[Tutor] use while loop

[denis]

----- Original Message -----
From: <Simonjthecat at aol.com>
To: <tutor at python.org>
Sent: Friday, April 23, 2004 5:13 AM
Subject: [Tutor] use while loop


> Ok.  I know this is really simple, but it's my first program so anybody
can
> pobably answer it for me.  This program finds the area of different
shapes.
> Right now, however, it only goes through one time and then exits the
program.  I
> want to put it in a loop so that after one area is given it will start
over
> and ask what shape you want again.  I figured out how to put it in a for
loop
> and get it to execute a given number of times.  I'm assuming I need it in
a
> while loop but just can't do it.  So anyway, here's what I've got so far.

(Nobody else answers ? I'll try to be clear...)

This is the typical case where a code section has to be executed once, and
only then a condition will tell if it should be repeted or not. In
pseudo-code it looks like :

loop
    <instruction_bloc>
    until exit_condition

or :

loop
    <instruction_bloc>
    while continue_condition

(where each condition is the logical negation (logical NOT) of the other
one.)
As python doesn't provide such a loop form with an end test, you need to use
the one you've got -- that is : the 'while' loop with a start test. And use
a double trick to transform it into what you want do :

while True :                        # (1) switch off start test
    <instruction_bloc>
    if  exit_condition : break      # (2) handmade end test


> import sys #for exit command
>
> print """
> Choose a shape from the list:
> 1) Triangle
> 2) Square
> 3) Circle
> """
> shape = raw_input("Which shape[1,2,3,quit]? ")
>
> # note must now test for character '1' not
> # number 1 coz raw_input returns strings not numbers
> if shape == '1':     # a triangle
>    ht = input('What is the height of your triangle? ')
>    base = input('How long is the base? ')
>    print "The triangle's area is: ", 0.5*base*ht
>
> elif shape == '2':   # square
>    side = input("How long are the square's sides? ")
>    print "The square's area is: ", side*side
>
> elif shape == '3':   # a circle
>    rad = input('What radius is your circle? ')
>    print "The circle's area is: ", 3.14159*rad*rad
>
> elif shape == 'quit':
>    sys.exit()
>
> else:
>    print "Sorry, You didn't enter a valid choice"

Either to simplify your loop or (logical inclusive OR) for trainig purpose,
you may put your area calculations into functions.

> I know this has a real simple fix.
> Thanx for the help
>
> Simon the Cat
>


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