Case Construct

Generalized Conditional

§ Conditional statement has two branches

§ It can be generalized to have multiple branches

§ There are problems where different computations are invoked for different values of an expression

§ expressions can have more than two values

Example

Problem: Given an 24-hour clock value, compute the

corresponding value in the AM-PM clock

Solution:

AM-PM depends upon the value in the 24-hour clock:

00:01 – 11:59 - AM

12 hours - noon

12:01 - 23:59 - PM

00:00 - midnight

The Program

The Program continued

case(0) !hour has the value 0

if (minute = 0) then print *,"12 midnight"

else print *, “0”,minute,“ AM"

case(12) !hour has the value 12

if (minute = 0) then print *,"12 noon"

else print *, "12:",minute,“ PM"

case default

print *, "error in hour“

end select

end program

General form

• Selection of branch decided by the variable in the select statement

• The general form is:

select case(exp)

case(sel1) block1

case(sel2) block2

...

case(seln) blockn

case default block(n+1)

end select

General Case statement

• The case expression is of type integer, character or logical

• The case selector can be:

– Integer or truth value

– Integer1 : integer2

– Integer1:

– : integer2

– A list of integers

• The case expression can not match more than one selector

The default case

• Default case is optional and is the last

– When none of the case selector match, this default case matches

– Should be used with care

– This case will remove run-time errors and hence may hide logical errors in the program

Execution of Case Statement

• First the expression in the select case statement is executed

• The first case branch following the select statement matching the evaluated value is selected

• The block of statements following the selected case is executed

• The control then moves to the statement that follows the select statement

• The default case matches any value and hence executed when no other case matches

The Cases

• Range of values can be given, eg.,

case(0:11), case(11:23)

• Implicit endpoints allowed

case(:j) - all values <= j

case(i:) - all values >=i

• i,j must be constants

• ranges and values for different cases must be disjoint

Nesting of Cases

• Select statements can be nested

• A select case statement appear in one or more blocks of the cases of another case statement.

• The inner select statement should `end' within the same block

• case statements apply to the innermost select statement

• Case statements may be labelled

Example

out: select case(x)

case(1)

in:select case(y)

case(0) block1

default block2

end select in

case(2)

block3

default

block4

end select out

A Calculator example

read *, x,y

read *, op

select case(op)

case(“+”)

print *, (x+y)

case(“*”)

print *, (x*y)

case(“/”) ! Some problem with this character

if (y==0) then

print *, “result undefined”

else

print *, (x/y)

case default

print *, “undefined operation”

end select

Exercises

1. There is some problem in am_pm_clock program. Find and Improve the program

2. Develop a program that converts from am-pm format to 24-hour format

3. Write a program to compute the number of months in a given month