Programming Forums - Loops in Python

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Dietrich

Jan 5th, 2007 12:45 PM

Loops in Python

This question came from a hijacked thread. I thought it important enough to put it in its own thread with the proper title.

What loops does Python offer and what are they used for?

I might add, that our friend Arevos gave one good answer here:
http://www.programmingforums.org/for...122207-30.html

DaWei

Jan 5th, 2007 12:49 PM

I don't think you can get a much better answer than his reply. Certainly his first sentence is virtually THE definitive answer to the second question. I agree with your reason for restating it here, however.

Dietrich

Jan 5th, 2007 1:56 PM

There are several ways to stop a while loop:

# the while loop needs a True condition, here x < 10 to run

# and will stop running on False, when x goes to 10 or above

x = 0

while x < 10:

    # do something with x

    print x

    # increment x by 1, same as x = x + 1

    x += 1



print



# 'while True' would be an endless loop, so you need to break it

# with a break condition (an if statement), stops when x is 10 or higher

x = 0

while True:

    # do something with x

    print x

    x += 1

    if x >= 10:

        break



"""

output in either case -->

0

1

2

3

4

5

6

7

8

9

"""

With the while loop you can do cute things like this:

# whittle down a string with while:

s = "internets"

# string s is True until it's empty

while s: 

    print s

    # each time around remove one char from the end

    s = s[:-1]



"""

output -->

internets

internet

interne

intern

inter

inte

int

in

i

"""

hydroxide

Jan 6th, 2007 5:09 AM

Another way of breaking a while loop - or even nested while loops -- is to use a flag. If you find yourself using this technique, though, you probably need to redesign:

flag1 = False

flag2 = False

while not (flag1 or flag2):

    for i in range(10):

        print i

        if i == 4:

            flag1 = True

            break

        elif i == 6:

            flag2 = True

            break

A sometimes useful trick for getting out of nested loops is to raise an exception - StopIteration is the clearest, (use sparingly - a redesign/restructure is normally better):

def myfunc(i, j):

    print i, j

    if i == j == 1:

        raise StopIteration

    

try:

    while True:

        for i in range(10):

            for j in range(10):

                myfunc(i, j)

except StopIteration:

    pass

-T.

Dietrich

Jan 6th, 2007 11:34 AM

Here is an example of using a recursive function as a while loop with a name:

def count_to_9(x):

    # do something with x

    print x

    x += 1

    if x >= 10:

        return

    count_to_9(x)



# start the loop at zero

count_to_9(0)



"""

output -->

0

1

2

3

4

5

6

7

8

9

"""

Note, this is just an example. Function calls are too time expensive to make this practical.

If you ever have to break out of some badly nested loops, put them into a function and use return to break out.

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