Stanford CS101: Code Variables Lecture 3

 

Code Variables

In this section, I want to add the idea of variables in code.

• A "variable" is like a box that holds a value
• x = 7;
• This stores the value 7 into the variable (i.e. box) x
• Later x in the code retrieve the value from the box
• x becomes a shorthand for 7 (whatever is in the box)
• Using = in this way is called "variable assignment"

Variables in CS101

• How we use variables in CS101:
• -Assign a value into a variable once
• -Then use that variable many times below
• A convenient shorthand in the code

x=7;

 print("lucky",x);

print("x is", x);

      

7
lucky 7
x is 7

 

• Experiments: try assigning (=) these values to x: 8, "hi"
• Store a value once, use it on several lines, saving repetition
• = in algebra is different, two things are equal forever
• = in code is simple, just puts a value in a box when run

Variables work as a shorthand -- we = assign a value into a variable, and then use that variable on later lines to retrieve that value. In the simplest case, this just works to avoid repeating a value: we store the value once, and then can use it many times. All computer languages have some form of variable like this -- storing and retrieving values.

Code Example - The Curse

Change the code below so it produces the following output. Use a variable to store the string "Alice" in a variable on the first line like x = "Alice";, then use the variable x on the later lines. In this way, changing just the first line to use the value "Bob" or "Zoe" or whatever changes the output of the whole program.

Alice Alice Alice
I had a crush on Alice

solution:

x = "Alice";
print(x, x, x);
print("I had a crush on", x);

Next: that's code with variables, now you try it.