beginner: reversing a string with pointers

Question

I'm trying to reverse a c string. I know of easier ways to do this but I was trying to practice some pointers. This compiles but returns a null string and I do not understand why. What am I missing here?

#include <stdio.h>
#include <stdlib.h>
int main(){
    char *forwards, *backwards;
    forwards = "hello";
    backwards = (char*) malloc( strlen(forwards) );
    int i;
    for( i = strlen(forwards) - 1; i >=0; --i )
            backwards[ strlen(forwards) - i ] = forwards[i];
    printf("%s", *backwards );
    return 0;
    }

Jack · Accepted Answer · 2016-04-15 01:42:58Z

2

You have multiple errors.

First of all the indices calculated are wrong, you should subtract 1 from the backwards index, eg:

for (i = strlen(forwards) - 1; i >= 0; --i )
{
  printf("%d -> %lu\n", i, strlen(forwards) - i - 1);
  backwards[ strlen(forwards) - i - 1] = forwards[i];
}

/* prints
4 -> 0
3 -> 1
2 -> 2
1 -> 3
0 -> 4

Mind that there is no reason to have the loop counting down, for (i = 0; i < length; ++i) would have worked exactly in the same way

The second error is that strlen doesn't include the NUL terminator so your backwards should be allocated as malloc(strlen(forwards)+1).

The third error is the fact that you are not setting the terminator on the reverse string, you must place it by hand, with something like:

backwards[strlen(forwards)] = '\0';

The last error is the fact that you are dereferencing the pointer in the printf, which you shouldn't do, so it should be

printf("%s\n", backwards);
               ^
               no * here

edited Apr 15, 2016 at 1:42

answered Apr 15, 2016 at 1:28

Jack

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1 Comment

Jonathan Leffler Over a year ago

And another problem is repeatedly calling strlen(forwards) in the loop when the value doesn't change.

ericbn · Accepted Answer · 2016-04-15 02:18:30Z

1

Your loop will result in:

backwards[1] = forwards[4];
backwards[2] = forwards[3];
backwards[3] = forwards[2];
backwards[4] = forwards[1];
backwards[5] = forwards[0];

You'll want to change your code to:

backwards[strlen(forwards) - i - 1] = forwards[i];

In C, strings must end with '\0', so make sure you do:

backwards = malloc(strlen(forwards) + 1);
// ...
backwards[strlen(forwards)] = '\0';

Also, to print the string:

printf("%s", backwards); // no dereferencing the pointer

edited Apr 15, 2016 at 2:18

answered Apr 15, 2016 at 1:21

ericbn

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4 Comments

ericbn Over a year ago

And a suggestion, instead of calling strlen(forwards) multiple times, assign it to a int len = strlen(forwards) and use the variable when needed.

mreff555 Over a year ago

Hey ericbn That worked, thanks. Why do I not need to dereference the pointer?

BLUEPIXY Over a year ago

And C-String is necessary to end with a null character.

Jack Over a year ago

@mreff555: dereferencing a char* yields a char which is a single character, not a string.

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