How would I use qsort to sort a struct with a char* member and a long member - I want to sort in order of the long member

[Angus Comber]

Hello

Here is my code so far. Is this correct/incorrect/along the right
lines/other?

#include <stdio.h>
#include <string.h>
#include <search.h>
struct mystruct
{
long nKey;
char szIP[20];
};

int compare( const void *long1, const void *long2 );

int main()
{
mystruct devlist[4];
devlist[0].nKey = 9;
strcpy(devlist[0].szIP, "192.168.1.1");
devlist[1].nKey = 2;
strcpy(devlist[1].szIP, "192.168.1.2");
devlist[2].nKey = 7;
strcpy(devlist[2].szIP, "192.168.1.3");
devlist[3].nKey = 1;
strcpy(devlist[3].szIP, "192.168.1.4");
qsort( (void *)devlist, (size_t)4, sizeof( char * ), compare );

// bsearch is the next thing to work out how to do!
// void *bsearch( const void *key, const void *base, size_t num, size_t
width, int
// ( __cdecl *compare ) ( const void *elem1, const void *elem2 ) );
return 0;
}

int compare( const void* val1, const void* val2 )
{
struct mystruct *sp1 = (mystruct*)val1;
struct mystruct *sp2 = (mystruct*)val2;
// IS THIS CORRECT?
return (int)(sp1->nKey - sp2->nKey);
}

Angus Comber
an***@NOSPAMiteloffice.com

[Leor Zolman]

On Thu, 5 Feb 2004 14:43:22 -0000, "Angus Comber"
<an***@iteloffice.com.PLEASENOSPAM> wrote:
Please use a short subject line, and put the complete text of your
question in the message body.

#include <stdio.h>
#include <string.h>
#include <search.h>
struct mystruct
{
long nKey;
char szIP[20];
};
So is this C or C++? If C++, fine. If C, in order to use "mystruct" as
a typename (without having to say "struct mystruct", add this:

typedef struct mystruct mystruct;

int compare( const void *long1, const void *long2 );

int main()
{
mystruct devlist[4];
To do the above in C, add the aforementioned typedef.
devlist[0].nKey = 9;
strcpy(devlist[0].szIP, "192.168.1.1");
devlist[1].nKey = 2;
strcpy(devlist[1].szIP, "192.168.1.2");
devlist[2].nKey = 7;
strcpy(devlist[2].szIP, "192.168.1.3");
devlist[3].nKey = 1;
strcpy(devlist[3].szIP, "192.168.1.4");
qsort( (void *)devlist, (size_t)4, sizeof( char * ), compare );

// bsearch is the next thing to work out how to do!
// void *bsearch( const void *key, const void *base, size_t num, size_t
width, int
// ( __cdecl *compare ) ( const void *elem1, const void *elem2 ) );
return 0;
}

int compare( const void* val1, const void* val2 )
{
struct mystruct *sp1 = (mystruct*)val1;
struct mystruct *sp2 = (mystruct*)val2;
// IS THIS CORRECT?
return (int)(sp1->nKey - sp2->nKey);
If you want to sort in descending order, yes. If you want to sort in
_ascending_ order, flip the operands around.

Overall, a good first stab if you've never used qsort before.

Have fun,
-leor
}

Leor Zolman
BD Software
le**@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[Angus Comber]

Hello

Thanks for that. I am using VC++ so it didn't complain at the mystruct
line!

However, real reason for replying is that on first call of compare function,
if I do this:

struct mystruct *sp1 = (mystruct*)val1;
struct mystruct *sp2 = (mystruct*)val2;
sp1->nKey is some wierd value - 775043377
sp2->nKey is 9

on the second call to compare the values are:
sp1->nKey is some wierd value - 775435825
sp2->nKey is 775043377

So I have definitely done something wrong!

Any ideas?

"Leor Zolman" <le**@bdsoft.com> wrote in message
news:58********************************@4ax.com...

On Thu, 5 Feb 2004 14:43:22 -0000, "Angus Comber"
<an***@iteloffice.com.PLEASENOSPAM> wrote:
Please use a short subject line, and put the complete text of your
question in the message body.

#include <stdio.h>
#include <string.h>
#include <search.h>
struct mystruct
{
long nKey;
char szIP[20];
};

So is this C or C++? If C++, fine. If C, in order to use "mystruct" as
a typename (without having to say "struct mystruct", add this:

typedef struct mystruct mystruct;

int compare( const void *long1, const void *long2 );

int main()
{
mystruct devlist[4];

To do the above in C, add the aforementioned typedef.

devlist[0].nKey = 9;
strcpy(devlist[0].szIP, "192.168.1.1");
devlist[1].nKey = 2;
strcpy(devlist[1].szIP, "192.168.1.2");
devlist[2].nKey = 7;
strcpy(devlist[2].szIP, "192.168.1.3");
devlist[3].nKey = 1;
strcpy(devlist[3].szIP, "192.168.1.4");
qsort( (void *)devlist, (size_t)4, sizeof( char * ), compare );

// bsearch is the next thing to work out how to do!
// void *bsearch( const void *key, const void *base, size_t num, size_t
width, int
// ( __cdecl *compare ) ( const void *elem1, const void *elem2 ) );
return 0;
}

int compare( const void* val1, const void* val2 )
{
struct mystruct *sp1 = (mystruct*)val1;
struct mystruct *sp2 = (mystruct*)val2;
// IS THIS CORRECT?
return (int)(sp1->nKey - sp2->nKey);

If you want to sort in descending order, yes. If you want to sort in
_ascending_ order, flip the operands around.

Overall, a good first stab if you've never used qsort before.

Have fun,
-leor

}

Leor Zolman
BD Software
le**@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[Leor Zolman]

On Thu, 5 Feb 2004 15:27:38 -0000, "Angus Comber"
<an***@iteloffice.com.PLEASENOSPAM> wrote:

Hello

Thanks for that. I am using VC++ so it didn't complain at the mystruct
line!

However, real reason for replying is that on first call of compare function,
if I do this:

struct mystruct *sp1 = (mystruct*)val1;
struct mystruct *sp2 = (mystruct*)val2;
sp1->nKey is some wierd value - 775043377
sp2->nKey is 9

on the second call to compare the values are:
sp1->nKey is some wierd value - 775435825
sp2->nKey is 775043377

So I have definitely done something wrong!
Sorry, my bad. I just ran it, it worked, and I didn't look hard enough
at the ordering...it was actually not in descending order.

Here's the problem: in your qsort call:

qsort( (void *)devlist, (size_t)4, sizeof( char * ), compare);

the 3rd parameter should be
sizeof (mystruct)
instead of
sizeof (char *)

I failed to double-check against my own mental checklist for args to
qsort:
"base, nel, width, compare"
-leor

Any ideas?

"Leor Zolman" <le**@bdsoft.com> wrote in message
news:58********************************@4ax.com.. .

On Thu, 5 Feb 2004 14:43:22 -0000, "Angus Comber"
<an***@iteloffice.com.PLEASENOSPAM> wrote:
Please use a short subject line, and put the complete text of your
question in the message body.

>
>#include <stdio.h>
>#include <string.h>
>#include <search.h>
>struct mystruct
>{
> long nKey;
> char szIP[20];
>};

So is this C or C++? If C++, fine. If C, in order to use "mystruct" as
a typename (without having to say "struct mystruct", add this:

typedef struct mystruct mystruct;

>
>int compare( const void *long1, const void *long2 );
>
>int main()
>{
> mystruct devlist[4];

To do the above in C, add the aforementioned typedef.

> devlist[0].nKey = 9;
> strcpy(devlist[0].szIP, "192.168.1.1");
> devlist[1].nKey = 2;
> strcpy(devlist[1].szIP, "192.168.1.2");
> devlist[2].nKey = 7;
> strcpy(devlist[2].szIP, "192.168.1.3");
> devlist[3].nKey = 1;
> strcpy(devlist[3].szIP, "192.168.1.4");
> qsort( (void *)devlist, (size_t)4, sizeof( char * ), compare );
>
> // bsearch is the next thing to work out how to do!
> // void *bsearch( const void *key, const void *base, size_t num, size_t
>width, int
> // ( __cdecl *compare ) ( const void *elem1, const void *elem2 ) );
> return 0;
>}
>
>int compare( const void* val1, const void* val2 )
>{
> struct mystruct *sp1 = (mystruct*)val1;
> struct mystruct *sp2 = (mystruct*)val2;
> // IS THIS CORRECT?
> return (int)(sp1->nKey - sp2->nKey);

If you want to sort in descending order, yes. If you want to sort in
_ascending_ order, flip the operands around.

Overall, a good first stab if you've never used qsort before.

Have fun,
-leor

>}

Leor Zolman
BD Software
le**@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

Leor Zolman
BD Software
le**@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[Al Bowers]

Angus Comber wrote:

Hello

Here is my code so far. Is this correct/incorrect/along the right
lines/other?

You are on the right tract but have a few errors.
#include <stdio.h>
#include <string.h>
#include <search.h>
search.h is not a Standard C header file. To declare functions qsort
and bsearch use the header stdlib.h
Replace the search.h include with
#include <stdlib.h>
struct mystruct
{
long nKey;
char szIP[20];
};

int compare( const void *long1, const void *long2 );

int main()
{
mystruct devlist[4]; struct mystruct devlist[4]; devlist[0].nKey = 9;
strcpy(devlist[0].szIP, "192.168.1.1");
devlist[1].nKey = 2;
strcpy(devlist[1].szIP, "192.168.1.2");
devlist[2].nKey = 7;
strcpy(devlist[2].szIP, "192.168.1.3");
devlist[3].nKey = 1;
strcpy(devlist[3].szIP, "192.168.1.4");
qsort( (void *)devlist, (size_t)4, sizeof( char * ), compare );
You are sorting the array of structs. sizeof(char *)is the wrong
element width. Make this:
qsort(devlist,4,sizeof *devlist,compare);
// bsearch is the next thing to work out how to do!
// void *bsearch( const void *key, const void *base, size_t num, size_t
width, int
// ( __cdecl *compare ) ( const void *elem1, const void *elem2 ) );
return 0;
}

int compare( const void* val1, const void* val2 )
{
struct mystruct *sp1 = (mystruct*)val1;
struct mystruct *sp1 = (struct mystruct *)val1;
or better:
const struct mystruct *sp1 = val1;
struct mystruct *sp2 = (mystruct*)val2;
same as above.
// IS THIS CORRECT?
return (int)(sp1->nKey - sp2->nKey);
This return statement has a potential problem in the operands in the
substraction may have values that could cause the long type to
overflow or underflow. It would be better to use something like:

return (sp2->nKey > sp1->nKey)?-1:(sp2->nKey != sp1->nKey);
}

--
Al Bowers
Tampa, Fl USA
mailto: xa******@myrapidsys.com (remove the x to send email)
http://www.geocities.com/abowers822/

[CBFalconer]

Angus Comber wrote:

Here is my code so far. Is this correct/incorrect/along the
right lines/other?
.... snip most code ...
int compare( const void* val1, const void* val2 )
{
struct mystruct *sp1 = (mystruct*)val1;
struct mystruct *sp2 = (mystruct*)val2;
// IS THIS CORRECT?
return (int)(sp1->nKey - sp2->nKey);
}

Almost. See revisions below:

int compare(const void* val1, const void* val2)
{
const struct mystruct *sp1 = val1;
const struct mystruct *sp2 = val2;
/* Preserve const, no useless error hiding casts */

/* // comments are not correct without a C99 compiler */

if (sp1->nKey > sp2->nKey) return 1;
else if (sp1->nKey < sp2->nKey) return -1;
else return 0;
/* subtraction may overflow, fouling everything */
} /* compare */

--
Chuck F (cb********@yahoo.com) (cb********@worldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net> USE worldnet address!

[Angus Comber]

I am, as they say in Peckham, a plonker - I should have noticed that!

Thank you.

Angus
"Leor Zolman" <le**@bdsoft.com> wrote in message
news:q2********************************@4ax.com...

On Thu, 5 Feb 2004 15:27:38 -0000, "Angus Comber"
<an***@iteloffice.com.PLEASENOSPAM> wrote:

Hello

Thanks for that. I am using VC++ so it didn't complain at the mystruct
line!

However, real reason for replying is that on first call of compare function,if I do this:

struct mystruct *sp1 = (mystruct*)val1;
struct mystruct *sp2 = (mystruct*)val2;
sp1->nKey is some wierd value - 775043377
sp2->nKey is 9

on the second call to compare the values are:
sp1->nKey is some wierd value - 775435825
sp2->nKey is 775043377

So I have definitely done something wrong!

Sorry, my bad. I just ran it, it worked, and I didn't look hard enough
at the ordering...it was actually not in descending order.

Here's the problem: in your qsort call:

qsort( (void *)devlist, (size_t)4, sizeof( char * ), compare);

the 3rd parameter should be
sizeof (mystruct)
instead of
sizeof (char *)

I failed to double-check against my own mental checklist for args to
qsort:
"base, nel, width, compare"
-leor

Any ideas?

"Leor Zolman" <le**@bdsoft.com> wrote in message
news:58********************************@4ax.com.. .

On Thu, 5 Feb 2004 14:43:22 -0000, "Angus Comber"
<an***@iteloffice.com.PLEASENOSPAM> wrote:
Please use a short subject line, and put the complete text of your
question in the message body.

>
>#include <stdio.h>
>#include <string.h>
>#include <search.h>
>struct mystruct
>{
> long nKey;
> char szIP[20];
>};

So is this C or C++? If C++, fine. If C, in order to use "mystruct" as
a typename (without having to say "struct mystruct", add this:

typedef struct mystruct mystruct;

>
>int compare( const void *long1, const void *long2 );
>
>int main()
>{
> mystruct devlist[4];

To do the above in C, add the aforementioned typedef.

> devlist[0].nKey = 9;
> strcpy(devlist[0].szIP, "192.168.1.1");
> devlist[1].nKey = 2;
> strcpy(devlist[1].szIP, "192.168.1.2");
> devlist[2].nKey = 7;
> strcpy(devlist[2].szIP, "192.168.1.3");
> devlist[3].nKey = 1;
> strcpy(devlist[3].szIP, "192.168.1.4");
> qsort( (void *)devlist, (size_t)4, sizeof( char * ), compare );
>
> // bsearch is the next thing to work out how to do!
> // void *bsearch( const void *key, const void *base, size_t num, size_t >width, int
> // ( __cdecl *compare ) ( const void *elem1, const void *elem2 ) );
> return 0;
>}
>
>int compare( const void* val1, const void* val2 )
>{
> struct mystruct *sp1 = (mystruct*)val1;
> struct mystruct *sp2 = (mystruct*)val2;
> // IS THIS CORRECT?
> return (int)(sp1->nKey - sp2->nKey);

If you want to sort in descending order, yes. If you want to sort in
_ascending_ order, flip the operands around.

Overall, a good first stab if you've never used qsort before.

Have fun,
-leor

>}

Leor Zolman
BD Software
le**@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

Leor Zolman
BD Software
le**@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[Richard Heathfield]

Leor Zolman wrote:

On Thu, 5 Feb 2004 14:43:22 -0000, "Angus Comber"
<an***@iteloffice.com.PLEASENOSPAM> wrote:

#include <stdio.h>
#include <string.h>
#include <search.h>
struct mystruct
{
long nKey;
char szIP[20];
};

So is this C or C++?

It's C. The newsgroup in which he posted it is a bit of a giveaway.

int compare( const void* val1, const void* val2 )
{
struct mystruct *sp1 = (mystruct*)val1;
struct mystruct *sp2 = (mystruct*)val2;
// IS THIS CORRECT?
return (int)(sp1->nKey - sp2->nKey);

If you want to sort in descending order, yes. If you want to sort in
_ascending_ order, flip the operands around.

Overall, a good first stab if you've never used qsort before.

You could have showed him a better way (I'll avoid your typedef, but not
because I don't think it's a good idea):

int compare(const void *val1, const void *val2)
{
const struct mystruct *sp1 = val1;
const struct mystruct *sp2 = val2;

return sp1->nKey < sk2->nKey ? -1: sp1->nKey > sp2->nKey;
}

This version avoids casting away constness, and doesn't invoke potential
overflow in the subtraction (and thus undefined behaviour).

--
Richard Heathfield : bi****@eton.powernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton