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Control statements

Control statements

Topics

• Conditional statements.
  • IF statement,

  • IF construct,

  • CASE construct.

• Repetition.

• DO constructs.

• GOTO.

Conditional statements

IF statement

• Form:

  IF( expression ) statement

  Only if expression is .TRUE. is statement executed.

• Example:

  LOGICAL :: test

  ...

  IF ( test ) WRITE(*,*) `test is true'

  IF ( x.LT.0 ) x=0

  IF ( a/=b ) a=b

  IF ( name=='Sam' ) &

  WRITE(*,*) `hello Sam...'

IF construct

• Single condition, single statement block:

block

ENDIF [name]

• Only if expression is .TRUE. is block executed.

• Single condition, alternative statement blocks:

  [name:] IF (expression) THEN

  block1

  ELSE [name]

  block2

  ENDIF [name]

• If expression is .TRUE.then block1 is executed, otherwise block2 is executed.

• Multiple conditions, alternative statement blocks:

block1

ELSEIF (expression2) THEN [name]

block2

...

[ELSE [name]

block]

ENDIF [name]

• if expression1 is .TRUE.then block1 is executed and control passed to ENDIF,

• otherwise expression2 is evaluated,

• if expression2 is .TRUE. then block2 is executed and control passed to ENDIF.

• Important - not all conditions in a construct need be evaluated!

• Example:

INTEGER :: n

...

IF ( n==1 ) THEN

discount = 0.0

ELSEIF ( n<=5 ) THEN

discount = 0.1

ELSEIF ( n>5 .AND. n<=10) THEN

discount = 0.15

ELSE

discount = 0.25

ENDIF

cost = cost-(cost*discount)

WRITE(*,*) `Invoice for', cost

CASE

• The select CASE construct is a structured way of selecting mutually exclusive actions, based on the value of a single expression.

• The SELECT CASE construct provides an alternative to the IF construct.

• Form:

CASE( value ) [name]

block

...

[CASE DEFAULT

block]

END SELECT [name]

• the result of expression may be character, integer or logical.

• value may take single value(s) or a range of values separated by a: (character and integer only), example:

min: !from min upward

:max !from max downward

min:max !between the limits

• Example:

season: SELECT CASE( month )

CASE(4,5)

WRITE(*,*) `Spring'

CASE(6,7)

WRITE(*,*) `Summer'

CASE(8:10)

WRITE(*,*) `Autumn'

CASE(11,1:3,12)

WRITE(*,*) `Winter'

CASE DEFAULT

WRITE(*,*) `not a month'

END SELCET season

Repetition

DO construct

• DO constructs (or `loops') contain executable statement(s) which are to be repeated.

• Form:

block

END DO [name]

• count is an integer variable,

• start is an integer variable (or expression) indicating the initial value of count,

• stop is an integer variable (or expresion) indicating the final value of count,

• step is an integer variable (or expression) indicating the increment (default is 1).

DO loops

Count control clause

• Examples:

... !some=1,2,3,4,5

END DO first

DO i=10,16,2

... !i=10,12,14,16

END DO

DO j=-7,4,3

... !j=-7,-4,-1,2

END DO

last: DO k=20,0,-5

... !k=20,15,10,5,0

END DO last

Transferring control

• Exit from loop with EXIT.

• Begin another iteration with CYCLE.

• By default EXIT and CYCLE apply to the inner-most loop containing the statement, but can refer to a specific, named loop.

• Example:

...

inner: DO k=1,5

IF ( a(k)==0 ) CYCLE

IF ( a(k)<0 ) EXIT outer

...

END DO inner

END DO outer

GOTO statement

Form

• The GOTO statement transfers control to the executable statement with label.
  • it is not possible to transfer control from outside to inside a construct.

• Example:

...

IF ( x.LT.1 ) GOTO 10

...

10 x = 1

• Beware! GOTO statements may be easy to use initially but very hard to maintain subsequently.