CEE15
Introduction to Computing for Civil Engineers
21
Chapter 3
Control Structures: Selective Execution
3.1. IF-THEN-ELSE-END IF Statement
3.1.1. IF-THEN-ELSE-END IF
The most general form of the
IF-THEN-ELSE-END IF
statement is the following:
IF (logical-expression) THEN
statements-1
ELSE
statements-2
END IF
where
statements-1
and
statements-2
are sequences of executable statements, and
logical-expression
is
a logical expression. The execution of this
IF-THEN-ELSE-END IF
statement goes as follows:
•
the
logical-expression
is evaluated, yielding a logical value
•
if the result is
.TRUE.
, the statements in
statements-1
are executed
•
if the result is
.FALSE.
, the statements in
statements-2
are executed
•
after finish executing statements in
statements-1
or
statements-2
, the statement following
END
IF
is executed
Examples:
•
The following code first reads in an integer into INTEGER variable Number. Then, if Number can
be divided evenly by 2 (i.e., Number is a multiple of 2), the WRITE(*,*) between IF and ELSE is
executed and shows that the number is even; otherwise, the WRITE(*,*) between ELSE and END
IF is executed and shows that the number is odd. Function MOD(x,y) computes the remainder of x
divided by y. This is the remainder (or modulo) function
INTEGER :: Number
READ(*,*)
Number
IF (MOD(Number, 2) == 0) THEN
WRITE(*,*)
Number, ' is even'
ELSE
WRITE(*,*)
Number, ' is odd'
END IF
•
The following program segment computes the absolute value of X and saves the result into variable
Absolute_X. Recall that the absolute value of x is x if x is non-negative; otherwise, the absolute
value is -x. For example, the absolute value of 5 is 5 and the absolute value of -4 is 4=-(-4). Also
note that the WRITE(*,*) statement has been intentionally broken into two lines with the
continuation line symbol &.
REAL
:: X, Absolute_X
X =
.....
IF (X >= 0.0) THEN
Absolute_X = X
ELSE
Absolute_X = -X
END IF

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