Question

I want to use an array of string the same as this:

char arr[][20] = {"username1", "username2"};

after i have not problem to get values, for example :

printf("%s", arr[0]); // for "username1"

i have problem to insert new string to this array, something like this!? :

arr[2] = "username3"; // or sprintf(arr[2], "%s", "username3");

Answer 1

The wrong way of doing this would be as follows:

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

int main(void) 
{
    char arr[][20] = {"username1", "username2"};
    printf("%s\n", arr[0]);
    printf("%s\n", arr[1]);
    printf("sizeof arr : %d \n", sizeof(arr)/sizeof(*arr));
    strcpy(arr[2],"username3");
    printf("%s\n", arr[2]);
    printf("sizeof arr : %d \n", sizeof(arr)/sizeof(*arr));
    return 0;
}

because, when you write something like the below

char arr[][20] = {"username1", "username2"};

The first dimension is automatically calculated based on the given initializer, in this case, it is 2 (i.e. index 0 and 1).

Later when you try to access (read or write), the third element i.e. index 2, it is a undefined behavior.

If you want to increase the first dimension of a 2d array, the correct way of doing that would be as follows:

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

#define LEN 20

int main(void) 
{
    char **arr = NULL;
    int n=2, n1;
    int i;

    if ((arr = malloc(sizeof(char *) * n)) == NULL) {
        printf("unable to allocate memory \n");
        return -1;
    }

    for (i=0; i<n ; i++) {
        if ((arr[i] = malloc(sizeof(char) * LEN)) == NULL) {
            printf("unable to allocate memory \n");
            return -1;
        }
    }

    strcpy(arr[0], "username1");
    strcpy(arr[1], "username2");

    printf("%s\n", arr[0]);
    printf("%s\n", arr[1]);

    /* add 5 more elements to arr */
    n1 = 5;

    if ((arr = realloc(arr, sizeof(char *) * (n + n1))) == NULL) {
        printf("unable to allocate memory \n");
        return -1;
    }

    for (i=n; i<n1+n ; i++) {
        if ((arr[i] = malloc(sizeof(char) * LEN)) == NULL) {
            printf("unable to allocate memory \n");
            return -1;
        }
    }

    strcpy(arr[2], "username2");
    strcpy(arr[3], "username3");
    strcpy(arr[4], "username4");
    strcpy(arr[5], "username5");
    strcpy(arr[6], "username6");

    /* if you uncomment the below, it will lead to undefined behavior */
    //strcpy(arr[7], "username7");

    for (i=0; i<n+n1 ; i++)
        printf("%d : %s \n", i, arr[i]);

    for (i=0; i<n+n1 ; i++) 
        free(arr[i]);
    free(arr);

    return 0;
}

$ ./a.out 
username1
username2
0 : username1 
1 : username2 
2 : username2 
3 : username3 
4 : username4 
5 : username5 
6 : username6 
$ 

Hope this helps!