returning more than one value

[Bill Cunningham]

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

 

[Stefan Ram]

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]

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?

 

[Bill Cunningham]

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]

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

 

[Bill Cunningham]

By using a language like Perl.


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

 

[Stefan Ram]

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]

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]

(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.

 

[Morris Keesan]

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;
}

 

[Keith Thompson]

But of course there are numerous ways to *simulate* returning multiple
values in C. If a tutorial mentions this without showing examples, it's
probably a bad tutorial.

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?

That's exactly what the function f does. main calls f and prints the
values.

The layout of the code makes it difficult to follow. Here's another
version. I've separated the declaration of the type "struct s" from the
definition of the function "f", and avoided re-using the name "s" for
multiple things.

include <stdio.h>

struct s {
int i;
double d;
};

struct s f( void )
{
struct s result;
result.i = 11;
result.d = 12.25;
return result;
}

int main( void )
{
const struct s returned_value = f();
printf( "%d %g\n", returned_value.i, returned_value.d );
return 0;
}

 

[Keith Thompson]

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

But functions can return values that *contain* multiple values,
particularly by returning a struct value (see examples elsethread).

On the other hand, this technique isn't commonly used unless the
multiple values are logically part of a single entity. It requires the
caller to create an object of the struct type to hold the result, then
extract the individual values from that object. It's often more
convenient for the caller to pass pointers to individual variables.

Here's a program I just posted (based on Stefan Ram's program) that uses
the struct technique:

#include <stdio.h>

struct s {
int i;
double d;
};

struct s f( void )
{
struct s result;
result.i = 11;
result.d = 12.25;
return result;
}

int main( void )
{
const struct s returned_value = f();
printf( "%d %g\n", returned_value.i, returned_value.d );
return 0;
}

And here's a version that doesn't use the struct:

#include <stdio.h>

void f( int *pi, double *pd )
{
*pi = 11;
*pd = 12.25;
}

int main( void )
{
int i;
double d;

f(&i, &d);

printf( "%d %g\n", i, d );
return 0;
}

As you can see, a lot of the extra code to pack the information into a
struct and then unpack it is no longer necessary.