = temp;
To the end of allocate2DArrayPtr will fix the problem.
Or you can reference your to temp like this:
int **&temp = *locArray;

Two-dimensional dynamic allocated arrays created inside a class in C++ [duplicate]

This question already has answers here:
What is The Rule of Three?
(8 answers)
Closed 7 years ago.
I'm relatively new at c++ and other object oriented languages as a whole (have done one semester of C classes and now I'm having c++ classes). I'm having problems concerning the making of a dynamically allocated two-dimensional array through a class with a classmate.
The exercise itself would be:
Prepare a class named "matrix" capable of storing two-dimensional
arrays dynamically allocated (for float variables). Remember that
information such as height and width have to be properly stored
somewhere, element pointers too.
The class needs to contain constructors that allow the creation of
objects using one of the following strategies: The creation of an
array consisting of MxN elements, example:
Array A (4, 5);
The creation of an empty array:
Array B;
The creation of an array that is a copy of another previous one:
Array C (A);
After some time trying to figure out why it wasn't working properly, our code is currently this:
obs: "Matriz" is how we call a two-dimensional array in our language.
Matriz.h
#pragma once
class Matriz{
public:
int l, c;
float** matriz;
void setL(int _l);
void setC(int _c);
int getL();
int getC();
Matriz();
Matriz(int _l, int _c);
Matriz(Matriz& m);
float **getMatriz();
float getElement(int pL, int pC);
void setElement(int pL, int pC, float value);
};
Matriz.cpp
#include "Matriz.h"
Matriz::Matriz(){
l = c = 0;
matriz = new float*[l];
for (int i = 0; i<l; i++) {
matriz[l] = new float[c];
}
}
Matriz::Matriz(Matriz& m){
l = m.getL();
c = m.getC();
matriz = new float*[l];
for (int i = 0; i<l; i++) {
matriz[l] = new float[c];
}
for (int i = 0; i<l; i++) {
for (int j = 0; j<l; j++) {
matriz[i][j] = m.matriz[i][j];
}
}
}
Matriz::Matriz(int _l, int _c){
l = _l;
c = _c;
matriz = new float*[l];
for (int i = 0; i<l; i++) {
matriz[l] = new float[c];
}
}
float **Matriz::getMatriz(){
return matriz;
}
int Matriz::getC(){
return c;
}
int Matriz::getL(){
return l;
}
void Matriz::setC(int _c){
c = _c;
}
void Matriz::setL(int _l){
l = _l;
}
float Matriz::getElement(int pL, int pC){
return matriz[pL][pC];
}
void Matriz::setElement(int pL, int pC, float value){
matriz[pL][pC] = value;
}
main.cpp
#include "stdafx.h"
int _tmain(int argc, _TCHAR* argv[])
{
int l = 2, c = 2;
float **m;
m = new float*[l];
for (int i=0; i<2; i++) {
m[i] = new float[c];
}
Matriz a(2, 2);
a.setC(2);
a.setL(2);
cout << " c = " << a.getC() << " l= " << a.getL() << "\n";
for (int i = 0; i<l; i++) {
for (int j = 0; j<c; j++) {
a.setElement(i, j, 0);
cout << " Elemento " << 1 << " " << 1 << " = " << a.getElement(l, c) << "\n";
}
}
a.setElement(0, 0, 1); // <- this is just for testing
system("pause");
}
iostream and the class header are both included at stdafx.h
Compiling it at MSVS 2013 breaks at
void Matriz::setElement(int pL, int pC, float value){
matriz[pL][pC] = value;
}
And we're not really sure why, the debugger is giving me
"Unhandled exception at 0x01092E27 in ConsoleApplication15.exe: 0xC0000005: Access violation writing location 0xCDCDCDD1."
We suspect however that something with the array is wrong, and when the program tries to write something into an element of it, it simply doesn't exist/can't be reached, making it impossible to change the value of that specific element.
We're thankful for any help or advice, fell free to suggest improvements or coding advices, learning new things is always good =).
I think your error is actually here: a.getElement(l, c). l and c are the bounds for the array, e.g. 2, when your largest index should only ever be 1.
The other serious flaw (pointed out by twsaef) is your constructor:
for (int i = 0; i<l; i++) {
matriz[l] = new float[c];
}
Should be
for (int i = 0; i<l; i++) {
matriz[i] = new float[c];
}
While I'm at it, this is redundant:
Matriz a(2, 2);
a.setC(2);
a.setL(2);
Because the constructor for Matriz will set l and c for you.
Also, what are you planning on doing with this:
float **m;
m = new float*[l];
for (int i=0; i<2; i++) {
m[i] = new float[c];
}
Currently it's not used for anything.
Then, as PaulMcKenzie pointed out, what will happen to your dynamically allocated memory when a Matriz instance goes out of scope?
As Matt McNabb pointed out, what if you need to resize a Matriz instance when setC() or setL() are called? At the moment, they only set the member variable and do nothing with the memory.

Referencing Elements of Class Arrays using C++

I am trying to switch the elements of a class array using pointers. It is not outputting what I want. I tried using pointers in the function, but it's not allowed. It's also not allowed to call the function onto the class object without using a pointer, since I declared the class object using a double pointer. I am not using this method simply to solve a small problem, but just to practice using this method for more difficult problems.
Here is my code:
#include <iostream>
#include <algorithm>
using namespace std;
class thing{
public:
int index;
int value;
thing();
private: int number;
};
thing::thing()
{
number = 0;
}
void arrange(thing array[]){
for(int i=0; i<19; ++i){
if(array[i].value<array[i+1].value){
swap(array[i], array[i+1]);
arrange(array);
}
}
}
int main(){
thing** things = new thing*[20];
for (int i=0; i < 20; ++i)
{
things[i] = new thing(); // default constructor
things[i]->index = i;
things[i]->value=rand() % 100;
}
cout << "The random array is: " << endl;
for(int i=0;i<20;++i){
cout << things[i]->value << endl;
}
arrange(*things);
cout << "The arranged array is: " << endl;
for (int i=0; i < 20; ++i)
{
cout << things[i]->value << endl;
}
return 0;
}
When you call arrange(*things), you're just passing the first element of things to the function, not the array. It should be array(things). Then the arrange function should be written to use pointers:
void arrange(thing* array[]){
for(int i=0; i<19; ++i){
if(array[i]->value<array[i+1]->value){
swap(array[i], array[i+1]);
arrange(array);
}
}
}
Here you create an array of pointers to thing:
thing** things = new thing*[20];
Here you dereference it and get a pointer to thing which is stored at thing[0]:
arrange(*things);
But this function declaration
void arrange(thing array[])
treats this pointer as an array of thing, so that *things points to it first element, which is absolutely not what it really is.
You should change your arrange() function to use correct type:
void arrange(thing* array[]){
for(int i=0; i<19; ++i){
if(array[i]->value<array[i+1]->value){
swap(array[i], array[i+1]);
arrange(array);
}
}
}
And call it as:
arrange(things);
Regarding using vectors, you don't need to use any pointers at all.
std::vector<thing> things(20);
for (int i=0; i < things.size(); ++i)
{
things[i].index = i;
things[i].value=rand() % 100;
}
arrange(things);
void arrange(std::vector<thing>& array){
for(int i=0; i + 1 < things.size(); ++i){
if(array[i].value<array[i+1].value){
swap(array[i], array[i+1]);
arrange(array);
}
}
}

dynamic allocation of int array in c++ and assigning value

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"