I want to display all entries of 2D array in c++ - c++

Question

I want to create a program that take size of rows and columns of 2D array from user, and then take all entries of array from user. And Finally display all the entries from array[0][0] to array[size][size].
My code is:
#include <iostream>
using namespace std;
int main()
{
int rows, columns;
int tom[rows][columns];
cout << "Size of array rows: ";
cin >> rows;
cout << "Size of array columns: ";
cin >> columns;
for(int count1 = 0; count1 < rows; count1++)
{
for(int count2 = 0; count2 < columns; count2++)
{
cout << "Enter entry of row " << count1 << " and column " << count2 << ": ";
cin >> tom[count1][count2];
}
}
for(int i = 0; i < rows; i++)
{
for(int j = 0; j < columns; j++)
{
cout << tom[i][j] << endl;
}
}
return 0;
}
Output is:
Size of array rows: 2
Size of array columns: 3
Enter entry of row 0 and column 0: 1
Enter entry of row 0 and column 1: 2
Enter entry of row 0 and column 2: 3
Enter entry of row 1 and column 0: 12
Enter entry of row 1 and column 1: 13
Enter entry of row 1 and column 2: 14
12
13
14
12
13
14
It should give output:
1
2
3
12
13
14
What is the problem?
Please help.

Answer 1

You can't dynamically create an array like this. This shouldn't even compile. And even if it did, you are creating the array before letting the user input the dimension. For the proper Approach use std::vector:
#include <iostream>
#include <vector>
int main()
{
int rows, columns;
std::cout << "Size of array rows: ";
std::cin >> rows;
std::cout << "Size of array columns: ";
std::cin >> columns;
std::vector<std::vector<int>> tom(rows, std::vector<int>(columns));
for (int count1 = 0; count1 < rows; count1++)
{
for (int count2 = 0; count2 < columns; count2++)
{
std::cout << "Enter entry of row " << count1 << " and column " << count2 << ": ";
std::cin >> tom[count1][count2];
}
}
for (int i = 0; i < rows; i++)
{
for (int j = 0; j < columns; j++)
{
std::cout << tom[i][j] << std::endl;
}
}
return 0;
}
output:
Size of array rows: 2
Size of array columns: 3
Enter entry of row 0 and column 0: 1
Enter entry of row 0 and column 1: 2
Enter entry of row 0 and column 2: 3
Enter entry of row 1 and column 0: 12
Enter entry of row 1 and column 1: 13
Enter entry of row 1 and column 2: 14
1
2
3
12
13
14
please don't use using namespace std; - read here why.

Answer 2

You initialized tom before actually getting the number of rows/columns.
#include <iostream>
int main()
{
int rows, columns;
std::cout << "Rows: ";
std::cin >> rows;
std::cout << "Columns: ";
std::cin >> columns;
int arr[rows][columns];
for(int i = 0; i < rows; i++)
{
for(int j = 0; j < columns; j++)
{
std::cout << "Enter the value for [" << i << "][" << j << "] : ";
std::cin >> arr[i][j];
}
}
for(int i = 0; i < rows; i++)
{
for(int j = 0; j < columns; j++)
{
std::cout << arr[i][j] << " ";
}
std::cout << std::endl;
}
return 0;
}

Answer 3

You cannot declare an array like this:
int tom[rows][columns];
since values of rows and columns are not known yet.
Instead you should initialize this array dynamically after asking for values.
Here is your code fixed:
#include <iostream>
using namespace std;
int main()
{
int rows, columns;
int **tom;
cout << "Size of array rows: ";
cin >> rows;
cout << "Size of array columns: ";
cin >> columns;
tom = new int*[rows];
for (int i = 0; i < rows; i++) {
tom[i] = new int[columns];
}
for(int count1 = 0; count1 < rows; count1++)
{
for(int count2 = 0; count2 < columns; count2++)
{
cout << "Enter entry of row " << count1 << " and column " << count2 << ": ";
cin >> tom[count1][count2];
}
}
for(int i = 0; i < rows; i++)
{
for(int j = 0; j < columns; j++)
{
cout << tom[i][j] << endl;
}
}
return 0;
}