CODEWITHSUNDEEP
cpointers
cud_programmer
cud_programmer

Reputation: 1264

Pointers in C, returning from functions?

I am trying to create a simple matrix multiplication program in C. To do this I have opted to use 2 functions: 1 to simply create matrices, and 1 to hold all the created matrices (for doing multiplication).

I have finally managed to work out how to pass an array from a function (or more specifically pass a pointer to the array from a function), however, I want the function matrixWelcome() below to be passed the number of rows & columns of the array. This is where I am stuck.

Currently gcc is giving me error invalid type argument of ‘unary *’ for line -> rows = *i;

My code is:

void matrixWelcome() {
    int selection;
    int a, b, c, d, *rows, *columns;
    int *matrix1[0][0], *matrix2[0][0];

    printf("Welcome to matrix mode\n");
    printf("Please select from the following: \n");
    printf("1 - Matrix multiplication");

    scanf("%d", &selection);

    switch (selection) {
        case 1:
            printf("Selected matrix multiplication...\n");
            printf("Please enter matrix 1\n");

            matrix1[*rows][*columns] = matrixInput(*rows, *columns);
            printf("%d\n", *matrix1[1][1]);
            a = *rows;
            b = *columns;

            printf("****ROWS = %d, COLUMNS = %d", a, b);

           // printf("Matrix 1 has %d rows and %d columns", rows, columns);
            printf("Please enter matrix 2\n");
           // matrix2 = matrixInput();
            break;

        default:
            printf("No input entered\n");
            break;
    }
}

int *matrixInput(int *rows, int *columns) {
    int i, j, x, y;
    int *array_pointer = malloc(5 * sizeof(int)); //grab some memory

    if (array_pointer == NULL) { // check that we have successfully got memory
        return NULL;
    }

    // Number of rows & columns for matrix 1
    printf("Enter number of rows: ");
    scanf("%d", &i);
    printf("\nEnter number of columns: ");
    scanf("%d", &j);

    rows = *i;
    columns = *j;

    // Initialise 2D array to hold values
    int matrix_input[i][j];

    printf("Enter values for matrix, starting at 1,1 and moving across 1st row; then move across 2nd row etc..");

    // loop to store values in matrix
    for (x=0; x<i; x++) {
        for (y=0; y<j; y++) {

            int value_entered;
            scanf("%d", &value_entered);
            matrix_input[x][y] = value_entered; 


        }
    }

    *array_pointer = matrix_input[i][j]; 

    // print out matrix - just to confirm
    printf("You've entered the following matrix: \n");
    for (x=0; x<i; x++) {
        for (y=0; y<j; y++) {
            printf("%4d", matrix_input[x][y]);
        }
        printf("\n");
    }
    return array_pointer;
}

Any help would be appreciated.

Upvotes: 2

Views: 528

Answers (2)

Carl Norum
Carl Norum

Reputation: 224844

i is an int. You don't need to dereference it. You do need to dereference rows, though - *rows = i will work just fine. Don't dereference when calling your function, though. Use:

 matrixInput(rows, columns);

since you want to pass the pointers.

Upvotes: 3

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

int *matrix1[0][0], *matrix2[0][0];

this is strange and incorrect (you should not declare global variables as 0-dimensionned arrays).

And your code is not complete. You forgot at least the #include <stdio.h> and also #include <stdlib.h>

And your code don't compile.

You should try to compile with all warnings enabled, e.g. on Linux with GCC with 

gcc -Wall -g cud.c -o cud

I'm getting 4 errors and more than 10 warnings.

Don't post a code till you've got no warnings and no more errors. Edit & improve your code till all errors and warning are gone. Then debug your code (with a debugger, like gdb on Linux).

Upvotes: 2

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