Do Loops

There will be many times in coding where you will want/need to do the same set of actions repeatedly, in which case you would want to use some sort of loop to concisely write and execute that portion of code. The do loop in Fortran is the simple looping operator that is controlled by an iterative counter.

The following is the syntax for a do loop in Fortran.

        DO n control_var = initial_val, end_val, step_val

             statement_sequence

 n      continue

where n is a statement number, control_var is the iterating variable (e.g., i or j), and initial_val, end_val, and step_val are integer or real values over which the loop will occur.

The statement number is how a Fortran program is told where the end of the loop sequence is located. It is good coding practice to indent all of the statement sequence contained within the do loop by the same amount of spaces (typically four). Doing so will make the code much more readable to you and especially anyone who is trying to read your code to figure out what it is actually doing.

NOTE: Indention is paramount to how the Python is formulated, so get used to indenting your code appropriately.

The do-loop body is executed as long as the initial value is less than or equal to the end value, for a positive step value. Fortran also allows for a negative step value, which means that the initial value must be greater than the end value in order for the loop to execute.

Simple Do Loop Examples

• Example 1

      DO 100 i = 1, 10

         PRINT *, i

 100  continue

In the example above the statement number, n, is 100, the control_var is i, the initial_val is 1, and the end_val is 10. This means that there is an implied step_val of 1. The result of the code should be

1
2
3
4
5
6
7
8
9
10

Since the loop will begin with the initial_val of 1 and continue until the control_var is greater than the end_val.

• Example 2

In this example we get rid of the statement number and use an 'END DO' to signify the end of the do loop. Additionally, we have specified a negative step_val, which means we want to count down from our initial_val.

        DO j = 200, 2, -5

          PRINT *, j

        END DO

The output of the above code should be

200
195
190
185
180
...
...
...
20
15
10
5

The execution of the loop will end when the value of the control_var is less than the end_val. In the above case the next value of j is zero (0), which is less than 2, so the program would exit the do loop and not execute the code within the do loop.