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comp.lang.c

returning more than one value

Bill Cunningham

5/28/2011 6:55:00 PM

I came across a tutorial the other day that said a function can return
more than one value. How is this done? Pointers. Can someone please show me
an example. Say I want to return a double or an int.

Bill
13 Answers

ram

5/28/2011 7:12:00 PM

0

"Bill Cunningham" <nospam@nspam.invalid> writes:
>I came across a tutorial the other day that said a function
>can return more than one value. How is this done?

By using a language like Perl.

>Say I want to return a double or an int.

A double or an int is one value. You want to return
a double /and/ an int.

Here, »f« does not return two values, but one value,
which happens to be a struct:

#include <stdio.h>

struct s { int i; double d; }f( void )
{ struct s s; s.i = 11; s.d = 12; return s; }

int main( void )
{ struct s const s = f(); printf( "%d %g\n", s.i, s.d ); }

You could also call a call-back function instead of the
return to pass multiple values.

Keith Thompson

5/28/2011 7:17:00 PM

0

"Bill Cunningham" <nospam@nspam.invalid> writes:
> I came across a tutorial the other day that said a function can return
> more than one value. How is this done? Pointers. Can someone please show me
> an example.

Isn't there an example in the tutorial?

> an example. Say I want to return a double or an int.

Don't you mean a double *and* and int? Have you misunderstood what it
means to return more than one value?

--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.ne...
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

Bill Cunningham

5/28/2011 7:27:00 PM

0

Keith Thompson wrote:

> Don't you mean a double *and* and int?

Oh yes.

Have you misunderstood what it
> means to return more than one value?

perhaps.

Bill

Bill Cunningham

5/28/2011 7:31:00 PM

0

Stefan Ram wrote:
> You could also call a call-back function instead of the
> return to pass multiple values.

What's that mean? If I can grasp it.

Bill

Joe Wright

5/28/2011 7:34:00 PM

0

On 5/28/2011 14:55, Bill Cunningham wrote:
> I came across a tutorial the other day that said a function can return
> more than one value. How is this done? Pointers. Can someone please show me
> an example. Say I want to return a double or an int.
>
> Bill

A function has a type and may (or not) return a value. Functions do not
return multiple values.

--
Joe Wright
"If you rob Peter to pay Paul you can depend on the support of Paul."

Bill Cunningham

5/28/2011 7:35:00 PM

0

Stefan Ram wrote:
> "Bill Cunningham" <nospam@nspam.invalid> writes:
>> I came across a tutorial the other day that said a function
>> can return more than one value. How is this done?
>
> By using a language like Perl.
>
>> Say I want to return a double or an int.
>
> A double or an int is one value. You want to return
> a double /and/ an int.
>
> Here, »f« does not return two values, but one value,
> which happens to be a struct:
>
> #include <stdio.h>
>
> struct s { int i; double d; }f( void )
> { struct s s; s.i = 11; s.d = 12; return s; }
>
> int main( void )
> { struct s const s = f(); printf( "%d %g\n", s.i, s.d ); }
>
> You could also call a call-back function instead of the
> return to pass multiple values.

What about not using printf to print the values but using return because
you want to return a value to be taken by another function as a parameter?

Bill

ram

5/28/2011 7:56:00 PM

0

"Bill Cunningham" <nospam@nspam.invalid> writes:
>Stefan Ram wrote:
>>You could also call a call-back function instead of the
>>return to pass multiple values.
>What's that mean? If I can grasp it.

#include <stdio.h>

struct visitor
{ int( *int_result )( int );
double( *double_result )( double ); };

void f( struct visitor const * const visitor )
{ visitor->int_result( 11 );
visitor->double_result( 12 ); }

int int_queue( int const arg )
{ static int value; int result = value;
value = arg; return result; }

double double_queue( double const arg )
{ static double value; double result = value;
value = arg; return result; }

int main( void )
{ struct visitor const visitor ={ int_queue, double_queue };
f( &visitor );
printf( "%d\n", visitor.int_result( 0 ));
printf( "%g\n", visitor.double_result( 0 )); } /* prints:
11
12 */

Shao Miller

5/28/2011 8:10:00 PM

0

On 5/28/2011 1:55 PM, Bill Cunningham wrote:
> I came across a tutorial the other day that said a function can return
> more than one value. How is this done? Pointers. Can someone please show me
> an example. Say I want to return a double or an int.

(As asked already:) Did the tutorial include an example?

A C function has a type which includes the specification of the type of
the returned value, if any. Thus a C function cannot sometimes return
an 'int' and sometimes a 'double'.

However, as already mentioned, you can return a struct (or a union, if
you only desire one or the other of 'double' and 'int'). Like:

union onion {
double d;
int i;
};

enum e_type {
gimme_double,
gimme_int,
e_types
};

union onion foo(enum e_type type) {
union onion value = {0};

switch (type) {
case gimme_double:
value.d = 3.14159;
break;
case gimme_int:
value.i = 42;
break;
}
return value;
}

int main(void) {
return foo(gimme_int).i;
}

Function-like macros in C can "return" any type as the macro can expand
to whatever you like, but those aren't functions, of course.

I hope this helps. :)

Bill Cunningham

5/28/2011 8:15:00 PM

0

Shao Miller wrote:
> On 5/28/2011 1:55 PM, Bill Cunningham wrote:
>> I came across a tutorial the other day that said a function can
>> return more than one value. How is this done? Pointers. Can someone
>> please show me an example. Say I want to return a double or an int.
>
> (As asked already:) Did the tutorial include an example?

No I must've misread or misunderstood something. I would have to go back
over the tutorial again. It's Beej's C tutorial. I find it and his
networking tutorial interesting.

> A C function has a type which includes the specification of the type
> of the returned value, if any. Thus a C function cannot sometimes
> return an 'int' and sometimes a 'double'.
>
> However, as already mentioned, you can return a struct (or a union, if
> you only desire one or the other of 'double' and 'int'). Like:
>
> union onion {
> double d;
> int i;
> };
>
> enum e_type {
> gimme_double,
> gimme_int,
> e_types
> };
>
> union onion foo(enum e_type type) {
> union onion value = {0};
>
> switch (type) {
> case gimme_double:
> value.d = 3.14159;
> break;
> case gimme_int:
> value.i = 42;
> break;
> }
> return value;
> }
>
> int main(void) {
> return foo(gimme_int).i;
> }
>
> Function-like macros in C can "return" any type as the macro can
> expand to whatever you like, but those aren't functions, of course.
>
> I hope this helps. :)


Morris Keesan

5/28/2011 8:33:00 PM

0

On Sat, 28 May 2011 14:55:08 -0400, Bill Cunningham <nospam@nspam.invalid>
wrote:

> I came across a tutorial the other day that said a function can
> return more than one value. How is this done? Pointers. Can someone
> please show me an example. Say I want to return a double or an int.

A function can only return one value as its return value, but it can
achieve the effect of multiple return values by taking pointers as
arguments and storing its "return" values in the locations pointed to
by that pointer. So you can't write anything like

//NOT VALID C
int, double myfunc(void) { return 1, 3.14159; }

void someotherfunc(void)
{
int eger; double talk;
eger, talk = myfunc();
}
//NOT VALID C


but you can "return" both an int and a double like this:

void myfunc(int *errupt, double *dutch)
{
*errupt = 3; // returning int value through "output parameter"
*dutch = 0.14159; // returning double value through "output param"
}

void sometherfunc(void)
{
int elligent;
double t;

myfunc(&elligent, &t);
printf("%d %f\n", elligent, t);
}

This technique is also useful even when you want to return just one value,
but want to have a way to signal success or failure, without reserving
any special return value to indicate failure:

/* Returns 0 for success, non-zero for failure
* Numerical result stored in *resultp
*/
int do_something(int *resultp)
{
// ...
if (/* some error condition */) return 1;
// ...
*resultp = result;
return 0;
}

--
Morris Keesan -- mkeesan@post.harvard.edu