4.3 Examples

Passing Arrays to Functions Using Pointers

When you pass an array to a function, the array is essentially passed as a pointer to its first element. This allows you to modify the array within the function.

Example: Modifying Array Elements in a Function

#include <stdio.h>

void double_elements(int *arr, int size) {
    for (int i = 0; i < size; i++) {
        arr[i] *= 2;  // Modify the array elements
    }
}

int main() {
    int arr[] = {1, 2, 3, 4, 5};
    int size = sizeof(arr) / sizeof(arr[0]);

    printf("Before doubling:\n");
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }

    // Pass the array to the function
    double_elements(arr, size);

    printf("\nAfter doubling:\n");
    for (int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }

    return 0;
}

Output:

Before doubling:
1 2 3 4 5
After doubling:
2 4 6 8 10

Sample Code 01 (Arrays & Function Pointers)

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

void print(int *pArr, int len)
{

    int i;
    printf("Printing the array:\n");
    for (i = 0; i < len; i++)
    {
        printf("%d ", pArr[i]);
    }
    printf("\n");
}

int sum(int *pArr, int len)
{
    int i;
    int total = 0;
    for (i = 0; i < len; i++)
    {
        total += pArr[i];
    }

    return total;
}

int max(int *pArr, int len)
{
    int i;
    int cur_max = pArr[0];
    int cur_max_i;
    for (i = 0; i < len; i++)
    {
        if (pArr[i] > cur_max)
            cur_max_i = i;
    }

    return cur_max_i;
}

/* Create a function pointer type */
typedef int (*PtrType)(int*, int);

PtrType ProcessCommandLineArg(char* argv[])
{
    PtrType fptr;
    if (strcmp(argv[1], "sum") == 0)
    {
        fptr = &sum;  /* type => int (*)(int *, int); */
    }
    else if (strcmp(argv[1], "max") == 0)
    {
        fptr = &max;
    }

    return fptr;
}

int main(int argc, char *argv[])
{
    /* ./prog sum 10 20 30 40 50 60 */

    int i;
    int len = argc - 2;
    int *pArr = malloc(sizeof(int) * len);

    /* Create a instance with the newly created type */
    /* This instance is a function pointer which can poitn to functions like sum and max */
    PtrType fptr;

    if (argc < 3)
    {
        printf("Invalid number of args: usage: <function_name> <arg1> <arg2> <arg3> ...\n");
    }
    else
    {
        /* Parse the numbers in the cmd line args into an integer array */
        for (i = 2; i < argc; i++)
        {
            pArr[i - 2] = atoi(argv[i]);
        }

        fptr = ProcessCommandLineArg(argv);
        printf("The result is: %d\n", (*fptr)(pArr, len));
    }

    free(pArr);
    return 0;
}