CS5Operators and Selection

You need addition, subtraction, multiplication, real division, and integer division with remainders.

• Integer division is a two-stage idea: 11 ÷ 2 gives 11 DIV 2 = 5 and 11 MOD 2 = 1, so 5 remainder 1.
• DIV drops the remainder; it doesn’t round. 19 DIV 4 is 4, not 5.
• Order: brackets first, then *, /, DIV, MOD, then + and -. Equal ones work left to right. Use brackets whenever you’re not sure.
• In Python, / always gives a real (float): 8 / 2 is 4.0.
• Never put a £ sign in a calculation: total = £5 * qty is an error.

print(3 + 4 * 2)
print((3 + 4) * 2)
print(20 / 8)
print(20 // 8)
print(20 % 8)

Real division

Division that can give an answer with a fractional part: 7 / 2 = 3.5.

Integer division (DIV, //)

Division that gives the whole-number part only: 7 DIV 2 = 3.

Remainder (MOD, %)

What is left over after integer division: 7 MOD 2 = 1.

DIV and MOD solve lots of small problems in exam programs.

• If n MOD k is 0, then n divides exactly by k.
• Odd numbers give n MOD 2 = 1.

totalMinutes = int(input("Minutes: "))
hours = totalMinutes // 60
minutes = totalMinutes % 60
print(f"{hours} h {minutes} min")

Relational operators compare two values. The result is always a Boolean: True or False.

• In Python, == compares and = assigns. In pseudo-code, = compares.
• Strings are compared character by character in alphabetical (character-code) order: 'adam' < 'adele' because d comes before e.
• ≤ and ≥ include the value itself: 4 <= 4 is True; 4 < 4 is False.
• In Python write >= and <= (the equals sign second).
• The pointed end of < or > points at the smaller value.

Relational operator

An operator that compares two values and gives True or False: =, ≠, <, >, ≤, ≥.

Boolean operators combine or reverse conditions in selection and iteration.

• Each comparison needs both sides. Write size == "S" or size == "M", never size == "S" or "M".
• Inside a range: mark >= 0 and mark <= 100. Outside a range: mark < 0 or mark > 100.
• Never write a chained comparison (two comparison operators in a row, as you might in maths). It is wrong or an error in other languages, and examiners discourage it.
• NOT is worked out before AND, and AND before OR. Use brackets to make the meaning clear: (a < 1) OR (a > 9).

age = 15
member = True
print(age >= 16 and member)
print(age >= 16 or member)
print(not member)

AND

True only when both conditions are True.

OR

True when at least one condition is True.

NOT

Reverses a condition: True becomes False and False becomes True.

A selection statement allows a block of code to be executed only when a certain condition is satisfied.

• Conditions are checked from the top. The first True one runs, and the rest are skipped. Exactly one branch of an IF … ELSE runs.
• Python uses a colon and indentation to show which lines are inside each branch; pseudo-code uses ENDIF. In the exam, use the vertical guide lines in the answer box to show indentation.
• elif is Python’s ELSE IF.

IF temp > 30 THEN
   OUTPUT 'Hot'
ENDIF

if temp > 30:
    print("Hot")

IF temp > 30 THEN
   OUTPUT 'Hot'
ELSE
   OUTPUT 'Not hot'
ENDIF

if temp > 30:
    print("Hot")
else:
    print("Not hot")

IF temp > 30 THEN
   OUTPUT 'Hot'
ELSE IF temp > 15 THEN
   OUTPUT 'Warm'
ELSE
   OUTPUT 'Cold'
ENDIF

if temp > 30:
    print("Hot")
elif temp > 15:
    print("Warm")
else:
    print("Cold")

IF … ELSE as a flowchart: exactly one of the two outputs happens

Selection

A selection statement allows a block of code to be executed only when a certain condition is satisfied.

Condition

An expression that is True or False, used to decide which branch runs.

When a selection statement appears inside another selection statement, it is nested.

• The inner IF is only checked when the outer condition is True.
• Nesting lets each situation have its own message. Some nested IFs can be joined into one condition with AND, but then you lose the separate messages.
• IF statements can be nested inside the THEN part, the ELSE part or both.

IF doorClosed THEN
   IF floor ≠ target THEN
      OUTPUT 'Moving'
   ELSE
      OUTPUT 'Arrived'
   ENDIF
ELSE
   OUTPUT 'Close the door'
ENDIF

doorClosed = True
floor = 2
target = 5
if doorClosed:
    if floor != target:
        print("Moving")
    else:
        print("Arrived")
else:
    print("Close the door")

Nested selection

A selection statement inside another selection statement.

Turn the words of a brief into the exact operator. The boundary value decides it.

• Removing NOT: flip the operator to its opposite. The opposite of < is ≥; the opposite of > is ≤; the opposite of = is ≠.
• So NOT (x < 10) is the same as x ≥ 10, not x > 10.
• Check any rewrite with the boundary value: both versions must give the same result for it.

When values fall into bands, the order of the conditions decides which band is chosen.

• With > or ≥, test the biggest threshold first. (With < or ≤, test the smallest first.)
• If a smaller threshold comes first, big values match it and the later branch can never run.
• Store the result in a variable in each branch and output it once after the IF. Then one output line covers every branch.
• Read every boundary in the brief: ‘6 or more’ needs >= 6.

Biggest threshold first: each value reaches only one output

speed = int(input("Speed in Mbps: "))
if speed >= 300:
    rating = "Ultrafast"
elif speed >= 30:
    rating = "Superfast"
else:
    rating = "Standard"
print(rating)

AQA lists IF THEN ELSE and SWITCH/CASE/MATCH as selection statements. Python 3.10 and later has match.

• match tests one value against several possibilities. case _ catches anything not matched above, like else.
• AQA’s marking guidance accepts match, but AQA’s own pseudo-code and Python always use IF … ELSE IF. Learn if/elif first; recognise match.

choice = input("Choose A, B or Q: ")
match choice:
    case "A":
        print("Add a record")
    case "B":
        print("Browse records")
    case "Q":
        print("Quit")
    case _:
        print("Invalid choice")

if choice == "A":
    print("Add a record")
elif choice == "B":
    print("Browse records")
elif choice == "Q":
    print("Quit")
else:
    print("Invalid choice")

match … case

Python 3.10+ selection that compares one value with several cases; case _ matches anything else.