CS 2120: Topic 2¶

Videos for this week:¶

Welcome back! This week we’ll cover a bit of background theory and get into variables, operations, printing to the console, and a few other things.

Before we start..

Do you have Python running on your computer? If not, see “Getting set up for CS 2120”

You can think of it as a recipe for the computer to do something that you want it to do
More formal: A sequence of instructions that specifies exactly how to perform a computation

Remark

Is the following statement true or false? This statement is false.

Sanity Check

Write a Python program that prints a message to the console.

Values are things that a program manipulates.
- Strings: “abcdef”
- Integers: 7, 42, 97
- Floating-point numbers: 3.792, 0.00005
To a computer, the integer 1 is not necessarily the same thing as the floating point number 1.0… because they have different types
Python will guess at the type of your value if you don’t tell it.

Can I ask Python to tell me its guess for the type of a value?

>>> type(12)
<type 'int'>
>>> type('Toronto Blue Jays')
<type 'str'>
>>> type(3.75)
<type 'float'>

Variables let you store values in a labelled (named) location
You store values into variables by using the assignment operator =
>>> a=5 >>> m='I like variables'
The ‘=’ symbol here corresponds to variable assignment… it doesn’t really mean the same thing as the ‘=’ sign in math.
In math when we write ‘a = 5’ we mean that ‘5’ and ‘a’ are equivalent.
In Python when we write
>>> a=5
… we’re telling the Python interpreter to create a variable named a and store the value 5 in it.

For example, we can add variables:

>>> a = 5
>>> b = 7
>>> a+b
12
>>> b=5
>>> a+b
10

Some rules for Python..
- start the name with a letter of the alphabet or an underscore (“_”)
- a variable name can’t start with a number
- a variable name can only be made up of alphanumeric characters and underscores
- variable names are case-sensitive

Try this

Create two variables, named maple and leafs, and set them to 19 and 67, respectively. Try adding them. What happened?

A statement tells Python to “do something”.
- for example, printing to the screen (i.e. print("a"))
it is an instruction that can be executed by the interpreter.

Operators are symbols that tell Python to perform computations on expressions.
- +, -, \*, /

Python follows the usual order of operations (B-E-D-M-A-S).
You can use () to customize your expressions:
>>> 2 + 5 * 2 12 >>> (2 + 5) * 2 14

Try this

Try mixing strings and integers with various operators.

The engine which runs Python (the interpreter) can run in 2 ways:
- immediate mode: you type a line of code and the interpreter responds with an output (like the code samples on this page)
- script mode: you type all of the code out (i.e. in a PyCharm script), run it, the interpreter responds with all of its output in the console window