Thread: Problem with passing 2D array by reference

Hi! I have a 2D array that I need to pass to a function, have its value modified within that function, and then use the altered array within the original function.

Here is a simplification of my code, along with a summary of my problem:

Code:

void function1(int arg1);
void function2(double myArray[4][4]);

void function1(int arg1){

   double myArray[4][4];
   int i = 0, j = 0;

  /* initialize array to 0 */ 
  for(i = 0; i < 4; i++){
     for(j = 0; j < 4; j++){
          myArray[4][4] = 0.0;
     }
   }
 
  function2(myArray); 
  /* print myArray values */ 

  /* problem: when I print out the resulting values of myArray here, they are wrong. How could they change from being correct within function2(), to being incorrect here? */ 
  

}

void function2(double myArray[4][4]){

  /* change values of myArray, and print myArray values; I've verified that *within* this function, the final myArray values *are* correct */ 

}

I've been referring to the following relevant C forum thread:
Passing a 2d array by Reference

I tried the double-pointer syntax suggested within that thread, as well, but I get compiler errors.

Any suggestions about what I'm doing wrong, that would result in the myArray[][] values being correct when they're changed within function2(), but then are wrong when printed within function1()?

Thanks in advance for your help!

Perhaps it has to do with the way you're not initialising the array properly to begin with?

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void function1(int arg1);
void function2(double myArray[4][4]);

void function1(int arg1){

   double myArray[4][4];
   int i = 0, j = 0;

  /* initialize array to 0 */
  for(i = 0; i < 4; i++){
     for(j = 0; j < 4; j++){
          myArray[i][j] = 0.0;  //!! NOT [4][4]
     }
   }

  function2(myArray);
  /* print myArray values */

  /* problem: when I print out the resulting values of myArray here, they are wrong. How could they change from being correct within function2(), to being incorrect here? */
  /* print again */
  for(i = 0; i < 4; i++){
     for(j = 0; j < 4; j++){
        printf("%f, ", myArray[i][j] );
     }
     printf("\n");
   }


}

void function2(double myArray[4][4]){
  /* change values of myArray, and print myArray values; I've verified that *within* this function, the final myArray values *are* correct */
   int i = 0, j = 0;

   /* assign */
  for(i = 0; i < 4; i++){
     for(j = 0; j < 4; j++){
          myArray[i][j] = i*j;
     }
   }

  /* print */
  for(i = 0; i < 4; i++){
     for(j = 0; j < 4; j++){
        printf("%f, ", myArray[i][j] );
     }
     printf("\n");
   }
}
int main ( ) {
  function1(3);
  return 0;
}

No, my example works just fine.

Perhaps you should post some real code which demonstrates the problem, and not a facsimile which doesn't.

And how exactly does that demonstrate the problem?

If I can't copy / paste / compile / run and see the problem, I can't help you.