How to create a dynamic array in C++11 with size from command line arguments?

Question

Program

#include <iostream>
#include <cstdlib>
#include <cstring>

using namespace std;

int main(int argc, char** argv)  
{
    if(argc < 2)
        exit(1);
    cout<<strtof(argv[1],NULL);
    int SIZE = (int) (strtof(argv[1],NULL)/8);
    int **arr = new int[SIZE][SIZE*4]();

    for(int i = 0; i < SIZE; i++) {
        for(int j = 0; j < SIZE*4; j++) {
            cout<<arr[i][j];
        }
    }

    return 0;
}

Input

32

Error

prog.cpp: In function ‘int main(int, char**)’: prog.cpp:13:39: error: array size in new-expression must be constant
     int **arr = new int[SIZE][SIZE*4]();
                                       ^
prog.cpp:13:39: error: the value of ‘SIZE’ is not usable in a constant expression prog.cpp:12:9: note: ‘int SIZE’ is not const
     int SIZE = (int) (strtof(argv[1],NULL)/8);
         ^~~~

Is there any other way, rather than using malloc or calloc to do this in C++11?

Code in Ideone

I want to be able to read and write binary data easily into files. — Tarun Maganti, Sep 09 '19 at 12:15
It is possible to create a 2D dynamic array however vectors are better and easier to use. — drescherjm, Sep 09 '19 at 12:15
Not in any sane way -- leave raw arrays to C, where they belong. — Quentin, Sep 09 '19 at 12:15
Related: https://stackoverflow.com/questions/936687/how-do-i-declare-a-2d-array-in-c-using-new — drescherjm, Sep 09 '19 at 12:17
Is there any informative binary i/o using vectors for reference? — Tarun Maganti, Sep 09 '19 at 12:18
Oh, it's possible, but you can't do it all at once. If you really want to `new` and `delete` your own arrays, you must do each one individually... One for the array of arrays, and one for each of the arrays... and then you must `delete[]` each one individually as well. Yuck! You're **much** better off with `std::vector>`. — Mark Storer, Sep 09 '19 at 12:18
This may also be helpful:[https://stackoverflow.com/questions/2076624/c-matrix-class](https://stackoverflow.com/questions/2076624/c-matrix-class) — drescherjm, Sep 09 '19 at 12:20
That seems like a straight-up duplicate of 936687, yeah. None of the high-scoring answers cover the whole "vector is better" thing very well, if at all, though I haven't read all the answers. — Mark Storer, Sep 09 '19 at 12:34

score 0 · Answer 1 · answered Sep 09 '19 at 13:50

If you really don't want to use vector, then maybe this working code can help.

#include <iostream>
#include <cstdlib>
#include <cstring>

using namespace std;

int main(int argc, char** argv)  
{
    if(argc < 2)
        exit(1);
    cout<<strtof(argv[1],NULL);
    int SIZE = (int) (strtof(argv[1],NULL)/8);
    int** arr = new int*[SIZE];

    for(int i = 0; i < SIZE; i++) {
        arr[i] = new int[ 4* SIZE];
    }

    for(int i = 0; i < SIZE; i++) {
        for(int j = 0; j < SIZE*4; j++) {
            cout<<arr[i][j];
        }
    }

    return 0;
}

How to create a dynamic array in C++11 with size from command line arguments?

Program

Input

Error

1 Answers1