Pen & Paper Analogy

Many new programmers have difficulty learning to think it terms of variables, but this is a fundamental skill for writing code. It may be helpful to think about variables in terms of writing information down by hand on a piece of paper or a whiteboard. Everything we can do with a variable can be represented this way, which lets you simulate small amounts of code by hand or think through the process in your head to better understand what's going on.

Declare and Initialize

If we declare a variable in a program (int x), this is equivalent to writing the label x on our paper. When we initialize that variable in code (x = 5), we would write the initial value on the paper beneath or next to x.

Example:

int x;
x = 5;
int y = 10;

x -> 5

y -> 10

Reassign

Reassigning the variable in code (x = 10) is equivalent to crossing out the old value of x and replacing it with the new value.

x = 11;
y = -4;
x = 3;

x -> 5 11 3

y -> 10 -4

Print

Printing the variable (println(x)) means we're communicating it to someone else. We could write it down on a separate piece of paper, or we could read the text out loud to the other person.

println(x);
println(y);

Variable record

x -> 5 11 3

y -> 10 -4

Output paper

3

-4

Use in a Calculation

If we use our variable in a calculation, we would write the expression with our variable, then substitute the variable's value as we simplify the expression one step at a time.

x = 10;
y = 5;
x = x + y;
y = x * 2 - 4;
println("x = " + x + " and y = " + y);

Variable record

x -> 5 11 3 10 15

y -> 10 -4 5 26

Scratch paper for calculations

Calculate x + y

x + y

10 + y

10 + 5

15

set x equal to 15

Calculate x * 2 - 4

x * 2 - 4

15 * 2 - 4

30 - 4

26

set y equal to 26

Calculate "x = " + x + " and y = " + y

"x = " + x + " and y = " + y

"x = " + 15 + " and y = " + y

"x = 15" + " and y = " + y

"x = 15 and y = " + y

"x = 15 and y = " + 26

"x = 15 and y = 26"

write "x = 15 and y = 26" to the output paper