Reading: Boolean Values and Expressions

This reading introduces Boolean (true/false) values and expressions in C.

Comparison Operators

Many (most?) programs later in this course will require the comparison of values, yielding a true or false conclusion. Such comparisons are called Boolean expressions. Simple comparisons may use these relations:

Notes:

• Programmers who may have programmed in other languages may mistakenly write = for ==. In C, the compiler will interpret the equal sign = as an assignment. Thus, the statement i = 3 does not compare i to 3 but rather assigns the value 3 to i.

• In C, the exclamation point ! means "not", so != means "not equal".

Combining Boolean Expressions

Beyond these simple connectives (<, <=, >, >=, ==, !=), compound Boolean expressions may be created with logical and (&&) and logical or (||).

Given Boolean values or expressions A and B,

• Logical and:

  • A && B is true if both A and B are true
  • A && B is false if either A or B is false, or if they are both false.

• Logical or:

  • A || B is true if either A or B is true, or if they are both true.
  • A || B is false if both A and B are false.

Boolean Values in C

Technically, C has no separate Boolean type. Rather Boolean results are considered integers.

• In C, a zero value is considered to be false.

• Any non-zero value is considered to be true.

In practice, the result of a comparison is normally 1 (true) or 0 (false). Thus, 1 < 2 and 4 > 3 both evaluate to 1.

A Common Error

It is easy to misuse the comparison operators. In mathematics, the expression:

    -2 < -1 < 0

makes sense and would evaluate to true. However, in C, the semantics of these expressions is different, and the above expression in C actually evaluates to false. Specifically, comparison operators associate left to right and evaluate to Boolean values, so -2 < -1 < 0 is evaluated as if parentheses were added to give (-2 < -1) < 0. The value returned from the first comparison is 1 (true) because -2 is less than -1. Next, 1 is compared to 0. 1 is not less than 0 so the expression evaluates to false.

To do multiple tests at once, use logical AND (&&), and logical OR (||) operators. For example, the previous example can be correctly expressed in C as:

     (-1 > -2) && (-1 < 0) 

Acknowledgments

Much of this reading evolved specifically to introduce Boolean data types and expressions. That material was written and edited by Henry M. Walker.

The last section of this reading is derived from material written by David Cowden, Dilan Ustek, and Henry M. Walker.