Making decisions

Making a choice

• if runs some code only when a condition is true.
• The condition is followed by a colon :.
• The lines that belong to the if are indented (4 spaces).

Comparisons

• A condition is usually a comparison that is True or False.
• == equal, != not equal, < > <= >=.
• Note: == tests equality; a single = stores a value.

temperature = 30
if temperature > 25:
    print("It is hot.")

if / elif / else

• elif ("else if") checks another condition if the first was false.
• else runs when none of the conditions were true.
• Python checks them in order and stops at the first true one.

hour = 14
if hour < 12:
    print("Morning")
elif hour < 18:
    print("Afternoon")
else:
    print("Evening")

The remainder

• a % b is what is left over when a is divided by b.
• year % 4 == 0 means the year divides by 4 exactly — nothing left over.
• 8 % 3 is 2, because 8 is two 3s with 2 left over.

Combine conditions

• 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 false to true.

day = "Sat"
if day == "Sat" or day == "Sun":
    print("Weekend!")

In Cambridge pseudocode

• Python if/elif/else matches the exam's IF … ELSE … ENDIF and CASE.

IF Score >= 50
  THEN
    OUTPUT "Pass"
  ELSE
    OUTPUT "Fail"
ENDIF

CASE OF Grade
    "A": OUTPUT "Top"
    "B": OUTPUT "Good"
    OTHERWISE OUTPUT "Keep trying"
ENDCASE

Common mistakes

• Compare with ==; a single = assigns.
• Write elif, not else if, and end each if / elif / else line with :.
• The indented block below if is what runs when the condition is true.

Now you try

• When you Run, type the input in the box under the code and press Enter.
• The checks type the input for you. Read it, then decide what to print.
• Match the output text exactly.