string reversal

[arnuld]

This is a typical interview questions. I have browsed archives and came
across solutions, both iterative and recursive. They work fine, posting
them here if there is some problem in code that I can't see:


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


void using_recursion(char* p, char* q);
void using_iteration(char* p);


int main(void)
{
/* can not reverse next commented array as it consists of const
characters */
/* char arr = "123" */
char arr[10] = {'1', '2', '3', '4', '5', '\0'};
char* p = arr;
printf("%s\n", arr);
printf("------------------------------\n");
using_recursion(p, (p + (strlen(p) - 1)));
printf("%s\n", arr);
using_iteration(p);
printf("%s\n", arr);

return 0;
}


void using_recursion(char* p, char* q)
{
char c;
if(NULL == p || NULL == q)
{
printf("Error: Invalid Args!\n");
}
else if( p < q)
{
c = *p;
*p = *q;
*q = c;
using_recursion(p + 1, q - 1);
}
}


void using_iteration(char* p)
{
size_t len = strlen(p);
if(NULL == p)
{
printf("Error: Invalid Args!\n");
}
else if(1 < len)
{
char* end = p + len;
char tmp = '\0';
while(end > p)
{
tmp = *p;
*p++ = *--end;
*end = tmp;
}
}
}

 

[Ike Naar]

void using_iteration(char* p)
{
size_t len = strlen(p);
if(NULL == p)
{
printf("Error: Invalid Args!\n");
}

You probably want to check for NULL==p *before* computing strlen(p) .

 

[Malcolm McLean]

void using_iteration(char* p)
{
  size_t len = strlen(p);
  if(NULL == p)
    {
      printf("Error: Invalid Args!\n");
    }
  else if(1 < len)
    {
      char* end = p + len;
      char tmp = '\0';
      while(end > p)
        {
          tmp = *p;
          *p++ = *--end;
          *end = tmp;
        }
    }

}

You can lose the if(1 < len), the code is still valid for the empty
string and the one character string, and it makes it clearer.

 

[arnuld]

You probably want to check for NULL==p *before* computing strlen(p) .

Ah... good catch

 

[Ben Bacarisse]

This is a typical interview questions. I have browsed archives and came
across solutions, both iterative and recursive. They work fine, posting
them here if there is some problem in code that I can't see:

void using_recursion(char* p, char* q);
void using_iteration(char* p);

You should really have the same API (to use a fancy term) for both.
That will show up the fact the your recursive solution is considerably
more complex then the iterative one and it might have eliminated the
problems below:

int main(void)
{
/* can not reverse next commented array as it consists of const
characters */
/* char arr = "123" */
char arr[10] = {'1', '2', '3', '4', '5', '\0'};
char* p = arr;
printf("%s\n", arr);
printf("------------------------------\n");
using_recursion(p, (p + (strlen(p) - 1)));

Two things:

(a) When p points to an empty string, p + strlen(p) - 1 is an invalid
pointer.

(b) Your expression is, arguably, worse because (strlen(p) - 1) is a
very large positive number when strlen(p) is zero.

printf("%s\n", arr);
using_iteration(p);
printf("%s\n", arr);

return 0;
}


void using_recursion(char* p, char* q)
{
char c;
if(NULL == p || NULL == q)

This is too late. When p == NULL you can't do the calculation that
computes q. You need a set-up function that tests p (and strlen(p))
and which calls this function when all is well.

{
printf("Error: Invalid Args!\n");

All of your functions do this sort of thing. I'd down-grade interview
candidates that printed error message from inside "utility" code like
this; and I'd do so a little more aggressively when the error was not
sent to stderr. Part of the challenge of C is to devise a way to signal
or handle errors in utility code like this. What's often done (and I'd
do so here) is to pass the buck to the caller -- i.e. to assume that
null pointers are not passed.

}

<snip>