HP Fortran Programmer's Reference (September 2007)

Execution control
Control constructs and statement blocks
Chapter 6142
A third form, combining elements of the other two, is also supported:
[ construct-name :] DO label index = init , limit [, step ]
Execution logic
The following execution steps apply to all three syntactic forms, except as noted:
1. The loop becomes active, and index is set to init .
2. The iteration count is determined by the following expression:
MAX( INT ( limit - init + step ) / step , 0 )
step is optional, with the default value of 1. It may not be 0.
Note that the iteration count is 0 if either of the following conditions is true:
• step (if present) is a positive number and init is greater than limit .
• step is a negative number and init is less than limit .
3. If the iteration count is 0, the construct becomes inactive and the normal flow of execution
resumes with the first executable statement following the END DO or terminal statement.
4. statement-block executes. (In the case of the old-style syntactic form, both
statement-sequence and terminal-statement execute.)
5. The iteration count is decremented by 1, and index is incremented by step , or by 1 if
step is not specified.
6. Go to Step 3.
NOTE To ensure compatibility with older versions of Fortran, you can use the
+onetrip compile-line option to ensure that, when a counter-controlled DO loop
is encountered during program execution, the body of the loop executes at least
once.
Examples
This example uses nested DO loops to sort an array into ascending order:
INTEGER :: scores(100)
DO i = 1, 99
DO j = i+1, 100
IF (scores(i) > scores(j)) THEN
temp = scores(i)
scores(i) = scores(j)
scores(j) = temp