How to pass Array to a Function in C

Whenever we need to pass a list of elements as argument to any functions in C language, it is prefered to do so using an array. But how can we pass an array as argument to a function? Let's see how its done.

Declaring Function with array as a parameter

There are two possible ways to do so, one by using call by value and other by using call by reference.

1. We can either have an array as a parameter.

   int sum (int arr[]);

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2. Or, we can have a pointers in the parameter list, to hold the base address of our array.

   int sum (int* ptr);

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   We will study the second way in details later when we will study pointers.

Returning an Array from a function

We don't return an array from functions, rather we return a pointer holding the base address of the array to be returned. But we must, make sure that the array exists after the function ends i.e. the array is not local to the function.

int* sum (int x[])
{
    // statements
    return x ;
}

We will discuss about this when we will study pointers with arrays.

Passing arrays as parameter to function

Now let's see a few examples where we will pass a single array element as argument to a function, a one dimensional array to a function and a multidimensional array to a function.

1. Passing a single array element to a function

Let's write a very simple program, where we will declare and define an array of integers in our main() function and pass one of the array element to a function, which will just print the value of the element.

#include<stdio.h>

void giveMeArray(int a);

int main()
{
    int myArray[] = { 2, 3, 4 };
    giveMeArray(myArray[2]);        //Passing array element myArray[2] only.
    return 0;
}

void giveMeArray(int a)
{
    printf("%d", a);
}

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2. Passing a complete One-dimensional array to a function

To understand how this is done, let's write a function to find out average of all the elements of the array and print it.

We will only send in the name of the array as argument, which is nothing but the address of the starting element of the array, or we can say the starting memory address.

#include<stdio.h>

float findAverage(int marks[]);

int main()
{
    float avg;
    int marks[] = {99, 90, 96, 93, 95};
    avg = findAverage(marks);       // name of the array is passed as argument.
    printf("Average marks = %.1f", avg);
    return 0;
}

float findAverage(int marks[])
{
    int i, sum = 0;
    float avg;
    for (i = 0; i <= 4; i++) {
        sum += marks[i];
    }
    avg = (sum / 5);
    return avg;
}

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3. Passing a Multi-dimensional array to a function

Here again, we will only pass the name of the array as argument.

#include<stdio.h>

void displayArray(int arr[3][3]);

int main()
{
    int arr[3][3], i, j;
    printf("Please enter 9 numbers for the array: \n");
    for (i = 0; i < 3; ++i)
    {
        for (j = 0; j < 3; ++j)
        {    
            scanf("%d", &arr[i][j]);
        }
    }
    // passing the array as argument
    displayArray(arr);
    return 0;
}

void displayArray(int arr[3][3])
{
    int i, j;
    printf("The complete array is: \n");
    for (i = 0; i < 3; ++i)
    {
        // getting cursor to new line
        printf("\n");
        for (j = 0; j < 3; ++j)
        {       
            // \t is used to provide tab space
            printf("%d\t", arr[i][j]);
        }
    }
}

Please enter 9 numbers for the array: 1 2 3 4 5 6 7 8 9 The complete array is: 1 2 3 4 5 6 7 8 9