C-2d array print function not working when I call it in main function-CodePudding

I wrote two functions, readMatrix which allocate a double pointer and take the input from keyboard and the second function is a print function.

When I call the print function in main function, it does not work completely.

This is the code :

void print(int** mat, int lin, int col) {
    int i,j;
    printf("Matrix is:\n");
    for(i=0; i<lin; i  ) {
        for(j=0; j<col; j  ) {
            printf("%d ", *(*(mat i) j));
        }
        printf("\n");
    }
}

int** readMatrix(int** mat, int lin, int col) {
    int x;
    int i,j;
    mat==(int**)malloc(lin*sizeof(int*));
    for(x=0; x<lin; x  ) {
        mat[x]=(int*)malloc(col*sizeof(int));
    }
    
    for(i=0; i<lin; i  ) {
        printf("Line %d: ", i);
        for(j=0; j<col; j  ) {
            scanf("%d",  &mat[i][j]);
        }
    }
}


int main()
{
    int lin, col;
    int i,j;
    printf("Enter number of lines: ");
    scanf("%d", &lin);
    printf("Enter number of cols: ");
    scanf("%d", &col);
    int** mat = readMatrix(mat,lin,col);
    print(mat,lin,col);
    return 0;
}

After I take the input, I get the message "Matrix is" but the matrix does not appear, why?

If I call the print function inside readMatrix function, it works, but why is not working if I call it in main?

Thanks.

CodePudding user response：

readMatrix takes mat as a parameter, but that is useless because it only receives the value of it, and it does not have a value yet. (To be able to change a variable in the calling routine, a routine must receive a pointer to it, not its value.)

readMatrix is declared to return int ** but does not have a return statement.

mat==(int**)malloc(lin*sizeof(int*)); uses ==, which compares values. It does not perform an assignment.

Remove the mat parameter from the declaration of readMatrix. Remove the mat argument from the call to readMatrix. Correct the == to =. Add a return statement that returns the value of mat from the allocation that was performed in readMatrix.

CodePudding user response：

The parameter mat is not used in the function readMatrix because its value id overwritten

int** readMatrix(int** mat, int lin, int col) {
    int x;
    int i,j;
    mat==(int**)malloc(lin*sizeof(int*));
    //...

Moreover the function returns nothing though its return type is not void.

So this record

int** mat = readMatrix(mat,lin,col);

invokes undefined behavior.

You need to declare the function at least lie

int** readMatrix( int lin, int col);

and at the end of the function to write

return mat;

For example

int** readMatrix(int lin, int col) {
    int **mat==(int**)malloc(lin*sizeof(int*));
    //...

    return mat;
}

And the function will be called like

int** mat = readMatrix(lin,col);

Also in main you should free the all allocated memory when the arrays is not needed any more.

For example

for ( i = 0; i < lin; i   )
{
    free( mat[i] );
}

free( mat );