The if statements |
An important part of any programming language are the
conditional statements. The most common such statement in Fortran is the
ifstatement, which actually has several forms. The simplest one
is the logical if
statement:
A D V E R T I S E M E N T
if (logical expression) executable
statement
This has to be written on one line. This example finds the
absolute value of x:
if (x .LT. 0) x = -x
If more than one statement should be executed inside the
if, then the following
syntax should be used:
if (logical expression) then
statements
endif
The most general form of the
if statement has the following form:
if (logical expression) then
statements
elseif (logical expression) then
statements
:
:
else
statements
endif
The execution flow is from top
to bottom. The conditional expressions are evaluated in sequence until one is
found to be true. Then the associated code is executed and the control jumps to
the next statement after the
endif.
Nested ifstatements
if
statements can be nested in several levels. To ensure readability, it is
important to use proper indentation. Here is an example:
if (x .GT. 0) then
if (x .GE. y) then
write(*,*) 'x is positive and x = y'
else
write(*,*) 'x is positive but x < y'
endif
elseif (x .LT. 0) then
write(*,*) 'x is negative'
else
write(*,*) 'x is zero'
endif
You should avoid nesting many levels of
if statements since
things get hard to follow.
