Share variables between functions in C99

Question

1. Taking in input from the main matrix_1, I need to create a new one and print it with only the odd numbers. Now I'm printing it from the function, but I need to print it from the main(); how can I do that?

#include <stdio.h>
#define len 4

void copiaDispari();

int main () {

    int i, j, matrix_1[len][len];

    for (i=0; i < len; i++){
        for (j=0; j < len; j++) {
            printf ("Inserisci il numero della matrice %d %d:", i, j);
            scanf ("%d", &matrix_1[i][j]);
        }
    }

    copiaDispari(matrix_1);

    for (i=0; i < len; i++){
        for (j=0; j < len; j++) {
            printf ("%d", matrix_2[i][j]);
        }
        printf("\n");
    }

    return 0;
}

void copiaDispari(int matrix_1[len][len]) {

    int matrix_2[len][len]= {{0,0,0,0},{0,0,0,0},{0,0,0,0},{0,0,0,0}}, i, j, l=0, m=0;


    for (i=0; i < len; i++){
        for (j=0; j < len; j++){
            if (matrix_1[i][j]%2!=0){
                if (l==4) {
                    l=0; m++;
                }
                matrix_2[m][l] = matrix_1[i][j];
                l++;
            }
        }
    }

    for (i=0; i < len; i++){
        for (j=0; j < len; j++) {
            printf ("%d", matrix_2[i][j]);
        }
        printf("\n");
    }

    return;
}

2. Imagine I have to pass 5 or 6 variables from a function void yeah() to another function void yessss(); how can I achieve it?
3. When should I use a void function, and when an int function?

Answer 1

1. The simplest approach is to allocate the second array in main() and pass it as an argument to the function (as well as the first array).

   int i, j, matrix_1[len][len], matrix_2[len][len] = { { 0 } };
   …
   copiaDispari(matrix_1, matrix_2);
   …
   void copiaDispari(int matrix_1[len][len], matrix_2[len][len])
   {
       …
   }
   

   Other techniques use dynamic memory management (malloc() et al), or encapsulate the array in a structure and pass the structure type around (you can return structures that contain arrays; you can't return arrays from functions), or (sometimes, but not often) using a static array in the function and returning a pointer to it (don't do this), or using a global variable (don't do this, either!).

2. Functions can have semi-indefinitely large numbers of parameters. C11 §5.2.4.1 Translation limits requires the compiler to accept function definitions and calls with at least 127 arguments. A mere 5 or 6 doesn't cause a compiler any angst. However, reducing the number of arguments is a good idea.

   If you want to return multiple values from a function, you can pass multiple pointers to the function and have it modify the variables via the pointers. Or you can wrap the set of variables into a structure and pass a pointer to a suitable structure, or you can return a suitable structure to the calling function.

3. When the function produces a value that will be used by the calling code, you should often not use a void function. You can pass pointers to variables and the function might modify those to return its answer. If the function can fail, you might have it return 0 on success and some other value on failure; that precludes it having the return type void, of course. Error handling is a major part of C programming. If a function doesn't return a value to indicate success or failure, it can be hard to know whether there a problems.

---

Incidentally, the forward declaration of the function should not be:

void copiaDispari();

In your original code, it should be:

void copiaDispari(int m1[len][len]);

In the revised code, it should be;

void copiaDispari(int m1[len][len], int m2[len][len]);

This will help prevent mistakes such as calling the function with the wrong number of arguments.

You should never write a function declaration with () for the argument list. That says the function takes an indeterminate number of arguments (but it isn't a variable argument list function like printf()). If the function takes no arguments, write int function(void); in the declaration, and for consistency, int function(void) { … } in the definition. Although you can write int function() { … } in the definition, this does not provide a prototype for the function and it can be miscalled in the same source file. Note that C++ differs from C in this area. Old C code (from before the C89/C90 standard was produced) uses the () notation; it didn't have a choice. Code written in this millennium should not use the () notation.

Answer 2

1. [rephrased] how to send data back to the calling function?

Basically you have two options

create the object to hold the data in the calling function, pass a pointer and have the called function use that

int main(void) {
    int src1[10][10] = {0};
    int src2[10][10] = {0};
    fx(src1, src2);
}

or use malloc() (and friends) in the called function then return that to the caller which will be responsible to free the resources.

typedef int m2[10][10];

int main(void) {
    int src[10][10] = {0};
    m2 *dst = fx(src);
    // use dst
    free(dst);
}

m2 *fx(int m[10][10]) {
    m2 *x = malloc(10 * 10 * sizeof (int));
    // work with m to change x values
    return x;
}

A third option, to be frowned upon (no example code), is to use global variables.

2. how to pass 5 or 6 variables between functions?

Basically, just specify them in the calling code, like

printf("%s bought %d %s for %.2f\n", name, quantity, "apples", quantity * price);

Or, if the variables are related, you can try grouping them in a struct and pass a pointer

struct GroupData {
    char name[100];
    int quantity;
    char item[100];
    double price;
};
struct GroupData x = {"John", 42, "apple", 0.31};
workwithdata(&x); // can also pass a copy of the struct
                  // workwithcopy(x);

3. when to use void function or int function?

use void when there is no need to use a value from the function

void beep(void); // sounds the bell

use int when you want to use a value from the function

int currenttemp(void); // returns the temperature