don't understand an array in function program

**ashlee** · 12-01-2010

Code:

#include<stdio.h>
void round (int values[5][5]);
int keep (int num[5][5]);

void main(void){
  int first[5][5];
  int p, q;

  for (p = 0; p < 5; ++p)
     for (q = 0; q < 5; ++q)
       first[p][q] = p+q;
  round(first);
}

void round (int values[5][5]){
  int u, v, t;
  for (u = 0; u < 5; ++u)
     for (v = 0; v < 5; ++v)
       values[u][v] = values[u][v]%3;
 t = keep(values);
 printf("t = %i", t);
}

int keep (int num[5][5]){
  int s, t, k = 0;
  for (s = 0; s < 5; ++s)
     for (t = 0; t < 5; ++t)
       k += num[s][t];
  return k;
}


i need some help getting this program.  
it looks like 
  for (p = 0; p < 5; ++p)
     for (q = 0; q < 5; ++q)
       first[p][q] = p+q;
  round(first);

is putting each array number in the "round function" five times. and "first[p][q] = p+q;" makes each array a number? [0][1] = 1 [2][2] = 4 [3][2] = 5 etc?
but what's confusing me is in the round function this: values[u][v] = values[u][v]%3; is making each array the number it is modulus to 3??? and then t = the keep function
which also loops five times just adds the numbers and stores as k and returns it back to what t is. i'm not seeing how this compiles or prints t = 25 at the end at all. could
someone possibly run me through this?

**Salem** · 12-01-2010

main calls round just once, not 5 times.

Extra braces added for clarity.

Code:

int /* Yes, really! */ main(void){
  int first[5][5];
  int p, q;

  for (p = 0; p < 5; ++p) {
     for (q = 0; q < 5; ++q) {
       first[p][q] = p+q;
    }
  }
  round(first);
  return 0;
}

**ashlee** · 12-01-2010

Hm whydo i keep getting 19

**Salem** · 12-01-2010

Your program doesn't call rand(), and has no user input.
It's going to give you the same answer each time you run it.

Use a debugger to step through the code, so you can see things like
- when functions are called, and when they return
- when variables are modified.

Or you can add printf statements at each end of the function, like

Code:

int keep (int num[5][5]){
  int s, t, k = 0;
  printf("Start of keep\n");
  for (s = 0; s < 5; ++s)
     for (t = 0; t < 5; ++t)
       k += num[s][t];
  printf("End of keep, return %d\n", k );
  return k;
}

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