'''
	while.py
    Jeff Ondich, 26 September 2018

    This sample program introduces the while-loop.
'''

# Since there are a lot of pieces to this program, you
# may want to use copy and paste to run just one piece
# at a time while you investigate each kind of loop
# structure.  That's up to you.

# 1. Run the loop below and see what happens.
# 2. What happens if you remove the "k = k - 1" line?
# 3. Can you make the loop count down by twos?
# 4. 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('Lift-off.')
print()

# Here's another while-loop.  Compare it to the for x in y
# loop above 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()


# Next: nested loops.  That is, loops inside loops.
# Try to predict what this loop will print.

print('====== nested loops, first example  ======')
for j in range(4):
    for k in range(3):
        print(j, k)
print()

# This one's trickier.  Can you predict it?  Can
# you explain why it does what it does?

print('====== nested loops, trickier ======')
for j in range(4):
    for k in range(j):
        print(j, k)
print()