Thanks Hans Mackowiak for your reply.

I tried using case, the real problem was. If iam giving user a list of 
items
for ex:
puts " press 1 for adding
       press 2 for subtraction
       press 3 for multiplication
       press 4 to exit"
var=STDIN.gets
case var
when 1
 sum=a+b
when 2
 sub=a-b
when 3
 pr=a*b
when 4
 exit
else "wrong entry"
end

after completing one task the user should get the list of items again to 
do one more task. which is possible using break in C

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