Question

I usually understand recursions pretty well, but because I’m new to C functions like strcpy and pointers I couldn’t figure out how this recursion reverses a string:

char *reverse(char *string)
{
    if (strlen(string) <= 1)
        return string;

    else
    {
        char temp = *string;

        strcpy(string, reverse(string+1));

        *(string+strlen(string)) = temp;

        return string;
    }
}

The strcpy part seems a little bit complicated to me, and also what’s the purpose of this line:
*(string+strlen(string)) = temp;?

I realize that after flipping the string you need to add the character that was at the beginning to the end of the string, but I’m not sure I understand the logic behind this code.

Answer 1

This code is extremely inefficient but what it does is:

1. Save the original first character
2. Recursively reverse the rest of the string (string+1 is a pointer to the second character in the string).
3. Copy the rest of the (reversed) string one character to the left.
4. Put the original first character at the end (*(string+strlen(string)) = temp;).

The *(string+strlen(string)) = temp; is equivalent to string[strlen(string)] = temp; if that is easier to understand.

I will not recommend using this code at all, since it is extremely inefficient — it copies the entire string (and measures its length twice) in every iteration, not to mention waste stack space.

A much better implementation would be:

void reverse(char *s) {
  char *e = s+strlen(s)-1;
  while (e > s) {
    char tmp = *s;
    *s=*e;
    *e=tmp;
    s++; e--:
  }
}