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Within Range() - Python

Find out more about adding flow control to your Python programswith the "for" and "while" loops, see how the range() function can be usedto generate number ranges, and learn all about list objects. And bring anold flame along for the ride.

TABLE OF CONTENTS:
  1. Python 101 (part 3): A Twist In The Tail
  2. Here Comes A Hero
  3. Making Friends And Influencing People
  4. We Don't Need Another Hero
  5. Looping The Loop
  6. Twist And Turn
  7. Within Range()
  8. Just Passin' Through
By: Vikram Vaswani, (c) Melonfire
Rating: starstarstarstarstar / 4
June 13, 2001

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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 returna range of numbers, given a starting and ending point. This range is alwaysreturned as a list - and as you'll see, this can combine quite effectivelywith 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 tothe 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, incombination with the "for" loop, can come in handy when you need to performa 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 theuser-specified number (num) by all numbers within the range 2 to (num). Ifno 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 withoutencountering a "break" statement even once.

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

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