2.2.1 - Sequence, selection and iteration

Sequence changes program state

Sequence executes instructions in their stated order, so an earlier assignment can change what a later instruction reads. A variable is a named place for a value; the collection of current values is the program's state. Assignment replaces a value. Equality tests whether two values are equal.

In OCR pseudocode, = assigns and == compares. A variable is declared on first assignment and takes the type of its value. Python also uses = and ==, but its names are bound to objects. These distinctions matter when translating an algorithm rather than copying its punctuation.

OCR prints a consistent pseudocode style so that you can read its algorithms. You do not need to memorise the exact syntax: your own pseudocode answers may use any style whose meaning a competent programmer can reasonably infer. Clear conditions, bounds and indentation communicate the logic.

boxes = 4
boxes = boxes + 2
cost = boxes * 3
print(str(cost))

The right side of boxes = boxes + 2 uses the old value 4, then assigns 6. cost becomes 18. Swapping the second and third instructions would calculate 12 instead: the same statements in a different sequence have different effects.

Input commonly arrives as text. int("12") converts suitable integer text to 12, float("2.5") converts to 2.5, and str(12) produces text for concatenation. A cast is not validation: Python int("twelve") fails rather than guessing a number. Specify the allowed input or check it before conversion.

minutes = int(input("Minutes: "))
hours = minutes // 60
remainder = minutes % 60
print(str(hours) + "h " + str(remainder) + "m")

For input 135, the quotient is 2 and remainder is 15. OCR DIV maps to Python // for these non-negative integer examples; MOD maps to %. Both use / for division, but Python / produces a floating-point result. OCR exponentiation ^ maps to Python **; Python ^ is a different, bitwise operation. Parentheses make intended order explicit. The examples here use non-negative integers, avoiding differing conventions for negative remainders.

Selection chooses a branch

Selection, also called branching, chooses instructions according to a Boolean condition. A comparison such as temperature < 0 has a true or false result; AND, OR and NOT combine or reverse conditions. Python spells these and, or and not.

if temperature < 0 then
    print("Ice risk")
elseif temperature <= 5 then
    print("Cold")
else
    print("Mild")
endif

Only the first true branch in this chain executes. At 0 the first test is false and the second true, giving Cold; at 5 it is still Cold; at 6 both tests are false, giving Mild. The order deliberately tests the narrower category first. Two separate if statements could both execute, unlike this mutually exclusive chain.

A range check age >= 12 AND age <= 17 requires both bounds. Using OR would accept every age: a value below 12 is still at most 17, while a value above 17 is still at least 12. Boundary tests expose that mistake.

OCR also uses switch/case to compare one expression against alternatives:

switch service:
    case "S":
        charge = 2
    case "E":
        charge = 5
    default:
        charge = -1
endswitch

The matching case executes; the default handles an unmatched service. This is suitable for discrete choices, whereas an if/elseif chain naturally handles ranges. A faithful Python translation here is if service == "S":, elif service == "E":, and else:, with the corresponding assignments indented underneath. Do not import a different language's case fall-through rules into this OCR example.

Iteration depends on bounds and timing

Iteration repeats a body of instructions. A count-controlled loop uses a known sequence of values; a condition-controlled loop repeats according to a test. Correctness depends on which values are included and when the test occurs.

total = 0
for i = 2 to 4
    total = total + i
next i

OCR's upper bound is inclusive: the body uses 2, 3 and 4. The totals after the three iterations are 2, 5 and 9. Python range(2, 5) gives these values because its stop value is excluded; range(2, 4) would omit 4.

A pre-test loop such as while tests before executing its body and can execute zero times. With count = 0, while count < 3, and count = count + 1 in the body, the values after the body are 1, 2, 3. The final failed test prevents a fourth iteration. Forgetting the update would leave the condition true forever.

OCR do ... until is a post-test loop: the body executes before the condition is tested. It executes at least once and stops when the until condition becomes true.

do
    choice = input("Enter S or E: ")
until choice == "S" OR choice == "E"

For entries X, E, two inputs are read. Python has no directly equivalent do/until statement; one explicit translation is:

while True:
    choice = input("Enter S or E: ")
    if choice == "S" or choice == "E":
        break

break exits this loop on the stopping condition. A pre-test alternative reads once before a while choice != "S" and choice != "E": loop, then reads again inside it. Simply writing while choice == "S" or choice == "E" reverses the intended continuation condition and may use a value before input has assigned it.

The sandbox below explores zero and one pass with a different program. Make each prediction before running; its final count and the loop-variable lifetime reveal different boundary behaviours.