"""indefinite-loops.py
By Jadrian Miles, adapted from Jeff Ondich.

A demonstration of various indefinite-loop constructs in
Python.
"""
# An indefinite loop just keeps going and going while
# some "continuation condition" is satisfied.  This
# condition is just an expression that evaluates to a
# boolean value, True or False.  As long as the
# expression is True, the loop keeps executing.  If
# it's ever False, though, the loop stops.

# There are lots of parts to this program.  It may be
# easier to work with if you comment out different
# all the lines, and then uncomment only one section at
# a time.  You could also copy each section in turn into
# a separate file, and run that little program instead.

# As always, you can modify this program to inspect its
# guts, too.  Try using type() to check the types of
# various things.


#################################################################
# -- Run the loop below and see what happens.
# -- What happens if you remove the "k = k - 1" line?
# -- Can you make the loop count down by twos?
# -- Can you make the loop count up by threes from 0 to 999?

print '====== a while-loop counting down ======'
k = 10
while k > 0:
    print k
    k = k - 1
print 'Boom.'
print


#################################################################
# Here's another while-loop.  Compare it to the "for x in y"
# loop from last time, where y was a string.

print '====== a while-loop and a string ======'
y = 'emus again'
k = 0
while k < len(y):
    print y[k]
    k = k + 1
print


#################################################################
# while-loops can do a lot of things other than simple
# counting.  What does this loop do?

print '====== a more sophisticated while-loop and a string ======'
y = 'emus again'
k = 0
s = ''
while k < len(y) and y[k].isalpha():
    s = s + y[k]
    k = k + 1
print 'The resulting string is:', s
print