C Programming For Beginners - Integer Expressions, Operators and Precedence

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.

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));}
}

C Programming – A Beginner’s Course

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