What Is a Variable?

A variable is a named storage location that holds a value. Think of it as a labeled box: the label is the variable name, and whatever is inside the box is the current value. You can check what's in the box, replace it with something new, or use the value to compute something.

In Python, you create a variable and give it a value using the assignment operator — a single equals sign (=):

score = 0
name  = "Alice"
level = 5
print(score)

score <- 0
name  <- "Alice"
level <- 5
DISPLAY(score)

x = 10
y = 3
print(x)

Updating Variables

Once a variable has a value, you can change it. Each new assignment completely replaces the old value. The previous value is gone — only the new one is stored.

score = 0
score = 10
score = 25
print(score)  # 25

score <- 0
score <- 10
score <- 25
DISPLAY(score)

Updating using the current value

One of the most important patterns — and most tested on the AP exam — is updating a variable based on its current value:

a = 3
a = a + 1  # 3+1=4
print(a)     # 4

a <- 3
a <- a + 1
DISPLAY(a)

x = 5
x = x * 2
x = x + 3
print(x)

How to Trace Code

Tracing is the skill of manually following code line by line and tracking what each variable holds at each step. It is the core skill tested in Big Idea 3 MCQs. Here is the method:

1. Draw a table — one column per variable
2. Process each line in order, top to bottom
3. For each assignment: evaluate the right side using current values, then update the variable
4. Fill in the table after each change

x = 4   # x=4
y = 7   # y=7
x = x + y  # x=11
y = x * 2  # y=22
print(y)    # 22

x <- 4
y <- 7
x <- x + y
y <- x * 2
DISPLAY(y)

a = 6
b = 4
a = a - b
result = a * b

Swapping Two Variables

A classic exam problem: swap the values of two variables. The naive approach destroys one value — you need a temporary variable to hold one value while you overwrite it:

# WRONG: loses original x
x = 10  # x=10
y = 20  # y=20
x = y      # x=20, 10 lost
y = x      # y=20 (wrong!)

# CORRECT: use temp
x    = 10
y    = 20
temp = x
x    = y
y    = temp
print(x, y)  # 20 10

// CORRECT swap:
x    <- 10
y    <- 20
temp <- x
x    <- y
y    <- temp
DISPLAY(x)
DISPLAY(y)

# a = 100, b = 200 before this runs:
a = b
temp = a
b = temp

Key Vocabulary