The C Programming Language, Ansi C
3.1 Statements and Blocks
Chapter 3 - Control Flow
The control-flow of a language specify the order in which computations are
performed. We have already met the most common control-flow constructions in
earlier examples; here we will complete the set, and be more precise about
the ones discussed before.
An expression such as
x = 0
or
i++
or
printf(...)
becomes a
statement
when it is followed by a semicolon, as in
x = 0;
i++;
printf(...);
In C, the semicolon is a statement terminator, rather than a separator as it
is in languages like Pascal.
Braces { and } are used to group declarations and statements together into a compound statement, or block, so that they are syntactically equivalent to a single statement. The braces that surround the statements of a function are one obvious example; braces around multiple statements after an if, else, while, or for are another. (Variables can be declared inside any block; we will talk about this in Chapter 4.) There is no semicolon after the right brace that ends a block.
The
if-else
statement is used to express decisions. Formally the syntax
is
if (
expression
)
statement
1
else
statement
2
where the
else
part is optional. The
expression
is evaluated;
if it is true (that is, if
expression
has a non-zero value),
statement
1
is executed. If it is false (
expression
is zero) and if there is an
else
part,
statement
2
is executed instead.
Since an if tests the numeric value of an expression, certain coding shortcuts are possible. The most obvious is writing
if (
expression
)
instead of
if (
expression
!= 0)
Sometimes this is natural and clear; at other times it can be cryptic.
Because the else part of an if-else is optional,there is an ambiguity when an else if omitted from a nested if sequence. This is resolved by associating the else with the closest previous else-less if. For example, in