How can I pass a matrix to a double pointer function?

Question

I have a problem with this program, the program must simply output the minimum value contained in a matrix, this must be done by the function that has as parameter the double pointer (**A) that would be the array , the problem is that after inserting the items , the program finishes and returns nothing.

(I don’t even appear written outuput instruction )

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

int minium_matrix_value(int **A, int rows, int columns) {
    int min=A[0][0];
    
    for(int i=0;i<rows;i++){
        for(int j=0;j<columns;j++)
           
            if(A[i][j]<min)
                min=A[i][j];
    }
    return min;
}

int main(){
    
   int** A;
   int i, j;
   int result;
   int sizer=100, sizec=100;
 
   A = (int**)calloc(sizer,sizeof(int*));
   A = (int**)calloc(sizec,sizeof(int*));
   printf("insert rows size");
   scanf("%d",&sizer);
   printf("insert columns size");
   scanf("%d",&sizec);

   printf("insert matrix elements");
   for( i=0; i < sizer; i++ )
       for( j=0; j < sizec; j++ ) {
           scanf("%d",((A+i)+j));
       }
    
    result=minium_matrix_value(A,sizer,sizec);
    printf("the minium elements is: %d",result);
   
    return 0;

}

C and C++ are different language. Would you mind choosing one? — MikeCAT, Nov 05 '20 at 19:14
Start with a much smaller matrix, like 2x2 and then print the elements after you read them in. Pretty soon you'll find your issue. — AndyG, Nov 05 '20 at 19:17
Why do you overwrite `A` immediately after the first assignemtn? And why do you take user input for `sizer` and `sizec` _after_ using them? Also note that `A` is only an array of uninitialized pointers, you never actually allocate an array of ints. — Lukas-T, Nov 05 '20 at 19:17
You are mixing an attempt to build a 'jagged array' (array of pointers to 1D arrays) with a 1D array indexed as 2D. The `scanf("%d",((A+i)+j))` is wrong. — Weather Vane, Nov 05 '20 at 19:18
The code is C code; it doesn't seem to use any C++ features. As such, tag it with just C, unless you particularly like to earn the ire of those who might otherwise help you. — Jonathan Leffler, Nov 05 '20 at 19:21

MikeCAT · Accepted Answer · 2020-11-05T22:49:41.947

1

You only allocated arrays to store pointers to rows. You have to allocate arrays for storing values of each rows.
Using the value of sizer should be after reading value into that/

     // move this after scanf()
     // A = (int**)calloc(sizer,sizeof(int*));
     // remove this, or memory leak occur
     // A = (int**)calloc(sizec,sizeof(int*));
     printf("insert rows size");
     scanf("%d",&sizer);
     printf("insert columns size");
     scanf("%d",&sizec);

     // move here
     A = calloc(sizer,sizeof(int*));

     printf("insert matrix elements");
     for( i=0; i < sizer; i++ ) { // add { to do multiple things
       A[i] = calloc(sizec,sizeof(int)); // allocate row array
       for( j=0; j < sizec; j++ ) {
        //scanf("%d",((A+i)+j));
        scanf("%d",&A[i][j]); // use array style, which should be easier to understand
       }
     } // add } to do multiple things

Also note that:

scanf("%d",((A+i)+j)); is wrong and it should be scanf("%d",(*(A+i)+j)); if you hate [] operator.
Casting result of malloc() family is discouraged in C.

edited Nov 05 '20 at 22:49

answered Nov 05 '20 at 19:18

MikeCAT

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Ok thanks i corrected the errors, but i didn't understand why it's not good to use calloc to allocate elements. With new [] what are you referring to? You mean there is a better way to allocate items? – Res Nov 05 '20 at 21:06
and one last thing I'm sorry, but why do you use a double pointer (int **) in the calloc for sizer allocation and only a pointer (int *) for sizec? – Res Nov 05 '20 at 21:14
@AbdallaElAwad Casting is for C++ support. I removed casting because C is selected. `(int**)` was used because the destination of assignment `A` is `int**` and `(int*)` was used because the destination `A[i]` is `int*`. – MikeCAT Nov 05 '20 at 22:50

How can I pass a matrix to a double pointer function?

1 Answers1