Thread: Passing a 2d array by Reference

I created a problem and solved it. Now i rewrote it and try to pass a variable named square by reference. Now im having trouble. What is the right way to pass a 2d array by reference?


Call by Value - Working

PHP Code:

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


#define SIZE 9


void createprintdata( int [SIZE][SIZE] );
int hasduplicate( int [SIZE][SIZE], int );
void printmax( int [SIZE][SIZE] );

int square[SIZE][SIZE] = { 0 };


int main()
{
    clrscr();
    createprintdata( square );
    getch();
    return 0;
}


void createprintdata( int location[SIZE][SIZE] )
{
    int genint = 0;
    int col = 0, row = 0;

    srand( time( 0 ) );

    for( col = 0; col != SIZE; col++ ){
        gotoxy( ( col + 2 ) * 5, 1 );
        printf( "(%d)", col + 1 );
    }


    for( row = 0; row != SIZE; row++ ){
        gotoxy( 2,  ( row + 2 ) * 2 );
        printf( "(%d)", row + 1 );

        for( col = 0, genint = 0; col != SIZE; col++ ){
            while( !genint || hasduplicate( location, genint ) ){
                genint = ( rand() % 99 ) + 1;
            }
            location[row][col] = genint;
            gotoxy( ( col + 2 ) * 5, ( row + 2 ) * 2 );
            printf( "%d", genint );
        }
    }
    gotoxy( 25, 22 );
    printmax( location );
}


int hasduplicate( int location[SIZE][SIZE], int i )
{
    int hasdup = 0;
    int row, col;

    for( col = 0; col != SIZE; col++ ){
        for( row = 0; row != SIZE; row++ ){
            if( location[row][col] == i ){
                hasdup = 1;
                break;
            }
        }
        if( hasdup ) break;
    }
    return hasdup;
}


void printmax( int location[SIZE][SIZE] )
{
    int row, col;
    int max = location[0][0], min = max, curval = min;

    for( row = 0; row != SIZE; row++ ){
        for( col = 0; col != SIZE; col++ ){
            curval = location[row][col];
            max = ( curval > max ? curval : max );
            min = ( curval < min ? curval : min );
        }
    }

    printf( "\nThe highest value generated is %d", max );
    printf( "\nThe lowest value generated is %d", min );
} 


Call by Reference - Not Working!!

PHP Code:

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


#define SIZE 9


void createprintdata( int ** );
int hasduplicate( int **, int );
void printmax( int ** );



int main()
{
    int square[12][12] = { 0 };
    clrscr();
    createprintdata( square );
    getch();
    return 0;
}


void createprintdata( int **location )
{
    int genint = 0;
    int col = 0, row = 0;

    srand( time( 0 ) );

    for( col = 0; col != SIZE; col++ ){
        gotoxy( ( col + 2 ) * 5, 1 );
        printf( "(%d)", col + 1 );
    }


    for( row = 0; row != SIZE; row++ ){
        gotoxy( 2,  ( row + 2 ) * 2 );
        printf( "(%d)", row + 1 );

        for( col = 0, genint = 0; col != SIZE; col++ ){
            while( !genint || hasduplicate( location, genint ) ){
                genint = ( rand() % 99 ) + 1;
            }
            location[row][col] = genint;
            gotoxy( ( col + 2 ) * 5, ( row + 2 ) * 2 );
            printf( "%d", genint );
        }
    }
    gotoxy( 25, 22 );
    printmax( location );
}


int hasduplicate( int **location, int i )
{
    int hasdup = 0;
    int row, col;

    for( col = 0; col != SIZE; col++ ){
        for( row = 0; row != SIZE; row++ ){
            if( location[row][col] == i ){
                hasdup = 1;
                break;
            }
        }
        if( hasdup ) break;
    }
    return hasdup;
}


void printmax( int **location )
{
    int row, col;
    int max = location[0][0], min = max, curval = min;

    for( row = 0; row != SIZE; row++ ){
        for( col = 0; col != SIZE; col++ ){
            curval = location[row][col];
            max = ( curval > max ? curval : max );
            min = ( curval < min ? curval : min );
        }
    }

    printf( "\nThe highest value generated is %d", max );
    printf( "\nThe lowest value generated is %d", min );
} 


In C all arrays are passed by reference, that is the variable is the address of the first element of the array. The trouble you may be having in the second program which uses pointer notation is with referencing each element via it's pointer.
You have to be careful when passing arrays to a function if you do not wish to change the values of the elements of an array. To pass an array to a function without changing the underlying values of the elements pass the array with the keyword const.

Originally Posted by bigtamscot

In C all arrays are passed by reference, that is the variable is the address of the first element of the array. The trouble you may be having in the second program which uses pointer notation is with referencing each element via it's pointer.
You have to be careful when passing arrays to a function if you do not wish to change the values of the elements of an array. To pass an array to a function without changing the underlying values of the elements pass the array with the keyword const.

I need to change the value in function createprintdata() so i didnt put the keyword const..

The problem im having with the second program is when i try calling printmax and printing the lowest and highest value generated it doesnt generate the correct values.

Did i accenditally changed the data in var square in function printmax?


Output

Code:

         (1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)


 (1)     4    70   71   84   57   29   7    74   35

 (2)     79   16   31   93   9    11   63   2    86

 (3)     88   49   52   56   57   66   68   14   19

 (4)     5    51   89   37   79   59   41   32   23

 (5)     17   40   13   75   8    74   33   89   43

 (6)     46   91   60   23   27   65   40   4    79

 (7)     38   37   69   5    62   60   61   7    1

 (8)     76   89   84   96   65   56   69   67   37

 (9)     23   14   44   45   48   30   34   6    7


The highest value generated is 48
The lowest value generated is 6

Edit: Seems like the createprintdata() is not working properly too. number 40 was generated twice.

When trying to get the highest and lowest values for an array of integers, it is a good idea to set the values of high and low prior to the loop where the minimum is the highest value possible and the maximum is the lowest value possible so that they both will change.

>int square[12][12] = { 0 };
>createprintdata( square );
This is fine, and actually passes the array by simulated reference (since C doesn't support that feature at all), but only if createprintdata expects a two dimensional array or anything equivalent.

>void createprintdata( int **location )
Okay, a pointer to a pointer is not equivalent to a two dimensional array. These will work:

Code:

void createprintdata ( int location[10][10] );

Code:

void createprintdata ( int (*location)[10] );

A pointer to a pointer will work if you simulate a two dimensional array with dynamic memory:

Code:

int **square = malloc ( 10 * sizeof *square );
int i;

for ( i = 0; i < 10; i++ )
  square[i] = malloc ( 10 * sizeof *square[i] );

createprintdata ( square );

Why all of this? Because, as you may have learned because you tried to take advantage of it, when an array is passed to a function, it is converted to a pointer to the first element. However, that rule only applies to the first dimension. So a[10][10] is converted to (*a)[10], not **a.

>Is it the same in C++ that all arrays are passed by reference?
Yes, but in C you have to be more careful to make the distinction between passing by reference, and passing by pointer, because C++ does support passing by reference, and it's different.