Community Data Science Workshops (Spring 2014)/Friday April 4th Tutorial: Difference between revisions

This is an interactive tutorial! As you go through this tutorial, any time you see something that looks like this:

 

 

you should type the expression at a Python prompt, hitting Return after every line and noting the output.

===Addition===

 

 

===Subtraction===

 

 

===Multiplication===

 

 

===Division===

 

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:

 

 

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:

 

 

What do you need to do to get the right answer? Use data types that understand decimals for each of the divisions:

 

 

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".

There's a helpful <b>function</b> (more on what a function is in a second) called <code>type</code> that tells you what kind of thing -- what <b>data type</b> -- Python thinks something is. We can check for ourselves that Python considers '1' and '1.0' to be different data types:

 

So now we've seen two data types: <b>integers</b> and <b>floats</b>.

 

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.

 

 

Giving a name to something, so that you can refer to it by that name, is called <b>assignment</b>. Above, we assigned the name 'x' to 4, and after that we can use <code>x</code> wherever we want to use the number 4.

out:

 

 

But if you assign 4 to a variable, nothing is printed:

 

 

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.

You can reassign variables if you want:

 

 

Sometimes reassigning a variable is an accident and causes bugs in programs.

 

 

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:

 

 

Note that the spacing doesn't matter:

 

 

and

 

 

are both valid Python and mean the same thing.

 

 

and

 

 

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.

So far we've seen two data types: <b>integers</b> and <b>floats</b>. Another useful data type is a <b>string</b>, which is just what Python calls a bunch of characters (like numbers, letters, whitespace, and punctuation) put together. Strings are indicated by being surrounded by quotes:

 

 

Like with the math data types above, we can use the <code>type</code> function to check the type of strings:

 

 

===String Concatenation===

You can smoosh strings together (called "concatenation") using the '+' sign:

 

 

 

How about concatenating different data types?

 

 

Hey now! The output from the previous example was really different and interesting; let's break down exactly what happened:

We saw above the we can concatenate strings:

 

 

works just fine.

However,

 

 

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.

We can convert an integer into a string ourselves, using the <code>str</code> function:

 

 

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.

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:

 

 

===Quotes===

We've been using double quotes around our strings, but you can use either double or single quotes:

 

 

Like with spacing above, use whichever quotes make the most sense for you, but be consistent.

You do have to be careful about using quotes inside of strings:

 

 

This gives us another <b>traceback</b>, for a new kind of error, a <code>SyntaxError</code>. When Python looks at that expression, it sees the string 'I' and then

We can use double quotes to avoid this problem:

 

 

One fun thing about strings in Python is that you can multiply them:

 

 

==Part 1 Practice==

 

1.

 

2.

 

==End of Part 1==

So far we've been learning at the interactive <b>Python interpreter</b>. When you are working at the interpreter, any work that you do gets printed to the screen. For example:

 

 

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 <b>print</b> function. Here's how it works:

 

 

 

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:

<li>Once you are in your Desktop directory, execute the contents of <code>nobel.py</code> by typing

 

 

at the terminal prompt.

<li>How do you print a multi-line string so that whitespace is preserved?</li>

</ol>

 

==Making choices==

 

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.

 

==End of Part 2==

 

[[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.