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Variables

caution

This is not entirely complete, you may notice some TODOs I have left for myself.

I'm still trying to figure out how I want to get diagrams and multi-image diagrams in here. But getting assignments out takes priority over that.

Assignment Updates​

Stuff due this week:

  • Reading 1: Due Thursday 2/16
  • Quiz 2: Due Thursday 2/16
  • Lab 2: Due Friday 2/17

Stuff due next week:

  • HW 1: Due Wednesday 2/22
  • Quiz 3: Due Thursday 2/23
  • Lab 3: Due Friday 2/24
  • [TODO: Reading? Presumably Thursday.]

Assignment Workflow​

Caveat that this is but one approach. In the fullness of time, everyone develops their own preferred workflows and ways of doing things.

But for the purposes of getting started, here is one workflow.

Making a Folder​

A lot of IDEs (Integrated Development Environments, fancy work for editors) have a sort of built-in assumption that you are working on the project level, where the project may have multiple files.

So often, the IDE is working in/opening the project folder. And so we a need a folder on our computer to do work in.

Open your computer's file manage (e.g. Finder on Mac and File Explorer on Windows).

Navigate to where you want to make this new folder. For example: I might have/make a school folder, and within that a CICS110 folder, and within that a HW folder.

Secondary click (right click on mouse, two-finger click on most trackpads, etc) in the finder where you want to folder to be to bring up a menu. Select either New Folder or New > Folder depending on your operating system.

Give the new folder a name, 1-variables, for instance. (You can secondary click on the folder to to bring up a menu that should have a Rename option, if need be.)

Opening a Folder​

We need to open that folder in our editor to do stuff in it (you can also make files directly, but I'm not demoing that approach specifically).

Launch VSCode.

Select File > Open Folder. This notation means, click the File menu and then select Open Folder from the dropdown.

This menu is likely in the upper-left area of the application or you entire screen (depending on your operating system).

Making a File​

Select File > New File ....

VSCode will ask for a name for the file, give it a name ending in .py, e.g., hello.py. You need to specify that .py extension.

Running a File​

Finally, if you installed the extension packet from the VSCode installation guide, then there should be a play-button-like triangle in the upper-right of the python file.

Simply click that button and VSCode will open a terminal and run the file for you.

Extra: Running the File via Terminal

You can deliberately open a terminal in VSCode via the menu Terminal > New Terminal.

In the terminal, typing python3 followed by a file name will run that file, e.g., python3 hello.py.

Parts of a Program​

A program can be considered in terms of the information is is keeping track of, and what it is doing with or based on that information.

The information, or data, or objects, correspond with the memory part of out model of a computer running a program.

What it does with that, or the logic, corresponds to the instructions part of out model of a computer running a program.

This lecture (and last) focusses on the data/objects portion.

Proto Memory Diagrams​

[TODO: I really need to figure out diagrams. I just don't have tiiiiiiiiiiime. Maybe once spring break hits.]

Variables​

Variables are a way to store and track information.

Some people find the analogy of labeled boxes helpful, where objects are put into boxes labeled by the variable name, and can then be referenced by that variable name.

For instance:

Variables
x = 2
y = 1.5
z = x + y
print(z)

So what does each line here do?

  • x = 2 assigns the int 2 to the variable x.
  • y = 1.5 assigns the float 1.5 to the variable y.
  • z = x + y first evaluate x + y, which is 2 + 1.5, which is 3.5 (a float); and then stores that in z.
  • print(z) prints z, which 3.5.

Importantly, variables can be overwritten.

Take the following:

Variables_Overwrite
x = 2
y = 1.5
z = x + y
print(z)
x = -3
print(x)
print(z)

So what does each line here do?

  • The first 4 lines are the same as before.
    • x is the int 2.
    • x is the float 1.5.
    • z is the float 3.5, which is printed.
  • x is set to the int -3.
  • x, which is now -3, is printed (so -3 is printed).
  • z, which is still 3.5, is printed (so 3.5 is printed).

There are some important rules to remember about variable names:

  • Names must be composed of only letters, numbers, and underscores _.
  • Names may not begin with a number (must start with letter or _).
  • Names are case-sensitive. E.g., Foo and foo are different variables.

Examples of legal variables names:

  • Foo
  • foo
  • f00
  • _foo
  • foo_bar_42

Examples of illegal variables names:

  • 314_Foo
  • foo!

Good Names​

Just because a name is legal doesn't mean is is "good". You would ideally like variables names to articulate something about how they are used.

For example, say I have a program that takes in a number representing temperature in degrees Fahrenheit and converts it to degrees Celsius. I could reasonable name that input variable fahrenheit, Fahrenheit, degrees_F, or even F. If I named it foo, potatoes_per_cubic_yoctogram, of a, that would be profoundly unhelpful to anyone subsequently viewing that code, my future self included.

The goal is to try to have that name make sense for the context it is in.

Memory Diagrams​

This is a diagram, go look at the slides for now.

[TODO: Figure out how to import multi-image diagrams]

The input() Function​

The input() function gets input from the user (i.e. whoever is running the code, generally us in this case).

The input() function may optionally take in a string, which is used as text to display when prompting the user.

The output, or return, of the input() function is whatever the user typed in, as a string.

Take for example this program (that perhaps looks familiar):

Hello_You
your_name = input("Please enter your name: ")
print("Hello, " + your_name)

Go ahead and run it on the website. Then run it VSCode. Notice the different ways you can be prompted for input.

The website pops up a dialogue box, but the need to enter input into the terminal is less obvious:

$ python3 hello.py 
Please enter your name: Jared
Hello, Jared

I typed in that "Jared" and hit the return/enter key.

Now see what it looks like without specifying the prompt for the input function.

Hello_You_Quite
your_name = input()
print("Hello, " + your_name)

The website still pops up a dialogue box, but there is no indication (other than the terminal not prompting for another command) of the need to enter text into the terminal:

$ python3 hello.py 
Jared
Hello, Jared

Looking Back​

We haven't explored functions yet, but with just the idea of function as black boxes taking inputs and giving outputs, we now have all the pieces to understand this program:

your_name = input("Please enter your name: ")
print("Hello, " + your_name)

The first line is your_name = input("Please enter your name: "):

  • "Please enter your name: " is a string, we pass that the the function input().
  • input("Please enter your name: ") then uses that string as a prompt to solicit input from the user, and returns that input.
  • your_name = input("Please enter your name: ") captures that returned input, and stores it in the variable your_name.
  • So if I entered Jared as the input, then your_name would hold "Jared".

The second line is print("Hello, " + your_name):

  • Again, your_name contains the string "Jared" in this step-through example.
  • The expression "Hello, " + your_name evaluates to "Hello, Jared".
  • print("Hello, " + your_name) then prints Hello, Jared to the terminal.
caution

From here on is post-lecture content, or notes about the lecture but not part of the lecture itself.

Instructor's Outline

This is an outline I had in the replit when demonstrating things.

############################
### Instructor's Outline ###
############################

# Making a Folder
#   Open Finder/File Navigater
#   Navigate to desired location
#     Could just use desktop
#   Seconary Click -> New Folder

# Opening Folder
#   Launch VSCode (or equivalent)
#   Navigate to folder you made

# Makeing a File
#   File -> New File
#   Give it a name ending in .py

# Run Script
#   (Make sure extension is installed)
#   Click the Triangle in the upper-right

# Return to slides

# input() Function
#   Solicits input from user
#   Can take in nothing or a string
#     Optional string is printed when soliciting input
#   Returns whatever user typed in (as a string)
#   Example: Hello, You
#   Example: Repeat n times

# Return to slides