How to initialize a 2D array in which the size is determined by argc and argv?

问题

I am working on code which will keep track of each time a specific element in an array is accessed. The array itself will be allocated dynamically based off of the inputs by the user, and so no functions that I have seen are what I'm looking for. To be more specific, how do I dynamically allocate the rows and columns of an array, and then initialize every element to 0? Ex.
./SIM A B

int* array_columns = malloc(atoi(argv[1]) * sizeof(int));
int* array_rows = malloc(atoi(argv[2]) * sizeof(int)); 
int array[*array_rows][*array_columns];

everything that I have seen requires knowing beforehand the number of elements in each row/column. Can anyone give any pointers on how to initialize this array to 0? edit: I have added the line where I attempt to establish the array

回答1:

This program allocates memory using command line parameters and creates a variable that can be accessed using array syntax. It uses calloc to initialize values to zero:

#include <stdio.h>
#include <stdlib.h>


int main(int argc, char *argv[])
{
    int **array;

    int length = atoi(argv[1]);
    int width = atoi(argv[2]);

    array = calloc(length, sizeof(int *));
    for (int i = 0; i < length; i++) {
        array[i] = calloc(width, sizeof(int));
    }

    for (int i = 0; i < length; i++) {
        for (int j = 0; j < width; j++) {
            printf("array[%d][%d] = %d\n", i, j, array[i][j]);
        }
    }

    for (int i = 0; i < length; i++) {
        free(array[i]);
    }
    free(array);

    return 0;
}

Compiled With

gcc -Wall -Werror -o scratch scratch.c

Output

[user@machine]: ./scratch 3 5
array[0][0] = 0
array[0][1] = 0
array[0][2] = 0
array[0][3] = 0
array[0][4] = 0
array[1][0] = 0
array[1][1] = 0
array[1][2] = 0
array[1][3] = 0
array[1][4] = 0
array[2][0] = 0
array[2][1] = 0
array[2][2] = 0
array[2][3] = 0
array[2][4] = 0

Note

I left out input validation and error checking to keep the example small.

Compiler Options

I use -Wall -Werror to turn on all warnings and treat them as errors. This means the compiler won't produce an executable unless all the causes of warnings are fixed. The -o scratch tells the compiler what to name the output file.

回答2:

It doesn't matter 1D, 2D or ND. You can use two strategies:

The first is to create a simple 1D array as

int *a = (int *)malloc(atoi(argv[1]) * atoi(argv[2]) * sizeof(int));

The second is to create an array of arrays as below:

int len = atoi(argv[1]);
int len2 = atoi(argv[2]);
int **a = (int **)malloc(len * sizeof(int *));

for (int i = 0; i < len; ++i) {
    a[i] = (int *)malloc(len2 * sizeof(int));
}

The first variant gives you a simple way to initialize and delete array, the second variant can allow to create bigger array because in this case memory chunks for child arrays can be reserved in different places of the memory.

To initialize it by some value use memset.

For 1D array:

memset(a, 0, sizeof(int) * len * len2);

For array of arrays:

for (int i = 0; i < len; ++i) {
    memset(a[i], 0, sizeof(int) * len2);
}

P.S. Some modern compilers allow you to initialize dynamic array as static but that code can have troubles when you will try to compile it in another environment.
P.P.S. Sorry for my English, I hope someone will fix it to human-readable English.

来源：https://stackoverflow.com/questions/55172407/how-to-initialize-a-2d-array-in-which-the-size-is-determined-by-argc-and-argv

标签

dynamic-memory-allocation