Variables and expressions

Variables hold values

• A variable stores a value so you can use it later.
• In Python you write score = 10. The = means "store on the left".
• You can change a variable by storing a new value into it.

score = 10
score = score + 5
print(score)

Arithmetic operators

• The usual operators are +, -, *, /.
• // gives whole-number division (no remainder); ** is power.
• % gives the remainder left after dividing.

print(17 // 5)   # whole 5s that fit = 3
print(17 % 5)    # what is left over = 2
print(2 ** 3)    # 2 to the power 3 = 8

Relational operators

• These compare two values and give True or False.
• == is "equal", != is "not equal".
• <, >, <=, >= compare size.

print(5 == 5)    # True
print(5 != 3)    # True
print(7 < 2)     # False

Logical operators

• and is True only when both sides are True.
• or is True when at least one side is True.
• not flips True to False and back.

print(True and False)   # False
print(True or False)    # True
print(not True)         # False

In AP CSP pseudocode

• The AP exam uses its own pseudocode, not Python. The ideas are the same.
• Assignment uses a left arrow. MOD is the remainder. DISPLAY prints.

Python                AP CSP pseudocode
score = 10            score ← 10
17 % 5                17 MOD 5
print(score)          DISPLAY(score)
a == b                a = b
a and b               a AND b

Common mistakes

• Give a variable a value before you use it.
• input() returns text — convert with int(...) for numbers.

Now you try

• Type your code below and press Run to see the output.
• Press Check answer to test it.