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→‎more choices: if and else
imported>Jesstess
 
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fish = "humuhumunukunukuapuaʻa"
name_length = len(fish)
fish + " is a Hawaiian fish whose name is " + str(name_length) + " characters long.")
</pre>
 
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Take a break, stretch, meet some neighbors, and ask the staff if you have any questions about this material.
 
[[File:Fireworks.png|200px]]
 
== Part 2: Printing==
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<ol>
<li>Download the file http://mit.edu/jesstess/www/BostonPythonWorkshop8/nobel.py by right-clicking on it and saying to save it as a ".py" file to your Desktop. The ".py" extension hints that this is a Python script.</li>
<li>Open a terminal prompt, and use the navigation commands (<code>dir</code> and <code>cd</code> on Windows, <code>ls</code>, <code>pwd</code>, and <code>cd</code> on OS X and Linux) to navigate to your homeDesktop directory. See [[Boston Python Workshop 8/Friday#Goal_.234:_practice_navigating_the_computer_from_a_terminal|navigating from a terminal]] for a refresher on those commands.</li>
<li>Once you are in your Desktop directory, execute the contents of <code>nobel.py</code> by typing
 
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<li>How do you print a multi-line string so that whitespace is preserved?</li>
</ol>
 
Let's get back to some interactive examples. Keep typing them out! You'll thank yourself tomorrow. :)
 
==Booleans==
 
Please return to the interactive Python interpreter for the rest of the tutorial. And remember: type out the examples. You'll thank yourself tomorrow. :)
 
[[File:Scales.png|100px]]
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</pre>
 
==FlowMaking Controlchoices==
 
We can use these expressions that <i>evaluate</i> to booleans to make decisions and conditionally execute code:.▼
 
[[File:Fork.png|100px]]
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====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):
▲We can use these expressions that <i>evaluate</i> to booleans to make decisions and conditionally execute code:
 
&nbsp;&nbsp;&nbsp;&nbsp;<code>if 6 > 5:</code><br />
<pre>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<code> print("Six is greater than five!")</code>▼
if 6 > 5:
print("Six is greater than five!")
</pre>▼
 
That wasis our first multi-line piece of code, and the way to entertype it at a Python prompt is a little different. First,Let's break down how to do this (type thethis out step by step):
 
<ol>
<code> if 6 > 5:</code>
<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 <b>code block</b>, 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>
part, and press Enter. The next line will have
<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 toone end the line, and press Entermore againtime to tell Python you are done with this code block. AllThe together,code itblock will looknow like this:execute.</li>▼
▲</preol>
 
All together, it will look like this:
<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 <b>code block</b>, and so long as we indent our code it should be a part of this code block.
 
Enter 4 spaces, and then type
 
▲<code> print("Six is greater than five!")</code>
 
▲Press Enter to end the line, and press Enter again to tell Python you are done with this code block. All together, it will look like this:
 
<pre>
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====more choices: <code>if</code> and <code>else</code>====
 
You can use the <b><code>elseif</code></b> keywordlets toyou execute some code whenonly theif a condition is <code>ifTrue</code>. expressionWhat isn'tif True.you Trywant this:to execute different code if a condition is <code>False</code>?
 
Use the <b><code>else</code></b> keyword, together with <code>if</code>, to execute different code when the <code>if</code> condition isn't <code>True</code>. Try this:
 
<pre>
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</pre>
 
Like with <code>if</code>, the code block under the <code>else</code> statementcondition must be indented so Python knows that it is a part of the <code>else</code> block.
 
====compound conditionals: <code>and</code> and <code>or</code>====
 
You can check multiple expressions together using the <b><code>and</code></b> and <b><code>or</code></b> keywords. If two expressions are joined by an <code>and</code>, they <b>both</b> have to be <code>True</code> for the overall expression to be <code>True</code>. If two expressions are joined by an <code>or</code>, as long as <b>at least one</b> is <code>True</code>, the overall expression is <code>True</code>.
 
Try typing these out and see what you get:
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====even more choices: <code>elif</code>====
 
If you haveneed to execute code conditional based on more than two cases, you can use the <b><code>elif</code></b> keyword to check more cases. You can have as many <code>elif</code> cases as you want; Python will go down the code checking each <code>elif</code> until it finds a <code>True</code> condition or reaches the default <code>else</code> block.
 
<pre>
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</pre>
 
You don't have to have an <code>else</code> block, if you don't need it. That just means there isn't default code to execute when none of the <code>if</code> or <code>elif</code> conditions are <code>True</code>:
 
<pre>
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