Functions and Integer Single Dimension Array

In this section, you are going to learn

What are the calling conventions of integer single dimension array ?

Call by Value

Call by Reference

Revisit Basics : Basics of Integer Single Dimension Array

Topics in this section,

Integer Single Dimension Array : Syntax

Integer Single Dimension Array : FAQs

Integer Single Dimension Array : fun(expression)

Rules for Call By Value

Rules for Call By Reference

Examples of Call by Value

Example 1 : Call by Value with [ ]

Example 2 : Call by Value with *

Examples of Call by Reference

Example 3 : Call by Reference WITH &a[ ]

Example 4 : Call by Reference WITH (a + x)

Example 5 : Pass full array to a function

Example 6 : Pass part of the array by reference

Example 7 : Address of full array and Pointer to an Array

Integer Single Dimension Array : Syntax

int array_name[Column];

Integer Single Dimension Array : FAQs

Consider a Integer Single Dimension Array

int a[10];

Let us answer few basic questions in this array

Integer Single Dimension Array : fun(expression)

If fun(x) is the function call, then fun(typeof(x)) is the prototype / definition

Function Call	Function Definition	Observations
fun(a[0])	void fun(int x) { }	Call by Value
fun(a[1])	void fun(int x) { }	Call by Value
fun(a[9])	void fun(int x) { }	Call by Value
fun(&a[0])	void fun(int *p) { }	Call by Reference
fun(&a[1])	void fun(int *p) { }	Call by Reference
fun(&a[9])	void fun(int *p) { }	Call by Reference
fun(*a)	void fun(int x) { }	Call by Value
fun(*(a + 1))	void fun(int x) { }	Call by Value
fun(*(a + 9))	void fun(int x) { }	Call by Value
fun(a)	void fun(int *p) { }	Call by Reference
fun(a + 1)	void fun(int *p) { }	Call by Reference
fun(a + 9)	void fun(int *p) { }	Call by Reference
fun(&a)	void fun(int (*p) [10] ) { }	Call by Reference

Rules for Call by Value

If Declaration has ONE dereference operator, and

Expression has ONE dereference operator [], and

Expression does not have &

then it is call by value

If Declaration has ONE dereference operators, and

Expression has ONE dereference operator *, and

Expression does not have &

then it is call by value

Rules for Call by Reference

If Declaration has ONE dereference operator, and

Expression has ONE dereference operators [] or *, and

Expression has ONE &

then it is call by reference

Example : &a[0]

If Declaration has ONE dereference operator, and

Expression has ZERO dereference operator [ ] or *, and

Expression has ZERO & operator

then it is call by reference

Example : a + 1, a + 4

If Declaration has ONE dereference operator, and

Expression has ZERO dereference operator [ ] or *, and

Expression has ONE & operator

then it is call by reference

Example : &a

Examples of Call by Value

Let us look at examples of Call by Value

Example 1 : Call by Value with [ ]

Example for Call By Value with [ ]

Step 1 : Define a integer array a

int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

Step 2 : Pass an individual integer a[2] to a function. Call by Value

fun(a[2]);

Individual array elements can be accessed using [ ]

In this case a[2] is third integer in the array

a[2] is fully dereferenced and there is no & symbol in fun(a[2]). Hence this is Call By Value

Step 3 : Define function fun

void fun(int x)
{

}

Step 4 : Change value of x inside function fun

void fun(int x)
{
        x = 100;
        printf("x = %d\n", x);
}

See the full program below

#include <stdio.h>

void fun(int x)
{
        x = 300;
        printf("x = %d\n", x);
}

int main(void)
{
        int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        fun(a[2]);

        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        printf("\n");

        return 0;
}

Output is as below

x = 300
a[0] = 1
a[1] = 2
a[2] = 3
a[3] = 4
a[4] = 5
a[5] = 6
a[6] = 7
a[7] = 8
a[8] = 9
a[9] = 10

Changing value of x inside function fun DOES NOT change a[2] in array a

Example 2 : Call by Value with *

Example for Call By Value with *

Step 1 : Define a integer array a

int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

Step 2 : Pass an individual integer *(a + 2) to a function. Call by Value

fun( *(a + 2) );

Individual array elements can be accessed using *

In this case *(a + 2) is third integer in the array

*(a + 2) is fully dereferenced and there is no & symbol in fun( *(a + 2) ). Hence this is Call By Value

Step 3 : Define function fun

void fun(int x)
{

}

Step 4 : Change value of x inside function fun

void fun(int x)
{
        x = 300;
        printf("x = %d\n", x);
}

See the full program below

#include <stdio.h>

void fun(int x)
{
        x = 300;

        printf("x = %d\n", x);
}

int main(void)
{
        int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        fun( *(a + 2) );

        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        return 0;
}

Output is as below

x = 300
a[0] = 1
a[1] = 2
a[2] = 3
a[3] = 4
a[4] = 5
a[5] = 6
a[6] = 7
a[7] = 8
a[8] = 9
a[9] = 10

Changing value of x inside function fun DOES NOT change *(a + 2) in array a

Remember a[2] and *(a + 2) are one and the same

Examples of Call by Reference

Let us look at examples of Call by Reference

Example 3 : Call by Reference WITH &a[ ]

Example for Call By Reference with &a[ ]

Step 1 : Define a integer array a

int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

Step 2 : Pass address of an individual integer &a[2] to a function. Call by Reference

fun( &a[2] );

Address of individual array elements can be accessed using &

In this case &a[2] is the address of third integer in the array

Since we are passing address of third integer to function fun, it is called call by reference with respect to third integer

Step 3 : Define function fun

void fun(int *x)
{

}

Step 4 : Change value of *x inside function fun

void fun(int *x)
{
        *x = 300;
        printf("*x = %d\n", *x);
}

See the full program below

#include <stdio.h>

void fun(int *x)
{
        *x = 300;
        printf("*x = %d\n", *x);
}

int main(void)
{
        int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        fun( &a[2] );

        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        return 0;
}

Output is as below

*x = 300
a[0] = 1
a[1] = 2
a[2] = 300
a[3] = 4
a[4] = 5
a[5] = 6
a[6] = 7
a[7] = 8
a[8] = 9
a[9] = 10

Changing value of *x inside function fun CHANGES a[2] in array a

Example 4 : Call by Reference WITH (a + x)

Example for Call By Reference with (a + x)

Step 1 : Define a integer array a

int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

Step 2 : Pass address of individual integer a + 2 to a function. Call by Reference

fun( a + 2 );

In this case a + 2 is the address of third integer in the array

Since we are passing address of third integer to function fun, it is called call by reference with respect to third integer

Step 3 : Define function fun

void fun(int *x)
{

}

Step 4 : Change value of *x inside function fun

void fun(int *x)
{
        *x = 300;

        printf("*x = %d\n", *x);
}

See the full program below

#include <stdio.h>

void fun(int *x)
{
        *x = 300;

        printf("*x = %d\n", *x);
}

int main(void)
{
        int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        fun( a + 2 );

        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        return 0;
}

Output is as below

*x = 300
a[0] = 1
a[1] = 2
a[2] = 300
a[3] = 4
a[4] = 5
a[5] = 6
a[6] = 7
a[7] = 8
a[8] = 9
a[9] = 10

Changing value of *x inside function fun CHANGES a[2] in array a

Example 5 : Pass full array array to a function

Step 1 : Consider a integer array

int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

Step 2 : Pass full array array to a function

fun( a );

Note that we are passing starting address of array

Hence function fun has read and write access to all Bytes of array

Step 3 : Define a function

void fun(int *x)
{
        x[0]  = 100;
        x[1]  = 200;
        x[2]  = 300;
        x[3]  = 400;
        x[4]  = 500;
        x[5]  = 600;
        x[6]  = 700;
        x[7]  = 800;
        x[8]  = 900;
        x[9]  = 1000;
}

function fun has access to all integers

See full program below

#include <stdio.h>

void fun(int *x)
{
        x[0]  = 100;
        x[1]  = 200;
        x[2]  = 300;
        x[3]  = 400;
        x[4]  = 500;
        x[5]  = 600;
        x[6]  = 700;
        x[7]  = 800;
        x[8]  = 900;
        x[9]  = 1000;
}

int main(void)
{
        int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        printf("----- Before call by reference -----\n");
        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        fun(a);

        printf("----- After call by reference -----\n");
        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        return 0;
}

Output is as below

----- Before call by reference -----
a[0] = 1
a[1] = 2
a[2] = 3
a[3] = 4
a[4] = 5
a[5] = 6
a[6] = 7
a[7] = 8
a[8] = 9
a[9] = 10

----- After call by reference -----
a[0] = 100
a[1] = 200
a[2] = 300
a[3] = 400
a[4] = 500
a[5] = 600
a[6] = 700
a[7] = 800
a[8] = 900
a[9] = 1000

Example 6 : Pass part of the array by reference

Step 1 : Consider a integer array

int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

Step 2 : Pass full array by reference

fun( a + 5 );

Note that we are passing part of the array by reference

In this case, we are passing address of 6th integer

Hence function fun has read and write access to Bytes 5, 6, 7, 8, 9 in forward direction

Hence function fun has read and write access to Bytes 0, 1, 2, 3, 4 in backward direction

Step 3 : Define a function

void fun(int *x)
{
        x[-5] = 100; // Same as a[0]
        x[-4] = 200; // Same as a[1]
        x[-3] = 300; // Same as a[2]
        x[-2] = 400; // Same as a[3]
        x[-1] = 500; // Same as a[4]
        x[0]  = 600; // Same as a[5]
        x[1]  = 700; // Same as a[6]
        x[2]  = 800; // Same as a[7]
        x[3]  = 900; // Same as a[8]
        x[4]  = 1000; // Same as a[9]
}

Note the relative access mechanism used inside function fun

See full program below

#include <stdio.h>

void fun(int *x)
{
        x[-5] = 100; // Same as a[0]
        x[-4] = 200; // Same as a[1]
        x[-3] = 300; // Same as a[2]
        x[-2] = 400; // Same as a[3]
        x[-1] = 500; // Same as a[4]
        x[0]  = 600; // Same as a[5]
        x[1]  = 700; // Same as a[6]
        x[2]  = 800; // Same as a[7]
        x[3]  = 900; // Same as a[8]
        x[4]  = 1000; // Same as a[9]
}

int main(void)
{
        int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        printf("----- Before call by reference -----\n");

        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        fun( a + 5 );

        printf("----- After call by reference -----\n");

        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        return 0;
}

Output is as below

----- Before call by reference -----
a[0] = 1
a[1] = 2
a[2] = 3
a[3] = 4
a[4] = 5
a[5] = 6
a[6] = 7
a[7] = 8
a[8] = 9
a[9] = 10

----- After call by reference -----
a[0] = 100
a[1] = 200
a[2] = 300
a[3] = 400
a[4] = 500
a[5] = 600
a[6] = 700
a[7] = 800
a[8] = 900
a[9] = 1000

Example 7 : Address of full array and Pointer to an Array

Step 1 : Consider a integer array

int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

Step 2 : Pass the address of array a to function fun

fun(&a);

Step 3 : Define the function fun

void fun( int (*ptr)[10] )
{

}

Note that int (*ptr)[10] is pointer to an array of 10 integers

Which means incrementing ptr will increment by 10 Bytes and decrementing ptr will decrement by 10 Bytes

Step 4 : Access individual integers inside function fun

void fun( int (*ptr)[10] )
{
        //Change individual integers
        (*ptr)[0] = 100;
        (*ptr)[1] = 200;
        (*ptr)[2] = 300;
        (*ptr)[3] = 400;
        (*ptr)[4] = 500;
        (*ptr)[5] = 600;
        (*ptr)[6] = 700;
        (*ptr)[7] = 800;
        (*ptr)[8] = 900;
        (*ptr)[9] = 1000;
}

See the full program below

#include <stdio.h>

void fun( int (*ptr)[10] )
{
        //Change individual integers
        (*ptr)[0] = 100;
        (*ptr)[1] = 200;
        (*ptr)[2] = 300;
        (*ptr)[3] = 400;
        (*ptr)[4] = 500;
        (*ptr)[5] = 600;
        (*ptr)[6] = 700;
        (*ptr)[7] = 800;
        (*ptr)[8] = 900;
        (*ptr)[9] = 1000;
}

int main(void)
{
        int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        printf("----- Before call by reference -----\n");

        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        fun( &a );

        printf("----- After call by reference -----\n");

        for (int i = 0; i < 10; i++)
        {
                printf("a[%d] = %d\n", i, a[i]);
        }

        return 0;
}

Output is as below

----- Before call by reference -----
a[0] = 1
a[1] = 2
a[2] = 3
a[3] = 4
a[4] = 5
a[5] = 6
a[6] = 7
a[7] = 8
a[8] = 9
a[9] = 10

----- After call by reference -----
a[0] = 100
a[1] = 200
a[2] = 300
a[3] = 400
a[4] = 500
a[5] = 600
a[6] = 700
a[7] = 800
a[8] = 900
a[9] = 1000