How to store a string in a multidimensional array in C

Question

I'm new at C and I'm trying to do an exercise which asks to insert some strings and then store them. First it requests a multidimensional array where we have for every row of the array a string, and then as an array of pointers. Here's the code for the first part. I don't know how to store into an array some strings that are not already written.

For the second one I have no idea since I've never done exercises with pointers before.

#include <stdio.h>

int main(){
int n; //number of strings
int x; //number of characters per string

    printf("How many strings do you want to insert?");
    scanf("%d", &n);

    if ((n >= 1) && (n <= 20)){
        printf("How many characters per string?");
        scanf("%d", &x);
        char str[x];

            if (x <= 10){
                for(int i = 0; i < n; i++){
                    printf("Insert a string:");
                    scanf("%s", str);
                    for(int j = 0; j < x; j++){
                        char arr[j];
                        arr[j] = str[x];
                        printf("%s", arr);
                    }
        }
            }
            else {
                printf("Error:the number of characters must be < 10");
            }
    }
    else {
        printf("Error: the number must be < 20");
    }

return 0;
}

Sorry, but I'm afraid you'll need to go back to your favorite book/tutorial and read a bit about how to declare and use arrays. — Hulk
– Hulk, Commented Apr 29, 2020 at 15:12
Yes, do some simpler things with pointers before in order to get familiar with the concept of pointers. — Jabberwocky
– Jabberwocky, Commented Apr 29, 2020 at 15:13
Hints: in C, char str[5]; declares an array of 5 characters. char str[x]; would be what is called a Variable Length Array, this is only supported by some versions of the C standard and probably not what you should use as a beginner (even if it might happen to work). You should be careful not to create an array with size zero, accessing that would be Undefined Behavior (you do this at char arr[j];). — Hulk
– Hulk, Commented Apr 29, 2020 at 15:21
I agree. I noticed a couple of mistakes and I was writing an answer listing them. Then I came to the part in which you copy the string to your array... but there's not any 2-D array.. — Roberto Caboni
– Roberto Caboni, Commented Apr 29, 2020 at 15:21

chux · Accepted Answer · 2020-04-29 17:53:27Z

... requests a multidimensional array where we have for every row of the array a string, and then as an array of pointers.

After getting the qualified number of strings, allocate an array of pointers to char.

if ((n >= 1) && (n <= 20)){
  char **string_list = calloc(n, sizeof *string_list);
  assert(string_list);  // or other error checking

(Notice no type in = calloc(n, sizeof *string_list);. Easier to code right, review and maintain.)

Read the strings in a working temp buffer. As "How many characters per string?" likely means the number of characters not including the null character, our str[] needs a +1 in size.

        // char str[x]; // too small
        char str[x+1];

Yet we know x <= 10 and can use a fixed buffer size and limit input length

      for(int i = 0; i < n; i++){
        char str[10+1];
        printf("Insert a string:");
        scanf("%10s", str);  // Notice the 10 - a width limit

        // TBD check if scanf() returned 1 and if str is longer than x

Now allocate a copy of the str

        string_list[j] = strdup(str);
        assert(string_list[j]);  // or other error checking
      }

Later, when done with string_list[], clean-up and free allocations.

for (int i=0; i<n; i++) {
  free(string_list[i]);
}
free(string_list);

What is weak about this:

It uses scanf() rather than fgets() and then parses, has minimal error checking, does not take in strings with spaces, does not handle over-long input, strdup() is not standard -yet, etc.

So the above is a baby step. Better code would handle the weak issues.

Thank you for your answer, it is necessary to usethe function calloc? Is there no other way?
@Schiele There are other ways. What is the concern about calloc() and what restrictions are there that prevent using calloc()?
It's just that this is an exercise which is not supposed to use this function (according to the exercise text)
Then use char **string_list = malloc(sizeof *string_list * n); memset(string_list, 0, sizeof *string_list * n);
The problem is that I can not use anything related to dynamic memory allocation

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