Loop structures: Difference between revisions

==[[Fortran]]==

===DO loop===

FORTRAN 77 only has one loop structure: the <tt>DO</tt> loop. It is very similar to the for loop of the C-style languages.

<lang fortran>C This will add the numbers from one through `N`. Naturally, the `10`

C label can be whatever label you want to use. Also note that indenting

C is optional. I indented for the sake of readability.

INTEGER I, N, TOTAL

READ (*,*) N

TOTAL = 0

DO 10 I = 1, N

TOTAL = TOTAL + I

WRITE (*,*) I, TOTAL

10 CONTINUE

C You can also control the step; this one will count backwards from `N`,

C two by two:

INTEGER I, N

READ (*,*) N

DO 20 I = N, 1, -2

WRITE (*,*) I

20 CONTINUE

C Nota bene: the two (or three) initialization values for the loop are

C only evaluated once! So, for example, this will count from one through

C ten, where it might lead to an infinite loop in other languages:

INTEGER I, J

J = 10

DO 30 I = 1, J

WRITE (*,*) I

J = J + 1

30 CONTINUE</lang>

===While loops===

FORTRAN 77 does not, semantically speaking, have <tt>while</tt> loops. However, you can easily simulate them using <tt>GOTO</tt> statements.

<lang fortran>C A while loop in FORTRAN 77 is really just a conditional with a goto.

40 IF (condition) THEN

:

:

GOTO 40

ENDIF</lang>

===Do-until and do-while loops===

<tt>Do-until</tt> and <tt>do-while</tt> loops are a little different, but also use <tt>GOTO</tt> statements.

<lang fortran>C A do-until loop is similar to the while loop:

50 CONTINUE

:

:

IF (.NOT. (condition)) GOTO 50

C A do-while loop is almost exactly the same as a do-until loop.

60 CONTINUE

:

:

IF (condition) GOTO 60</lang>

===For loops===

If you really need to have a <tt>for</tt> loop where the range can change during the loop, you can simulate that with a <tt>while</tt> loop.

<lang fortran>C This is a rewrite of the example labelled `30`, so that it actually

C will lead to an infinite loop:

INTEGER I, J

J = 10

I = 1

70 IF (I .LE. J) THEN

WRITE (*,*) I

J = J + I

GOTO 70

ENDIF</lang>