Python 101 (part 3): A Twist In The Tail - Within Range()
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While on the topic of the "for" loop, it's worth mentioning the range() function, a built-in Python function whose sole raison d'etre is to return a range of numbers, given a starting and ending point. This range is always returned as a list - and as you'll see, this can combine quite effectively with the "for" loop in certain situations.

>>> range(30,40)
[30, 31, 32, 33, 34, 35, 36, 37, 38, 39]
>>>

You can omit the first argument to have Python generate a range from 0 to the specified end point.

>>> range(40)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39]
>>>

You can skip certain numbers in the range by adding an optional "step" argument (by default, this is 1).

>>> range (25,500,25)
[25, 50, 75, 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375,
400, 425, 450, 475]
>>> range (100,1,-10)
[100, 90, 80, 70, 60, 50, 40, 30, 20, 10]
>>>

What does this have to do with anything? Well, the range() function, in combination with the "for" loop, can come in handy when you need to perform a series of actions a specified number of times,

>>> for x in range(1,12):
...     print "It is now", x, "o'clock"
...
It is now 1 o'clock
It is now 2 o'clock
It is now 3 o'clock
It is now 4 o'clock
It is now 5 o'clock
It is now 6 o'clock
It is now 7 o'clock
It is now 8 o'clock
It is now 9 o'clock
It is now 10 o'clock
It is now 11 o'clock
>>>

or to generate indices (corresponding to list elements) in a "for" loop.

>>> flavours = ["Strawberry", "Blueberry", "Blackcurrant", "Pineapple",
"Mango", "Grape", "Orange", "Banana"]
>>> for temp in range(2,5): print flavours[temp]
...
Blackcurrant
Pineapple
Mango
>>>

Here's a more interesting example.

#!/usr/bin/python

# get a number
num = input("Gimme a number: ")

# for loop
for count in range(2,num):
        # if factor exists, num cannot be prime!
        if num % count == 0:
                print num, "is not a prime number."
                break
else:
        # if we get this far, num is prime!
        print num, "is a prime number."

This is a simple piece of code to test whether or not a number is prime. A "for" loop is used, in conjunction with the range() function, to divide the user-specified number (num) by all numbers within the range 2 to (num). If no factors are found, it implies that the number is a prime number.

Here's what it looks like:

Gimme a number: 23
23 is a prime number.
Gimme a number: 45
45 is not a prime number.
Gimme a number: 11
11 is a prime number.
Gimme a number: 111
111 is not a prime number.

As with the "while" loop, Python allows you to add an "else" clause to a "for" loop; it is executed only if the loop is completed without encountering a "break" statement even once.

>>> More Python Articles          >>> More By Vikram Vaswani, (c) Melonfire