基本的C指针问题

Question

It has been a while since I last programmed C, seems I have forgotten everything in the meantime... I have a very simple pointer question. Assuming that I have a function that computes a sum through loop iteration. Not only should this function return the loop counter but also the sum it computed. Since I can just return one value I assume the best I could do for the sum is declaring a pointer. Can I do that like this:

   int loop_function(int* sum)
   {
    int len = 10; 

    for(j = 0; j < len; j++) 
    {
      sum += j;
    }

   return(j);
   }   

   ....


   int sum = 0;
   loop_function(&sum);
   printf("Sum is: %d", sum);

Or do I need to define an extra variable that points to sum which I pass then to the function?

Many thanks, Marcus

Answer 1

What you have is correct except for this line:

sum += j;

Since sum is a pointer, this increments the address contained in the pointer by j , which isn't what you want. You want to increment the value pointed to by the pointer by j , which is done by applying the dereference operator * to the pointer first, like this:

*sum += j;

Also, you need to define j somewhere in there, but I imagine you know that and it's just an oversight.

Answer 2

Write

*sum += j;

What you are doing is incrementing the pointer (Probably not what you wanted)

Answer 3

It should be:

*sum += j;

so you increment the value pointed to instead of the local pointer variable.

Answer 4

You don't need to create extra variable but in the loop you need to do

*sum += j

Answer 5

You have to write *sum + = j;

This suffices

Answer 6

You could also declare a simple struct to hold the two values, then pass in a pointer to an instance of that structure. That way you could manipulate both variables and they'd retain their values after the function returns.

struct loop_sum_struct {
    int loop_ctr;
    int sum;
} loop_sum;

loop_function(&loop_sum);

void loop_function(loop_sum_struct *s) {
    int len = 10;
    for(s->loop_ctr = 0; s->loop_ctr loop_ctr++) {
        s->sum += s->loop_ctr;
    }
}

Answer 7

How about this ?

As you are already returning int. So, another way is to just change the signature to

int loop_function(int sum)

and at the time of calling

int sum = 0;

...

sum = loop_function(sum);