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cpointersmatrix
Arpit
Arpit

Reputation: 12797

Getting a garbage value while printing matrix in C

I learned C programming language three years back and now when i am revisiting it after the experience of java and c# i'm facing some issues with pointers. So i tried to write a simple matrix addition program but i don't know why i'm getting some strange values while printing the matrices.

Code:

#include <stdio.h>

int* sumOfMat(int* m1,int* m2)
{
    printf("Matrix A: \n");
    printMat(m1);
    printf("Matrix B: \n");
    printMat(m2);
    int mat3[3][3];
    int row=0,col=0,k=0,sum=0;
    for(;row<3;row++)
    {
        col=0;
        for (;col<3 ;col++ )
        {
            sum=(*m1+*m2);
            m1++;
            m2++;
            mat3[row][col]=sum;
        }
    }

    printf("Result: \n");
//    printMat(mat3);  //this statement is giving me a correct output.
    return mat3;
}

void printMat(const int m[3][3])
{
    int row,col;
    for (row=0;row<3 ;row++ )
    {
        for (col=0;col<3 ;col++ )
        {
            printf("%d\t",m[row][col]);
        }
        printf("\n");
    }

}


int main(void) {
int mat1[3][3]={{1,2,3},{4,5,6},{7,8,9}};
int mat2[3][3]={{1,2,3},{4,5,6},{7,8,9}};
//add
printf("Sum of the metrices : \n");
int* x=sumOfMat(&mat1,&mat2);
printMat(x);   // this call is providing me some garbage values at some locations.
return 0;
}

Output:

Success  time: 0 memory: 2292 signal:0
Sum of the metrices : 
Matrix A: 
1   2   3   
4   5   6   
7   8   9   
Matrix B: 
1   2   3   
4   5   6   
7   8   9   
Result: 
2   134514448   134514448   
8   10  12  
14  16  -1216458764

Demo

Question: Why i'm getting this error and how to correct it.

Upvotes: 0

Views: 1668

Answers (3)

haccks
haccks

Reputation: 106032

Never return a pointer to an automatic variable: 

int *f(void)
{
    int i;
    ....
    return &i;
}

The variable i doesn't exist once f returns, so the pointer to it will be invalid.
In your case mat3 is an automatic variable and sumOfMat() returns a pointer to mat3 which does't exist once sumOfMat() returns.
One of the solution for your problem is, define mat3 as global variable.

Further I pointed out one major problem with your code:

Return type ofsumOfMat(int* m1,int* m2) is pointer to integer (int *) while you are returning mat3 which is of type int(*)[3].
I got lots of warnings while compiling your code. I corrected most of them.
Modified code: 

#include <stdio.h>
void printMat(int *m);
int* sumOfMat(int* m1,int* m2);
int mat3[3][3];

int* sumOfMat(int* m1,int* m2)
{
    printf("Matrix A: \n");
    printMat(m1);
    printf("Matrix B: \n");
    printMat(m2);
    //int mat3[3][3];
    int row=0,col=0,sum=0;
    for(;row<3;row++)
    {
        col=0;
        for (;col<3 ;col++ )
        {
            sum=(*m1+*m2);
            m1++;
            m2++;
            mat3[row][col]=sum;
        }
    }

   printf("Result: \n");
   //    printMat(mat3);  //this statement is giving me a correct output.
   return mat3[0];
}

void printMat(int *m)
{
    int row;
    for (row=1;row<=9 ;row++)
        {
            printf("%d\t",*m++);
            if(row%3 == 0)
                printf("\n");
        }
   /* {
        for (col=0;col<3 ;col++ )
        {
            printf("%d\t",m[row][col]);
        }
        printf("\n");
    }*/

}


int main(void) {
int mat1[3][3]={{1,2,3},{4,5,6},{7,8,9}};
int mat2[3][3]={{1,2,3},{4,5,6},{7,8,9}};
//add
printf("Sum of the metrices : \n");
int* x=sumOfMat(&mat1[0][0],&mat2[0][0]);
printMat(x);   // this call is providing me some garbage values at some locations.
return 0;
}

Upvotes: 2

Manoj Awasthi
Manoj Awasthi

Reputation: 3520

It is because you are returning mat3 which is a local variable defined as 

int mat3[3][3];

To correct this either do a dynamic allocation of mat3 using malloc OR pass it as a out variable in the function sumOfMat as third variable.

Upvotes: 0

TooTone
TooTone

Reputation: 8126

the line 

int mat3[3][3];

allocates your 2d array on the stack

You're returning a pointer to this array.

return mat3;

Unfortunately when the function call ends the stack is unwound and the memory for the array is no longer there, so you have a pointer to garbage.

One solution is to allocate the array in your main function and pass it into sumOfMat as a parameter.

Upvotes: 5

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