In all programming languages there are three general types of structures: sequence, selection, and looping. A sequence structure contains steps that are executed sequentially while a selection structure tests a condition to determine which steps are to be executed. Looping, the ability to repeat a block of statements, is the primary structure used to solve massive computations. It is used for mathematical techniques requiring iterative processing and, also, the processing of different data sets using the same, or similar, actions. It is in applications requiring a loop structure that computers are most useful and where programming is essential to solving the problem.

Example

Problem Statement

Compute the average of a set of numbers.

Input/Output Description

The user is prompted for and enters the number of values to be averaged.

The output is the average of the set of numbers.

Algorithm

1. Prompt for and get the number of values in the set of numbers to be averaged.
2. Initialize the value of sum to 0.0.
3. Enter a number from the set.
4. Add the number to sum.
5. Repeat steps two and three until all of the numbers are entered.
6. Divide the total by the number of values in the set.
7. Display the average.

CODE

c FORTRAN 77
 
c This program averages a series of numbers input from the keyboard.
 
        PROGRAM average
 
c Type variables.
        REAL data, sum, avg
        INTEGER num,i
 
c Prompt for and enter the number of numbers to average.
        PRINT *,'Enter the number of numbers to average.'
        READ *,num

c Initialize sum to 0.0 
        sum = 0.0

c The loop goes from 1 to number of values to average.
        DO 100 i = 1, num
 
c Prompt for and enter a number.
           PRINT *,'Enter a value for the number'
              READ *,data
 
c Add the number to the total.
              sum = sum + data
100     CONTINUE
 
c Calculate the average.
        avg = sum/real(num)
 
c Print the results.
        PRINT *,'The average = ',avg
 
        STOP
        END

Sample Output

(4,23,45,34,67 entered.)

Enter the number of numbers to average.
4
Enter a value for the number
23
Enter a value for the number
45
Enter a value for the number
34
Enter a value for the number
67
The average =     42.2500

The syntax of a FORTRAN 77 DO loop is

DO label k = start#, final#, [increment]

instruction block

label CONTINUE

where

DO marks the beginning of the loop

label is the label number of the CONTINUE statement that marks the end of the loop

k is a variable used as the loop counter; its value may not be changed inside the loop

start# is the initial value given to the loop counter

final# is the value that determines when the DO loop is completed

increment represents the value to be added to the loop counter each time the loop is executed.

There are five steps involved in the execution of a DO loop

• initialize the loop counter,
• execute the instruction block,
• increment the loop counter,
• compare the value of the loop counter to the final#, and,
• when the value of the counter variable is greater than the final#, program control is passed to the statement following the continue statement.

In the average program example, the loop counter is initialized with the statement DO 100 i = 1, num. The final# is the variable num that is entered from the keyboard. The instruction block reads the number from the keyboard and adds it to the total. The loop counter is incremented at the end of the loop. The control of the program returns to the statement DO 100 i = 1, num where the value of the loop counter is compared to final#. When the value of i is greater than num, program control is passed to the statement "avg = sum/real(num)".

Start#, final#, and increment may be constants, variables whose values have previously been assigned a value, or expressions. If there is not an increment value, it is assumed to be 1. The increment may be positive or negative.

Example 1

DO 20 i = 2,10,2

PRINT *, i

20 CONTINUE

Output

2

4

6

8

10

Example 2

DO 20 n = 25,5,-5

PRINT *,n

20 CONTINUE