Question

I have a few questions regarding array of strings in C.

I have an array char *string. I have a char *string and then I split every 4 characters in a array of strings called sep_str. So for example if char *string = 'The sum';, then char **sep_str is:

0: |_| --> "The "
1: |_| --> "Sum"

My first question is, in an array of strings in C (so array of array of chars), will there be a null terminating character at the end of each sep_str[i], or just at the last position of sep_str? Here is how I copy string into an array of strings:

    for (int i = 0; i < str_length; i++) {
        sep_str[i/4][i%4] = *ptr;
        ptr++;
    }

My second question is, how would I reverse the elements of each string in sep_str? Here’s how I did it, but I feel like it is stepping out of the array of the substring. (so out of the element of the sep_str):

// Reverse each element in the array
    char temp;

    for (int i = 0; i < num_strs; i++) {

        for (int j = 0, k = 4; j < k; j++, k--) {

            temp = sep_str[i][j];
            sep_str[i][j] = sep_str[i][k];
            sep_str[i][k] = temp;
        }
    }

Answer 1

The copy of the strings sounds good to me. Since each string has always 4 chars, you can avoid the null terminator \0. Alternatively you need to declare sep_str as a 5x(lenght/4) matrix, to store the \0 char at the end of each string.

To reverse a string you need to iterate from the start to the middle of the string, replacing the i-th char with the length-i-1-th. You need to replace the inner for replacing k=3 to k=2.

You also need to take care of the last string, since the lenght might not be multiple of four.

char temp;

for (int i = 0; i < (num_strs - 1); i++) {
    for (int j = 0, k = 3; j < k; j++, k--) {
        temp = sep_str[i][j];
        sep_str[i][j] = sep_str[i][k];
        sep_str[i][k] = temp;
    }
}

if (num_strs > 0) {
    for (int j = 0, k = strlen(sep_str[i]) - 1; j < k; j++, k--) {
        temp = sep_str[i][j];
        sep_str[i][j] = sep_str[i][k];
        sep_str[i][k] = temp;
    }
}