Trying to "copy" one string in reverse

Question

I've been trying to "copy" one string to another, in reverse. It kindof works, but it prints some weird symbols. I've tried setting char copy[length2] but that makes the program not run at all.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#define ARR_SIZE 50

int main()
{
    char string[ARR_SIZE];
    printf("Enter char array!\n");
    fgets(string, ARR_SIZE, stdin);

    string[strlen(string) - 1] = '\0';
    int length = (strlen(string) - 1);
    int length2 = (strlen(string) - 1);

    printf("%s\t%d\n", string, length);

    for (int i = 0; i <= length; i++)
    {
        printf("INDEX = %d   CHAR = %c\n", i, string[i]);
    }
    
    printf("%d", length2);
    char copy[ARR_SIZE];
    
    for (int i = 0; i <= length2; i++)
    {
        copy[i] = string[length];
        length--;
    }

    


    printf("\n%s", copy);
}

@thebusybee: It's not nonsense; `fgets` always puts in the NUL terminator when it reads a line, so `string[strlen(string) - 1] = '\0';` is basically Perl's `chop`; it unconditionally removes the final character from the string (presumably assumed to be a newline). It's going to trigger undefined behavior if you're at EOF though (better not hit Ctrl-D immediately, or run with `< /dev/null`); the array won't be initialized at all (`fgets` doesn't NUL it on EOF/error); even if it was zeroed, the code would write one character *before* the array. And it would misbehave if no newline was entered. — ShadowRanger, Nov 02 '21 at 15:46
@thebusybee I'm afraid your comment is wrong/misleading. `string[strlen(string) - 1]` just removes the `\n` at the end provided there is one. OTOH this is not the best method for removing the `\n`. The best method is shown here: https://stackoverflow.com/a/28462221/898348 — Jabberwocky, Nov 02 '21 at 15:59
your `length` and `length2` variables are misleading. `strlen` does _not_ include the `NUL` terminator in its calculation, so saying `length = strlen(..) - 1` is one too few of that actual string length. — yano, Nov 02 '21 at 16:01

score 1 · Accepted Answer · answered Nov 02 '21 at 16:10

These are the minimal modifications I'd make to your code:

#include <stdio.h>
#include <string.h>
// remove unneeded headers

#define ARR_SIZE 50

int main(void)
{
    char string[ARR_SIZE];
    printf("Enter char array!\n");
    fgets(string, ARR_SIZE, stdin);

    string[strlen(string) - 1] = '\0';
    // remove the -1 on the string length calculation, the NUL terminator is not
    // included in strlen's return value
    int length = strlen(string);
    // no sense in calling strlen twice
    int length2 = length;

    // fixed `length` now prints the correct length
    printf("%s\t%d\n", string, length);

    // change from <= to <. The array indices where the characters live are
    // [0, length-1].
    for (int i = 0; i < length; i++)
    {
        printf("INDEX = %d   CHAR = %c\n", i, string[i]);
    }
    
    // fixed `length2` now prints the correct length
    printf("%d", length2);
    char copy[ARR_SIZE];
    
    for (int i = 0; i < length2; i++)
    {
        // last character in `string` lives at the `length`-1 index
        copy[i] = string[length-1];
        length--;
    }

    // `length2` is the index after the last char in `copy`, this needs
    // to be NUL terminated.
    copy[length2] = '\0';

    // prints the reversed string
    printf("\n%s", copy);
}

Demo

score 0 · Answer 2 · answered Nov 02 '21 at 15:39

Use functions.
Terminate strings with null character \0 or simply 0.

char *copyreverse(char *dest, const char *src)
{
    size_t len = strlen(src);
    const char *end = src + len - !!len;
    char *wrk = dest;

    while(len--)
        *wrk++ = *end--;
    *wrk = 0;
    return dest;
}

int main()
{
    char dest[10];
    char *src = "hello";

    printf("`%s` reversed `%s`\n", src, copyreverse(dest, src));
}

Trying to "copy" one string in reverse

2 Answers2