Learn to Program with C_ Learn to Program using the Popular C Programming Language ( PDFDrive )

Chapter 4 ■ Programs with Selection Logic
4.2.3 NOT, !
If p is some Boolean expression, then NOT p reverses the truth value of p. In others words, if p is
true then NOT p is false; if p is false then NOT p is true. In C, the symbol for NOT is the exclamation
mark, !. Using the example above, since
n >= 1 && n <= 12
tests for valid n, the condition NOT (n >=1 && n <= 12) tests for invalid n. This is written in C as
!(n >= 1 && n <= 12)
This is equivalent to n < 1 || n > 12. Those familiar with de Morgan’s laws will know that
not (a and b) = (not a) or (not b)
and
not(a or b) = (not a) and (not b)
In general, if p and q are Boolean expressions, we have the following:
• p && q is true when both p and q are true and false, otherwise;
• p || q is true when either p or q is true and false only when
both p and q are false;
• !p is true when p is false and false when p is true.
This is summarized in the following table (with T for true and F for false):
P q && || !p
T T T T F
T F F T F
F T F T T
F F F F T
Most of the programs in this book will use simple conditions. A few will use compound
conditions.
4.2.3.1 The data type bool in C99
The original C standard and the later ANSI C standard did not define a Boolean data type.
Traditionally, C has used the concept of the value of an expression to denote true/false. A numeric
expression can be used in any context where a true/false value is required. The expression is
considered true if its value is nonzero and false if its value is zero.
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