9.4: Conditional Controlled Looping

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A conditional controlled loop repeats a series of one or more Fortran statements based on a condition. As such, the loop may execute an indeterminate number of times.

One form of the conditional loop is:

do while (conditional expression)
    <fortran statement(s)>
end do

In this form, the conditional expression is re-checked at the top of the loop on each iteration.

A more general format of the conditional loop is:

do
    <fortran statement(s)>
end do

As is, this loop will continue forever. Probably not so good. A selection statement, such as an IF statement, and an exit statement would be used to provide a means to terminate the looping.

For example,

do
    <fortran statement(s)>
    if (conditional expression) exit
    <fortran statement(s)>
end do

Would stop looping only when the conditional expression evaluates to true. The exit statement can be used multiple times in different locations as needed. An IF statement, in any form, can be used for either the exit or cycle statements.

For example, a conditional loop could be used to request input from the user and keep re-prompting until the input is correct.

integer :: month

do
    write (*,*) "Enter month (1-12): "
    read (*,*) month
    if (month >= 1 .and. month <= 12) exit
    write (*,*) "Error, month must be between 1 and 12."
    write (*,*) "Please re-enter."
end do

This will keep re-prompting an unlimited number of times until the correct input (a number between 1 and 12) is entered.

Since a counter controlled DO loop requires an integer loop counter, another use of conditional loops would be to simulate a real counter. For example, to display the values from 1.5 to 4.5 stepping by 0.25, the following conditional loop could be used.

real :: currValue = 1.5

do while (currValue <= 4.5)
    write (*,*) "Value = ", currValue
    currValue = currValue + 0.25
end do

The values of currValue and amount of the increment, set as 0.25, can be adjusted as needed.