Loops |
For repeated execution of
similar things, loops are used. If you are familiar with other
programming languages you have probably heard about for-loops, while-loops,
and until-loops. Fortran 77 has only one loop construct, called the
do-loop.
A D V E R T I S E M E N T
The
do-loop corresponds to what is known as a for-loop
in other languages. Other loop constructs have to be simulated using the
if and
goto statements.
do-loops
The do-loop
is used for simple counting. Here is a simple example that prints the cumulative
sums of the integers from 1 through n (assume n has been assigned a value
elsewhere):
integer i, n, sum
sum = 0
do 10 i = 1, n
sum = sum + i
write(*,*) 'i =', i
write(*,*) 'sum =', sum
10 continue
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The number 10 is a statement
label. Typically, there will be many loops and other statements in a single
program that require a statement label. The programmer is responsible for
assigning a unique number to each label in each program (or subprogram). Recall
that column positions 2-5 are reserved for statement labels. The numerical value
of statement labels have no significance, so any integer numbers can be used.
Typically, most programmers increment labels by 10 at a time.
The variable defined in the
do-statement is
incremented by 1 by default. However, you can define any other integer to be the
step. This program segment prints the even numbers between 1 and 10 in
decreasing order:
integer i
do 20 i = 10, 1, -2
write(*,*) 'i =', i
20 continue
The general form of the
do loop is as follows:
do label var = expr1, expr2, expr3
statements
label continue
var is the loop variable (often called the loop
index) which must be integer. expr1 specifies the initial value of
var, expr2 is the terminating bound, and expr3 is the
increment (step).
Note: The
do-loop variable must
never be changed by other statements within the loop! This will cause great
confusion.
Many Fortran 77 compilers allow
do-loops to be closed
by the enddo
statement. The advantage of this is that the statement label can then be omitted
since it is assumed that an
enddo closes the nearest previous
do statement. The
enddo construct is
widely used, but it is not a part of ANSI Fortran 77.
while-loops
The most intuitive way to write a
while-loop is
while (logical expr) do
statements
enddo
or alternatively,
do while (logical expr)
statements
enddo
The statements in the body will
be repeated as long as the condition in the
while statement is true. Even though this syntax is
accepted by many compilers, it is not ANSI Fortran 77. The correct way is to use
if and
goto:
label if (logical expr) then
statements
goto label
endif
Here is an example that calculates and prints all the
powers of two that are less than or equal to 100:
integer n
n = 1
10 if (n .le. 100) then
n = 2*n
write (*,*) n
goto 10
endif
until-loops
If the termination criterion is at the end instead of the
beginning, it is often called an until-loop. The pseudocode looks like this:
do
statements
until (logical expr)
Again, this should be implemented in Fortran 77 by using
if and
goto:
label continue
statements
if (logical expr) goto label
Note that the logical expression in the latter version
should be the negation of the expression given in the pseudocode!
