DO Loop

 

            Section 5 of the FORTRAN95 Manual introduces the topic of REPETITION in FORTRAN. One way to repeat one or more statements is to use DO loop.

 

The syntax of DO loop:

                        DO variable = initial_value, final_value[, increment]

                                    [statements]

                        END DO

or

                        DO label variable = initial_value, final_value[, increment]

                                    [statements]

            label     CONTINUE

 

If increment is not specified, it is assumed to be 1.

 

For example

PROGRAM LINES

! Illustration of DO-loops

IMPLICIT NONE

INTEGER L ! a counter

DO L = 1, 100 ! start of repeated section

PRINT *, L, ' I must not talk in class'

END DO ! end of repeated section

END PROGRAM LINES

or

PROGRAM LINES

! Illustration of DO-loops

IMPLICIT NONE

INTEGER L ! a counter

DO 100 L = 1, 100 ! start of repeated section

PRINT *, L, ' I must not talk in class'

100   contunue ! end of repeated section

END PROGRAM LINES

 

Here is the flowchart of this program; the DO loop is inside the dotted lines.

 

 

            To simplify the flowchart and show the DO loop more clearly, a new box can be used for a Do loop.

            It is easy to use a DO loop to calculate a formula which uses ∑ or an addition of a series of numbers. For example, 1+2+…..+100 or :

                        sum = 0

            DO I = 1, 100, 1

            sum = sum + i

            END DO

 

            Do loops can be nested, one inside another.

For example

PROGRAM LINES

! Illustration of DO-loops

IMPLICIT NONE

INTEGER L, K ! counters

DO L = 1, 100 ! start of repeated section

      DO K = 1, 100 ! start of repeated section

            PRINT *, L,K, ' I must not talk in class'

      END DO ! end of repeated section - K

END DO ! end of repeated section - L

END PROGRAM LINES

 

            Here is the flow chart for two nested Do loops:

 

Flowchart: Display: L, K, ‘ I must not talk in class’Flowchart: Preparation: DO K=1,100

loop
 

Exercises

            1. (optional) Draw flowchart of the following programs 1) - 3).

            2.  What are the outputs of the following programs 1) - 3)?

            3. (optional) Check your answers by running the programs. Use the program name as the FORTRAN file name (with the extension .f95) and run the programs on the Salford Plato IDE. (Remember to add your name, group, course and date at the beginning of each program in comment lines.)

 

1)                    

Program prog11

do 30 i = 10,2,-2

write(*,*) i

30    continue

stop

end

 

2)

 

PROGRAM NESTED

! Illustration of nested DO-loops

IMPLICIT NONE

INTEGER I, J                  ! loop counters

DO I = 1, 6                   ! start of outer loop

PRINT *, 'Outer loop with I = ', I

DO J = 1, 3                   ! start of inner loop

PRINT *, ' I, J = ', I, J

END DO

PRINT *                       ! a blank line

END DO                        ! end of repeated section

END PROGRAM NESTED

 

3)

 

PROGRAM XLOOP

! Illustration of non-integer values

IMPLICIT NONE

INTEGER I                     ! loop counter

REAL DX                       ! increment

REAL X0                       ! non-integral initial value

REAL X                        ! value to be output

X0 = 0.5                      ! set initial value

DX = 0.3                      ! set increment

DO I = 1, 10                  ! start of repeated section

X = X0 + (I - 1) * DX         ! actual value to be output

PRINT *, X

END DO                        ! end of repeated section

END PROGRAM XLOOP

 

            4. Use a DO loop to display a temperature conversion table, between -100 C˚ to 100 C˚ at every 20 C˚, as shown:

                        Degrees Fahrenheit   = Degrees Celsius ´ (9/5) + 32

           

C

F

-100

-148

-80

-112

-60

-76

-40

-40

-20

-4

0

32

20

68

40

104

60

140

80

176

100

212

FORTRAN file name: C2F.f95