Python Programming for Biology: Bioinformatics and Beyond

The ‘

while

’ loop

In addition to the for loop that operates on a collection of items, there is a while loop that

simply repeats while some statement is True:

while conditionalExpression:

# codeBlock

The  codeBlock  is  repeated  over  and  over  until  the  conditionalExpression  evaluates  to

False. Note that if the tested expression never evaluates to False then you have an ‘infinite

loop’, which is naturally not helpful. Sometimes you can use a while loop to avoid having

to  initially  generate  a  collection  of  items  to  then  loop  over.  For  example,  you  could

generate a series of numbers by doubling a value at each iteration, until a limit is reached:

value = 1

while value < 32:

# maybe do something with the value

value *= 2

# value is 32 at the end

What happens here is that the value is doubled in each loop and once it gets to 32  the

while test fails (32 is not less than 32) and that last value is preserved. Note that if the test

were instead value <= 32 then we would get one more doubling and the value would reach

64.