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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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The string manipulate is exactly the same as before. The only difference is that you need to use <b>print</b> to print results to the screen:
 
<code> <code>h + w</code>
 
becomes
 
<code> <code>print(h + w)</code>
 
We'll see more examples of the print function in the next section.
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[[File:Treasure_map.png|100px]]
 
Until now we've been executingusing commandsthe at theinteractive Python promptinterpreter. This is great for mathlearning and experimenting, shortbut bitsyou ofcan't code,easily andsave testingor edit your work. For longerbigger ideasprojects, itwe'sll easierwrite toour store thePython code in a file. Let's get some practice with this!
 
<ol>
<li>Download the file http://mit.edu/jesstess/www/BostonPythonWorkshop7BostonPythonWorkshop8/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 commandterminal 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_command_prompt_practice_navigating_the_computer_from_a_terminal|navigating from a command promptterminal]] for a refresher on those commands.</li>
<li>Once you are in your homeDesktop directory, execute the contents of <code>nobel.py</code> by typing
 
<pre>
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</pre>
 
at athe commandterminal prompt.
 
<code>nobel.py</code> introduces two new concepts: comments and multiline strings.</li>
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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 />
<code> ...</code>
&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>
▲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>Press Enter one more time to tell Python you are done with this code block. The code block will now execute.</li>
 
▲</preol>
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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