[Newbie] qsort question

[=?iso-8859-1?q?Alexander__D=FCnisch?=]

hi

I've written the follwing program:
=====================================
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int comparestr(const void *a, const void *b)
{
return strcmp((char*)a, (char*)b);
}

int main()
{
int len = 6;
char* names[] = {"Hansi", "Peter", "Ronny",
"Klaus", "Willy", "Michi"};
qsort(names, len, sizeof(char*), comparestr);
for(int i = 0; i < len; i++)
printf("%s ", names);
printf("\n");
return 0;
}
=====================================

but it doesn't seem to be sorting my array as I expected.
It prints:
Hansi Peter Ronny Klaus Willy Michi

What is my mistake?

Thanks for your help.
Best regards.
Alex

 

[Richard Heathfield]

Alexander Dünisch said:

hi

I've written the follwing program:
=====================================

but it doesn't seem to be sorting my array as I expected.
It prints:
Hansi Peter Ronny Klaus Willy Michi

What is my mistake?

You made several, I'm afraid, but the most important was that you forgot
that your comparison function accepts pointers to the objects stored in the
array, not the objects themselves. Since the objects that were stored are
pointers to char, the comparison function will get pointers to pointers to
char.

Here's a working version:

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

int comparestr(const void *vs1,
const void *vs2)
{
char *const *s1 = vs1;
char *const *s2 = vs2;

return strcmp(*s1, *s2);
}

int main(void)
{
size_t len = 6;
size_t i = 0;
const char *names[] = { "Hansi", "Peter", "Ronny",
"Klaus", "Willy", "Michi"
};

qsort(names, len, sizeof names[0], comparestr);
for(i = 0; i < len; i++)
printf("%s ", names);
printf("\n");
return 0;
}

 

[Knemon]

hi

I've written the follwing program:

[included in the modified program below]

but it doesn't seem to be sorting my array as I expected.
It prints:
Hansi Peter Ronny Klaus Willy Michi

What is my mistake?

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

int comparestr(const void *a, const void *b)
{
return strcmp((char *) a, (char *) b);
}

int replacement_comparestr(const void *a, const void *b)
{
return strcmp(*(char **) a, *(char **) b);
}

int main()
{
int len = 6;
char *names[] = { "Hansi", "Peter", "Ronny",
"Klaus", "Willy", "Michi"
};
printf("Attempt to sort with OP's comparestr\n");
qsort(names, len, sizeof(char *), comparestr);
for (int i = 0; i < len; i++)
printf("%s ", names);
printf("\n\n");

printf("Attempt to sort with replacement_comparestr\n");
qsort(names, len, sizeof(char *), replacement_comparestr);
for (int i = 0; i < len; i++)
printf("%s ", names);
printf("\n");

return 0;
}



Attempt to sort with OP's comparestr
Hansi Peter Ronny Klaus Willy Michi

Attempt to sort with replacement_comparestr
Hansi Klaus Michi Peter Ronny Willy

 

[=?iso-8859-1?q?Alexander__D=FCnisch?=]

int replacement_comparestr(const void *a, const void *b)
{
return strcmp(*(char **) a, *(char **) b);
}

Thanks a lot.
Alex