How to reverse a string using command line?

Question

I am trying to reverse a string using command line argument but I don't know why am I getting segmentation fault for this .

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

int main(int argc, char *argv[]) {
    char *str = argv[1];
    char *rev;

    int i, j, k;

    for (i = 0; str[i] != '\0'; i++); {
        k = i - 1;
    }
    for (j = 0; j <= i - 1; j++) {
        rev[j] = str[k]; 
        k--;
    }
    printf("The reverse string is %s\n", rev);

    return 0;
}

you never initialize `char *rev;` to point at a valid memory location — UnholySheep, Oct 12 '17 at 17:07
You aren't allocating any storage for `rev`, and even if you were, you aren't copying the terminating `'\0'`. Also, why did you place the initial assignment to `k` in a block? It's very misleading, given the preceding one-line `for` loop. And why are you using the loop? You already have `len`, so `k` is just `len - 1`. — Tom Karzes, Oct 12 '17 at 17:08
Because `rev` doesn't point anywhere meaningful, it's just an uninitialized pointer. Use `malloc()` or a variable-length array --- or a fixed-size array if an upper bound for string length is ok. — , Oct 12 '17 at 17:08
Aside about user input: assume nothing / check everything. In this case you have accessed `argv[1]` (through `str`) without checking `argc >= 2`. — Weather Vane, Oct 12 '17 at 17:15
Well, now that it's been reformatted, it's clear that you believe the first assignment to `k` is inside the preceding `for` loop. It isn't, nor does it need to be. The semicolon terminates the `for` loop, and the block that follows it outside of the loop. — Tom Karzes, Oct 12 '17 at 17:24

haccks · Accepted Answer · 2017-10-12T17:15:08.327

1

You have to allocate memory for reverse string and make rev to point to that location.

 char *rev = malloc(strlen(argv[1]) + 1); // 1 extra space to store null character.

And terminate the reversed string at the end

rev[j] = '\0';

edited Oct 12 '17 at 17:15

answered Oct 12 '17 at 17:08

haccks

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2

Or just reverse the whole thing in place... – Eugene Sh. Oct 12 '17 at 17:09
I initialized but now it doesn't print any output – Oct 12 '17 at 17:11

score 0 · Answer 2 · answered Oct 12 '17 at 17:15

0

After the for loop

    for(j = 0; j <= i-1; j++)
    {
        rev[j] = str[k];
        k--;
    }
    //Add this line
    rev[j]='\0'; //always terminate character array with null.
    printf("The reverse string is %s\n",rev);

answered Oct 12 '17 at 17:15

It's a good point, but `char *rev` has no memory allocated! – Weather Vane Oct 12 '17 at 17:17
yeah!! We need to allocate the memory. I forgot to add it, – Oct 12 '17 at 17:26

chqrlie · Answer 3 · 2017-10-13T20:34:06.863

0

There are multiple problems in the code:

There is an extra ; in the line for(i = 0; str[i] != '\0'; i++); {
you store characters to rev but this pointer is uninitialized, the behavior is undefined.

You can just reverse the string in place this way:

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

int main(int argc, char *argv[]) {
    if (argc > 1) {
       char *str = argv[1];
       int i, j, len = strien(str);

       for (i = 0, j = len - 1; i < j; i++, j--) {
           char c = str[i];
           str[i] = str[j];
           str[j] = c;
       }
       printf("The reverse string is %s\n", str);
    }
    return 0;
}

edited Oct 13 '17 at 20:34

answered Oct 12 '17 at 17:24

chqrlie

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Would `for (i = 0, j = len; i < j; i++, j--) {` make it more readable – Ed Heal Oct 12 '17 at 17:27
@EdHeal: `for (i = 0, j = len - 1; i < j; i++, j--) {` is indeed more readable. – chqrlie Oct 13 '17 at 20:33

score -1 · Answer 4 · answered Oct 12 '17 at 17:38

You can use recursion. For example

#include <stdio.h>

int main( int argc, char * argv[] ) 
{
    if ( !( argc < 2 ) )
    {
        if ( !*argv[1] )
        {
            printf("The reverse string is " ); 
        }
        else
        {
            ++argv[1];
            main( argc, argv );
            --argv[1];
            putchar( *argv[1] );
        }
    }

    return 0;
}

How to reverse a string using command line?

4 Answers4