C Programming: Integer Expressions and Precedence of Operators
Integer expressions
In C programming, an integer constant (e.g. 23, 0, -245) is the simplest example of an integer expression. However, most of the time, we write integer expressions by combining constants and variables with the following arithmetic operators:
- add
- subtract
* multiply
/ divide
% find remainder
For example, suppose we have the following declaration:
int a, b, c;
then the following are all valid expressions:
a + 39
a + b - c * 2
b % 10 //the remainder when b is divided by 10
c + (a * 2 + b * 2) / 2
The operators +, - and * all give the expected results. However, / performs integer division; if there is any remainder, it is thrown away. We say integer division truncates. Thus 19 / 5 gives the value 3; the remainder 4 is discarded.
But what is the value of -19 / 5? The answer here is –3. The rule is that, in C, integer division truncates towards zero. Since the exact value of –19 ÷ 5 is –3.8, truncating towards zero gives –3.
The % operator gives the remainder when one integer is divided by another; for example,
19 % 5 evaluates to 4;
j % 7 gives the remainder when j is divided by 7;
You can use it to test, for instance, if a number j is even or odd. If j % 2 is 0 then j is even; if j % 2 is 1, j is odd.
Precedence of operators
C evaluates an expression based on the usual precedence of operators: multiplication and division are done before addition and subtraction. We say that multiplication and division have higher precedence than addition and subtraction. For example, the expression
5 + 3 * 4
is evaluated by first multiplying 3 by 4 (giving 12) and then adding 5 to 12, giving 17 as the value of the expression.
As usual, we can use brackets to force the evaluation of an expression in the order we want. For example,
(5 + 3) * 4
first adds 5 and 3 (giving 8), and then multiplies 8 by 4, giving 32.
When two operators which have the same precedence appear in an expression, they are evaluated from left to right, unless specified otherwise by brackets. For example,
24 / 4 * 2
is evaluated as
(24 / 4) * 2
(giving 12) and
12 - 7 + 3
is evaluated as
(12 - 7) + 3
giving 8. However,
24 / (4 * 2)
is evaluated with the multiplication done first, giving 3, and
12 - (7 + 3)
is evaluated with the addition done first, giving 2.
In C, the remainder operator % has the same precedence as multiplication (*) and division (/).
Exercise: What is printed by the following program? Verify your answer by typing and running the program. Note: use greater than and less than signs to properly enclose ‘stdio.h’ instead of parantheses.
#include (stdio.h) main() { int a = 15: int b = 24; printf("%d %d\n", b - a + 7,b - (a + 7)); printf("%d %d\n", b - a - 4, b - (a - 4)); printf("%d %d\n", b % a / 2, b % (a / 2)); printf("%d %d\n", b * a / 2, b * (a / 2)); printf("%d %d\n", b / 2 * a, b / (2 * a)); } }
References
C Programming – A Beginner’s Course
Related programming references
This post is part of the series: C Programming for Beginners
A straightforward introduction to Programming in C for people with no previous programing experience.
- C Programming For Beginners - Part 1
- C Programming For Beginners - Part 2
- C Programming for Beginners – Part 3
- C Programming for Beginners – Part 4
- C Programming For Beginners - Part 5
- C Programming For Beginners - Part 6
- C Programming For Beginners - Data Types
- C Programming For Beginners - Part 8
- C Programming For Beginners - Part 9
- C Programming For Beginners - Part 10
- C Programming For Beginners - Part 11
- C Programming For Beginners - Part 12
- C Programming For Beginners - Part 13
- C Programming For Beginners - Part 14
- C Programming For Beginners - Integer Data Types
- C Programming for Beginners - Part 16
- C Programming For Beginners - Integer Expressions, Operators and Precedence
- C Programming For Beginners - Part 18
- C Programming For Beginners - Printing Double and Float
- C Programming For Beginners - Mixing double, float and int