returning more than one value

Discussion in 'C Programming' started by Bill Cunningham, May 28, 2011.

  1. 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
     
    Bill Cunningham, May 28, 2011
    #1
    1. Advertising

  2. Bill Cunningham

    Stefan Ram Guest

    "Bill Cunningham" <> 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.
     
    Stefan Ram, May 28, 2011
    #2
    1. Advertising

  3. "Bill Cunningham" <> 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) <http://www.ghoti.net/~kst>
    Nokia
    "We must do something. This is something. Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"
     
    Keith Thompson, May 28, 2011
    #3
  4. 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, May 28, 2011
    #4
  5. 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
     
    Bill Cunningham, May 28, 2011
    #5
  6. Stefan Ram wrote:
    > "Bill Cunningham" <> 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
     
    Bill Cunningham, May 28, 2011
    #6
  7. Bill Cunningham

    Stefan Ram Guest

    "Bill Cunningham" <> 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 */
     
    Stefan Ram, May 28, 2011
    #7
  8. Bill Cunningham

    Shao Miller Guest

    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. :)
     
    Shao Miller, May 28, 2011
    #8
  9. 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. :)
     
    Bill Cunningham, May 28, 2011
    #9
  10. On Sat, 28 May 2011 14:55:08 -0400, 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.


    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 --
     
    Morris Keesan, May 28, 2011
    #10
  11. "Bill Cunningham" <> writes:
    > Stefan Ram wrote:
    >> "Bill Cunningham" <> 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.


    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.

    >>> 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?


    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 (The_Other_Keith) <http://www.ghoti.net/~kst>
    Nokia
    "We must do something. This is something. Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"
     
    Keith Thompson, May 28, 2011
    #11
  12. Joe Wright <> writes:
    > 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.

    >
    > 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.

    --
    Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
    Nokia
    "We must do something. This is something. Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"
     
    Keith Thompson, May 28, 2011
    #12
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Mohitz
    Replies:
    11
    Views:
    528
    Diego Martins
    Jul 3, 2007
  2. Minkoo Seo

    returning more than one value

    Minkoo Seo, Mar 1, 2006, in forum: Ruby
    Replies:
    1
    Views:
    100
    Timothy Goddard
    Mar 1, 2006
  3. Bob Rashkin
    Replies:
    5
    Views:
    85
    Dennis Lee Bieber
    Dec 23, 2013
  4. Steven D'Aprano
    Replies:
    0
    Views:
    116
    Steven D'Aprano
    Dec 23, 2013
  5. Replies:
    3
    Views:
    98
    Gary Herron
    Dec 23, 2013
Loading...

Share This Page