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Community Data Science Workshops (Spring 2014)/Friday April 4th Tutorial: Difference between revisions
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{{CDSW Moved}}
Welcome to the Friday tutorial!
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This is an interactive tutorial! As you go through this tutorial, any time you see something that looks like this:
a = "Hello"
you should type the expression at a Python prompt, hitting Return after every line and noting the output.
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===Addition===
2 + 2
===Subtraction===
4 - 2
===Multiplication===
2 * 3
===Division===
4 / 2
Hey now! That last result is probably not what you expected. What's going on here is that integer divison produces an integer. You need a number that knows about the decimal point to get a decimal out of division:
1.0 / 2
This means you have to be careful when manipulating fractions. If you were doing some baking and needed to add 3/4 of a cup of flour and 1/4 of a cup of flour, we know in our heads that 3/4 + 1/4 = 1 cup. But try that at the Python prompt:
3/4 + 1/4
What do you need to do to get the right answer? Use data types that understand decimals for each of the divisions:
3.0/4 + 1.0/4
3.0/4.0 + 1.0/4.0
The two previous expressions produce the same result. You only need to make one of the numbers in each fraction have a decimal. When the Python interpreter goes to do the division, it notices that one of the numbers in the fraction cares about decimals and says "that means I have to make the other number care about decimals too".
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[[File:Geometry.png|150px]]
There's a helpful
type(1)
type(1.0)
So now we've seen two data types: '''integers''' and '''floats'''.
By the way, what is a "function"? Here are the important ideas about functions:
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* A function encapsulates a useful bit of work and gives that work a name.
* You provide input to a function and it produces output. For example, the <code>type</code> function takes data as an input, and produces what type of data the data is (e.g. an integer or a float) as output.
* To use a function, write the name of the function, followed by an open parenthesis, then what the function needs as input (we call that input the
* Programmers have a lot of slang around functions. They'll say that functions "take" arguments, or that they "give" or "pass" arguments to a function. "call" and "invoke" are both synonyms for using a function.
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===Command history===
Stop here and try hitting the Up arrow on your keyboard a few times. The Python
==Variables==
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A lot of work gets done in Python using variables. Variables are a lot like the variables in math class, except that in Python variables can be of any data type, not just numbers.
type(4)
x = 4
x
type(x)
2 * x
Giving a name to something, so that you can refer to it by that name, is called
Variables can't have spaces or other special characters, and they need to start with a letter. Here are some valid variable names:
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out:
4
But if you assign 4 to a variable, nothing is printed:
x = 4
You can think of it as that something needs to get the output. Without an assignment, the winner is the screen. With assignment, the output goes to the variable.
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You can reassign variables if you want:
x = 4
x
x = 5
x
Sometimes reassigning a variable is an accident and causes bugs in programs.
x = 3
x *
Order of operations works pretty much like how you learned in school. If you're unsure of an ordering, you can add parentheses like on a calculator:
(2 * x) - (1 * y)
Note that the spacing doesn't matter:
x = 4
and
x=4
are both valid Python and mean the same thing.
(2 * x) - (1 * y)
and
(2*x)-(1*y)
are also both valid and mean the same thing. You should strive to be consistent with whatever spacing you like or a job requires, since it makes reading the code easier.
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[[File:Letter.png|100px]]
So far we've seen two data types:
"Hello"
"Python, I'm your #1 fan!"
Like with the math data types above, we can use the <code>type</code> function to check the type of strings:
type("Hello")
type(
type("1")
===String Concatenation===
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You can smoosh strings together (called "concatenation") using the '+' sign:
"Hello" + "World"
name = "Jessica"
"Hello " + name
How about concatenating different data types?
"Hello" + 1
Hey now! The output from the previous example was really different and interesting; let's break down exactly what happened:
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<code>TypeError: cannot concatenate 'str' and 'int' objects</code>
Python is giving us a
There are many kinds of Python errors, with descriptive names to help us humans understand what went wrong. In this case we are getting a <code>TypeError</code>: we tried to do some operation on a data type that isn't supported for that data type.
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We saw above the we can concatenate strings:
"Hello" + "World"
works just fine.
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However,
"Hello" + 1
produces a <code>TypeError</code>. We are telling Python to concatenate a string and an integer, and that's not something Python understands how to do.
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We can convert an integer into a string ourselves, using the <code>str</code> function:
"Hello" + str(1)
Like the <code>type</code> function from before, the <code>str</code> function takes 1 argument. In the above example it took the integer 1. <code>str</code> takes a Python object as input and produces a string version of that input as output.
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There's another useful function that works on strings called <code>len</code>. <code>len</code> returns the length of a string as an integer:
len("Hello")
len("
fish = "humuhumunukunukuapuaʻa"
name_length = len(fish)
fish + " is a Hawaiian fish whose name is " + str(name_length) + " characters long."
===Quotes===
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We've been using double quotes around our strings, but you can use either double or single quotes:
'Hello'
Like with spacing above, use whichever quotes make the most sense for you, but be consistent.
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You do have to be careful about using quotes inside of strings:
'I'm a happy camper'
This gives us another
<code>m a happy camper'</code>
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We can use double quotes to avoid this problem:
"I'm a happy camper"
One fun thing about strings in Python is that you can multiply them:
"A" * 40
"
h = "Happy"
(h + b) * 10
==Part 1 Practice==
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1.
total = 1.5 - 1/2
total
type(total)
2.
a = "quick"
b = "brown"
c = "fox jumps over the lazy dog"
"The " + a * 3 + " " + b * 3 + " " + c
==End of Part 1==
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== Part 2: Printing==
So far we've been learning at the interactive
h = "Hello"
h + w
will display "HelloWorld".
Another place that we will be writing Python code is in a file. When we run Python code from a file instead of interactively, we don't get work printed to the screen for free. We have to tell Python to print the information to the screen. The way we do this is with the
h = "Hello"
print(h + w)
my_string = "Alpha " + "Beta " + "Gamma " + "Delta"
print(my_string)
The string manipulate is exactly the same as before. The only difference is that you need to use
<code>h + w</code>
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<ol>
<li>Download the file http://mako.cc/teaching/2014/
<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 Desktop 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
python nobel.py
at the terminal prompt.
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<li>How do you print a multi-line string so that whitespace is preserved?</li>
</ol>
==Making choices==
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<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>>>></code>. This is Python telling us that we are in the middle of a
<li>Press the spacebar 4 times to indent.</li>
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</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
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>====
Use the
<pre>
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====compound conditionals: <code>and</code> and <code>or</code>====
You can check multiple expressions together using the
Try typing these out and see what you get:
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====even more choices: <code>elif</code>====
If you need to execute code conditional based on more than two cases, you can use the
<pre>
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If color had been "purple", that code wouldn't have printed anything.
====In summary: the structure of if/elif/else====
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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.
==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]]
So far, the code we've written has been <i>unconditional</i>: no choice is getting made, and the code is always run. Python has another data type called a '''boolean''' that is helpful for writing code that makes decisions. There are two booleans: <code>True</code> and <code>False</code>.
True
type(True)
False
type(False)
You can test if Python objects are equal or unequal. The result is a boolean:
0 == 0
0 == 1
Use <code>==</code> to test for equality. Recall that <code>=</code> is used for <i>assignment</i>.
This is an important idea and can be a source of bugs until you get used to it: '''= is assignment, == is comparison'''.
Use <code>!=</code> to test for inequality:
"a" != "a"
"a" != "A"
<code><</code>, <code><=</code>, <code>></code>, and <code>>=</code> have the same meaning as in math class. The result of these tests is a boolean:
1 > 0
2 >= 3
-1 < 0
.5 <= 1
You can check for containment with the <code>in</code> keyword, which also results in a boolean:
"H" in "Hello"
"X" in "Hello"
Or check for a lack of containment with <code>not in</code>:
"a" not in "abcde"
"Perl" not in "Python Workshop"
==End of Part 2==
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[[File:Champagne.png|100px]][[File:Party.png|125px]]
Take a break, stretch, meet some neighbors, and ask the staff if you have any questions about this material.
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