Using a variable to represent a function in C

Question

This has the functionality I want (and it works)

#include <stdio.h>
//includes other libraries needed

int foo();

int main() 
{
    while(true) 
    {

        while(foo()==1) 
        {
            //do something
        }

        //does other unrelated things

    }

}

int foo() 
{
    // Returns if a switch is on or off when the function was called 
    // on return 1;
    // off return 0;
}

However, I would like this to happen:

#include <stdio.h>
//includes other libraries needed

int foo();

int main() 
{
    while(true) 
    {
        //THIS IS THE PROBLEM
        int something = foo();

        while(something==1) 
        {
            //do something
        }

        //does other unrelated things

    }

}

int foo() 
{
    // Returns if a switch is on or off when the function was called 
    // on return 1;
    // off return 0;
}

How can I have the something variable update each time the inner while loop is called? I know it has something to do with & or * for references and pointers but I haven't been able to find an example online about this.

Also I cannot edit anything within the foo() function.

If such a thing were possible, what are you gaining from it? — Jim Buck
– Jim Buck, Commented Oct 7, 2009 at 0:09
There is some confusion in the answers regarding what you're asking. Would you like the variable to represent the function itself, or the result of the function? — Greg Hewgill
– Greg Hewgill, Commented Oct 7, 2009 at 0:22
The result of the function at the time the variable is accessed. — 827
– 827, Commented Oct 7, 2009 at 2:44

rlbond · Accepted Answer · 2009-10-07 00:12:03Z

8

I think this is what you mean: it uses the function pointer to represent the function foo. Later it assigns it to a function bar:

#include <stdio.h>
//includes other libraries needed

int foo();
int bar();

int main() 
{
    while(true) 
    {
        int (*something)() = &foo;

        while(something()==1) 
        {
                something = &bar;
        }

        //does other unrelated things

    }

}

answered Oct 7, 2009 at 0:12

rlbond

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1 Comment

user180247 Over a year ago

Depending on the details, it may be worth considering a for loop - "for (something = &foo; something () == 1; something = &bar);"

R Samuel Klatchko · Accepted Answer · 2009-10-07 00:11:45Z

5

Since foo() is returning a value, you need to call the function again to get the latest value. The concise way to do that in C is to do the assignment and check together:

while ((something = foo()) == 1)
{
     // do something
}

answered Oct 7, 2009 at 0:11

R Samuel Klatchko

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Comments

Ryu · Accepted Answer · 2009-10-07 00:14:24Z

1

To represent a function in a variable use a function pointer

 int (*something)() = &foo;

edited Oct 7, 2009 at 0:14

answered Oct 7, 2009 at 0:07

Ryu

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Comments

JRL · Accepted Answer · 2009-10-07 00:14:24Z

0

How about this for your loop:

while(true) 
{
    int something = foo();
    // do whatever here

    if (!something) continue;
    do 
    {
      //do something
    } while(foo()==1)

    //does other unrelated things
}

This is assuming, from your example, that you want to have the something variable to do stuff with before the loop, and don't want a new call to foo() before going into your loop.

If not, then as mentionned in another post you can just do while(something = foo()) == 1)

answered Oct 7, 2009 at 0:14

JRL

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