Python 101 (part 2): If Wishes Were Pythons - If Only...

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Why do you need to know all this? Well, comparison operators come in very useful when building conditional expressions - and conditional expressions come in very useful when adding control routines to your code. Control routines check for the existence of certain conditions, and execute appropriate program code depending on what they find.

The first - and simplest - decision-making routine is the "if" statement, which looks like this:

if condition:
do this!

The "condition" here refers to a conditional expression, which evaluates to either true or false. For example,

if hearing spooky noises:
call Ghostbusters!

or, in Python lingo:

if spooky_noises == 1:
callGhostbusters()

If the conditional expression evaluates as true, all statements within the indented code block are executed. If the conditional expression evaluates as false, all statements within the indented block will be ignored, and the lines of code following the "if" block will be executed.

One of the unique things about Python is that it does not require you to enclose conditional statement blocks within curly braces, like most other languages. Instead, Python identifies code blocks according to indentation; all Python statements with the same amount of indentation are treated as though they belong to the same code block.

If, however, the conditional block consists of a single statement, Python also allows you to place it on the same line as the "if" statement. Consequently, the example above could also be written as

if spooky_noises == 1: callGhostbusters()

Here's a simple program that illustrates the basics of the "if" statement.

#!/usr/bin/python
# ask for a number
alpha = input("Gimme a number! ")
# ask for another number
beta = input("Gimme another number! ")
# check
if alpha == beta:
print ("Can't you read, moron? I need two *different* numbers!")
print ("Go away now!")

And they say that the popular conception of programmers as rude, uncouth hooligans is untrue!

Next: Tying Up The Loose Ends >>
 

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