Python 101 (part 3): A Twist In The Tail - Looping The Loop (
Page 5 of 8 )
For those of you unfamiliar with the term, a "loop" is a programming
construct that allows you to execute a set of statements over and over
again, until a pre-defined condition is met. It's one of the most basic
constructs available in a programming language, and comes in handy when you
need to perform a repetitive task over and over again.
Unlike its counterparts, which offer a variety of different loop variants,
Python keeps things simple with only two types of loops: the "while" loop
and the "for" loop. The former is simpler to read and understand, and it
usually looks like this:
while (condition):
do this!
In English, this would roughly translate to
while (living with Mom and Dad):
curfew is 11 PM
while the Python equivalent would look like this
while (livingWithParents == 1):
curfew = 2300
As with conditional statements, so long as the specified condition remains
true, the indented code block will continue to execute. However, as soon as
the condition becomes false - you move out and get your own place, say -
the loop will be broken and the indented statements will stop executing.
You'll notice that Python uses indentation to decide which statements
belong to the "while" block - you probably remember this from last time. As
with the "if" statement, if the code block consists of only a single
statement, Python allows you to place it on the same line as the "while"
statement. For example,
while (livingWithParents == 0): curfew = "Huh? What curfew?"
is a perfectly valid "while" loop.
Here's a simple example which demonstrates the "while" loop.
#!/usr/bin/python
# initialize a variable
response = ""
# while loop
while response != "y":
# keep asking the question
response = raw_input("Would you like to receive unsolicited
commercial email from people you don't know, advertising products you have
no interest in, on a regular basis (once every 15 minutes)? [y/n] ")
print "Thank you for your cooperation!"
And here's the output.
Would you like to receive unsolicited commercial email from people you
don't know, advertising products you have no interest in, on a regular
basis (once every 15 minutes)? [y/n] n
Would you like to receive unsolicted commerial email from people you don't
know, advertising products you have no interest in, on a regular basis
(once every 15 minutes)? [y/n] n
Would you like to receive unsolicted commerial email from people you don't
know, advertising products you have no interest in, on a regular basis
(once every 15 minutes)? [y/n] n
Would you like to receive unsolicted commerial email from people you don't
know, advertising products you have no interest in, on a regular basis
(once every 15 minutes)? [y/n] y
Thank you for your cooperation!
Python allows you to add an "else" clause to your "while" loop as well;
this clause is executed if the loop is executed without encountering a
single "break" statement (more on this later.)
Consequently, the example above could be rewritten to read:
#!/usr/bin/python
# initialize a variable
response = ""
# while loop
while response != "y":
# keep asking the question
response = raw_input("Would you like to receive unsolicited
commercial email from people you don't know, advertising products you have
no interest in, on
a regular basis (once every 15 minutes)? [y/n] ")
else:
print "Thank you for your cooperation!"
How about something a little more constructive?
#!/usr/bin/python
# get a number
num = input("Gimme a number: ")
# assign the number to a "temp" variable
tmpnum = num
factorial = 1
# calculate the factorial
while (num != 1):
factorial = factorial * num
num = num - 1
print "The factorial of", tmpnum, "is", factorial
In case you flunked math class, the factorial of a number X is the product
of all the numbers between 1 and X. And here's what the output looks like:
Gimme a number: 7
The factorial of 7 is 5040
And if you have a calculator handy, you'll see that
7*6*5*4*3*2*1 = 5040
Once the user enters a number, a "while" loop is used to calculate the
product of that number and the variable "factorial" (initialized to 1) -
this value is again stored in the variable "factorial". Next, the number is
reduced by 1, and the process is repeated, until the number becomes equal
to 1. At this stage, the value of "factorial" is printed.
