Home Computational Physics FORTRAN FORTRAN Loops

Fortran 77 has only one loop construct, called the do-loop. 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.
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

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 statemen tlabels. 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
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 dostatement. The enddo construct is widely used, but it is not a part of ANSI Fortran 77.

The most intuitive way to write a while-loop is
while (logical expr) do
or alternatively,
do while (logical expr)

The statements in the body will be repeated as long as the condition in the while statement is true. Event hough this syntax is accepted by many compilers, it is not ANSI Fortran 77. The correct way is to use ifand goto:
label if (logical expr) then
goto label

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

If the termination criterion is at the end instead of the beginning, it is often called an until-loop. The pseudo code looks like this:
until (logical expr)
Again, this should be implemented in Fortran 77 by using if and goto:
label continue
if (logical expr) goto label

Note that the logical expression in the latter version should be the negation of the expression given in the pseudo code!