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Boston Python Workshop 7/Friday/Tutorial: Difference between revisions
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<pre>
print "I'm a happy camper"
</pre>▼
<pre>▼
</pre>
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c = "fox jumps over the lazy dog"
print "The " + a * 3 + " " + b * 3 + " " + c
</pre>▼
<pre>▼
</pre>
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</pre>
That was our first multi-line piece of code, and the way to enter it at a Python prompt is a little different. First, type the
<code> if 6 > 5:</code>
Enter 4 spaces, and then type <code>print "Six is greater than five!"</code>. Hit enter to end the line, and hit enter again to tell Python you are done with this code block. All together, it will look like this:▼
part, and press Enter. The next line will have
<code> ...</code>
as a prompt, instead of the usual <code>>>></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
<pre>
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</pre>
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:
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====more choices: <code>if</code> and <code>else</code>====
You can use the <code>else</code> keyword to execute code
<pre>
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<b>Remember that '=' is for assignment and '==' is for comparison.</b>
====In summary: the structure of if/elif/else====
Here's a diagram of <code>if/elif/else</code>:
[[File:If-elif-else.png]]
Do you understand the difference between <code>elif</code> and <code>else</code>? When do you indent? When do you use a colon? If you're not sure, talk about it with a neighbor or staff member.
==Writing Functions==
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<b>Step 1: write a function signature</b>
A <b>function signature</b> tells you how the function will be called. It starts with the keyword <code>def</code>, which tells Python that you are defining a function. Then comes a space, the name of your function, an open parenthesis, the comma-separated input <b>
<code>def myFunction():</code>
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<code>def myFunction(myList, myInteger):</code>
<b>Step 2: do useful work inside the function</b>
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Underneath the function signature you do your useful work. Everything inside the function is indented, just like with if/else blocks, so Python knows that it is a part of the function.
You can use the variables passed into the function as
<pre>
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Functions don't have to return anything, if you don't want them to. They usually return something because we usually want to be able to assign variables to their output.
====What is the difference between <code>print</code> and <code>return</code>?====
Think for a moment about the differences between <code>print</code> and <code>return</code>:
* <code>print</code> prints output to the screen so your eyes can see it.
* <code>return</code> is used to hand off a value from inside a function to a variable outside the function.
For example:
▲<pre>
def add(x, y):
print x + y
▲</pre>
will print <code>x + y</code> to the screen so your eyes can see it.
▲<pre>
def add(x, y):
return x + y
▲</pre>
will hand off <code>x + y</code> from inside the function to outside the function. This allows you to do something like:
<pre>
result = add(5, 6)
print result
</pre>
Does that make sense? If not, talk about it with a neighbor or staff member.
==End of Part 2==
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