Programming In C: Using a Pointer to Change the Value of a Variable

Declaring (Initializing) Pointers

As we learned in the previous lesson, a pointer contains the address of a variable. Different types of variables have different types of pointers. For example, an integer variable will have an integer pointer; a float variable will have a float pointer.

Pointers can be variables too. You can declare a pointer at the beginning of a program, just like you declare a regular variable. There are three things you must do:

1. specify the type of pointer (int, float, etc.)
2. use a star (asterisk) * in front of the variable name, to show that it’s a pointer
3. set the variable value at NULL, so it can be filled in later

Put these three things together, and you can declare a pointer, like this:

int *point = NULL;

Now let’s try a program that declares a pointer, then uses it to show the pointer’s address:

#include<stdio.h> void main( ) { int year; int *point = NULL; printf(“What year is it? “); scanf(“%d”, &year); point = &year; printf(“\nThe year is %d.”, year); printf(“\nThe year’s address is %p.”, &year); printf(“\nThe pointer’s address is %p.”, &point); printf(“\nThe pointer’s value is %p, which is the same as the year’s address.”, point); }

Notice this line: point = &year; We are setting the pointer to equal the address where the variable ‘year’ is stored. Later in the program, we use the variable ‘point’ to show the pointer’s address: printf(“\nThe pointer’s address is %p.”, &point);

Type this source code in your editor and save it as point.c then compile it, link it, and run it.

Incrementing Pointers

We can increment pointers by using ++point. However, the computer will NOT simply add 1 to the pointer’s address.

When you increment a pointer, the computer will jump to the next block of memory. If you are using an int variable, which takes up 2 bytes of memory, then the computer will increment by2, to reach the next block.

If you are using a long variable, which takes up 4 bytes of memory, then the computer will increment by 4, to reach the next block.

Thus, when you increment a pointer, you must keep in mind what type of variable you are using. For example, let’s increment an int variable:

#include<stdio.h> void main( ) { int year = 1776; int *point = NULL; point = &year; printf(“\nThe year is %d.”, year); printf(“\nThe address of year is %p.”, &year); printf(“\nThe pointer is the same as the address: %p.”, point); ++point; printf(“\nAfter incrementing the pointer, year’s address is %p.”, point); ++point; printf(“\nIncrement again and year’s address is %p.”, point); }

Type this source code in your editor and save it as incint.c then compile it, link it, and run it.

Now let’s try it with a long variable:

#include<stdio.h> void main( ) { long salary = 75000; long *point = NULL; point = &salary; printf(“\nJesse’s salary is %ld.”, salary); printf(“\nThe address of salary is %p.”, &salary); printf(“\nThe pointer is the same as the address: %p.”, point); ++point; printf(“\nAfter incrementing the pointer, salary’s address is %p.”, point); ++point; printf(“\nIncrement again and salary’s address is %p.”, point); }

Type this source code in your editor and save it as inclong.c then compile it, link it, and run it.

Using a Pointer to Change a Variable’s Value

We can use pointers in many ways. For example, we can use a pointer to change the value of a variable. Let’s learn more about pointers.

When we use *point, we are dereferencing the pointer. This means that we are giving the value of the variable, instead of the value of the pointer.

To change the value of a variable, we must do four things:

1. declare a variable with a value
2. declare a pointer with the value NULL
3. make the pointer equal the address of the variable
4. dereference the pointer and make it equal another number

Put these four things together, and you can change a variable’s value like this:

int kids = 4;
int *point = NULL;
point = &kids;
*point = 6;

This changes the value of kids from 4 to 6. Let’s write a program that uses this:

#include<stdio.h> void main( ) { int kids = 4; int *point = NULL; printf(“\nThe Smiths have %d children.”, kids); point = &kids; *point = 6; printf(“\nThe Larks have %d children.”, kids); }

Type this source code in your editor and save it as chvar.c then compile it, link it, and run it. Your result will be:

The Smiths have 4 children.
The Larks have 6 children.