Editing Community Data Science Course (Spring 2015)/Day 1 Tutorial (section)

====if statements====

The simplest way to make a choice in Python is with the <code>if</code> keyword. Here's an example (don't try to type this one, just look at it for now):

&nbsp;&nbsp;&nbsp;&nbsp;<code>if 6 > 5:</code><br />
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<code>print("Six is greater than five!")</code>

That is our first multi-line piece of code, and the way to type it at a Python prompt is a little different. Let's break down how to do this (type this out step by step):

<ol>
<li>First, type the<br />
<br />
&nbsp;&nbsp;&nbsp;&nbsp;<code>if 6 > 5:</code><br />
<br />
part, and press Enter. The next line will have <code>...</code> as a prompt, instead of the usual <code>&gt;&gt;&gt;</code>. This is Python telling us that we are in the middle of a '''code block''', and so long as we indent our code it should be a part of this code block.</li>

<li>Press the spacebar 4 times to indent.</li>
<li>Type<br />
<br />
&nbsp;&nbsp;&nbsp;&nbsp;<code>print("Six is greater than five!")</code><br /><br /></li>
<li>Press Enter to end the line. The prompt will still be a <code>...</code></li>
<li>Press Enter one more time to tell Python you are done with this code block. The code block will now execute.</li>
</ol>

All together, it will look like this:

<pre>
>>> if 6 > 5:
...      print("Six is greater than five!")
... 
Six is greater than five!
</pre>

What is going on here? When Python encounters the <code>if</code> keyword, it <i>evaluates</i> the <i>expression</i> following the keyword and before the colon. If that expression is '''True''', Python executes the code in the indented code block under the <code>if</code> line. If that expression is '''False''', Python skips over the code block.

In this case, because 6 really is greater than 5, Python executes the code block under the if statement, and we see "Six is greater than five!" printed to the screen. Guess what will happen with these other expressions, then type them out and see if your guess was correct:

<pre>
if 0 > 2:
     print("Zero is greater than two!")
</pre>

<pre>
if "banana" in "bananarama":
    print("I miss the 80s.")
</pre>