Lesson 3.9. Increment and decrement operators

Lesson 3.8 Summary Lesson 3.10

In C, we use loops to repeat blocks of instructions. In these loops, there is often an index which counts the number of times the loop has been executed. Each time the loop is executed, this index is incremented, i.e. 1 is added to it. We could write an instruction like :

i = i + 1;

or:

i += 1;

This operation is so frequent, that the designers of Ccreated operators specially devoted to it:

Operator	name	Example	Equivalent
`i++`	post-increment	`a = i++;`	`a=i; i=i+1;`
`i--`	post-decrement	`a = i--;`	`a=i; i=i-1;`
`++i`	pre-increment	`a = ++i;`	`i=i+1; a=i;`
`--i`	pre-decrement	`a = --i;`	`i=i-1; a=i;`

In the case of post-increment, the increment is performed after the instruction. In the case of a pre-increment, the increment is performed first. This difference is essential, because it changes the result of the statements.

Post-increment

Consider the following example of a post-increment:

int a = 5, x;
x = 2 * a++;

In the case of post-increment (a++), the increment will be done at the end, after the multiplication (*) and the assignment (=). Here are the equivalent instructions:

// Multiplication
x = 2 * a;
// Post-increment
a = a + 1;

After the execution of the instructions, x is 10 and a is 6:

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Pre-increment

Let's go back to the previous example, modifying it with a pre-increment:

int a = 5, x;
x = 2 * ++a;

In the case of pre-increment, (++a) the increment is done first, before multiplication (*) and assignment (=). Here are the equivalent instructions:

// Pre-increment
a = a + 1;
// Multiplication
x = 2 * a;

After the execution of the instructions, x is 12 and a is 6:

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Exercises

Exercise 1

In the following code, merge the two operations on a single line with an increment operator:

short x, a=-6;

// MERGE THE TWO FOLLOWING LINES 
// WITH AN INCREMENT OPERATOR
a = a + 1;
x = 16 + a;

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

Do the exercise

Solution

Exercise 2

In the following code, merge the two operations on a single line with an increment operator:

short x, a=-6;

// MERGE THE TWO FOLLOWING LINES 
// WITH AN INCREMENT OPERATOR
x = 16 + a;
a = a + 1;

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

Do the exercise

Solution

Exercise 3

In the following code, merge the three operations on a single line with increment operators:

short x, a=0, b=2;

// MERGE THE THREE FOLLOWING LINES 
// WITH AN INCREMENT OPERATOR
a = a + 1;
x = a + b;
b = b - 1;

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

Do the exercise

Solution

Quiz

Which are the incrementing operators?

i++

++i

i--

--i

Check

Which is the pre-decrement operator?

i++

++i

i--

--i

Check Bravo! The pre-decrement operator is --i.

What is the equivalent statement to x = x + 1;?

x++

x--

x = x++;

x = x--;

Check Bravo! x++ already assign the result to variable x, no need to write x = x++;

Merge these two instructions into one:

a = a + 1;
x = a % b;

x = a % b++;

x = a++ % b;

x = a % ++b;

x = ++a % b;

Check Bravo! The increment of a is executed first, this is a pre-increment.

Merge these two instructions into one:

x = a * b;
a = a - 1;

x = a * b--;

x = a * --b;

x = --a * b;

x = a-- * b;

Check Bravo ! The increment of a is at the end, this is a post-decrement.

Merge these two instructions into one:

x = a * 2;
a = a + 2;

It is not possible.

x = a++ * 2;

x = a++++ * 2;

x = a++2 * 2;

Check

Merge these two instructions into one:

x1 = x1 - 1;
x = x1 + x2;
x2 = x2 + 1;
x = x + x3;
x3 = x3 - 1;

It is not possible.

x = --x1 + x2++ + --x3;

x = --x1 + x2++ + x3--;

x = --x1 + ++x2 + x3--;

Check

Lesson 3.8 Summary Lesson 3.10

Lesson 3.9. Increment and decrement operators

Post-increment

Pre-increment

Exercises

Exercise 1

Exercise 2

Exercise 3

Quiz

See also