Algorithms — control structures and documenting algorithms

Any algorithm, however big, can be built from just three constructs.

Enrichment (not examinable by name): this idea is called the structured programming theorem.

1. Sequence: steps run in order, from top to bottom.
2. Selection: choose between paths based on a condition (IF, CASE).
3. Iteration: repeat steps while a condition holds, or a set number of times (FOR, WHILE, REPEAT).

Here is a sequence in pseudocode. The arrow ← means "becomes". Total ← Price * 2 works out Price times 2 and stores the result in a variable called Total:

OUTPUT "Enter the price"
INPUT Price
Total ← Price * 2
OUTPUT Total

The lines run once each, from top to bottom, in the order they are written.

Each construct matches a shape in a flowchart. Nothing else, such as a jump to another line, is needed.

A flowchart draws an algorithm as boxes joined by arrows. Each shape has one job:

• Oval: the start or the end.
• Parallelogram: an INPUT or an OUTPUT.
• Rectangle: a process step, such as a calculation.
• Diamond: a decision. It holds the question for selection, or the test for a loop. A loop has an arrow going back up to an earlier box.

Selection asks a question and then picks one path. Here is the layout:

IF condition THEN
    // statements
ELSE
    // statements
ENDIF

• ELSE is optional. Leave it out if there is nothing to do when the condition is FALSE.
• Every IF needs its own ENDIF.
• Cambridge pseudocode has no separate keyword for a second test after ELSE. Instead, put a new IF inside the ELSE, with its own ENDIF.

CASE is for one variable compared with several fixed values:

CASE OF Choice
    1 : statements
    2 : statements
    OTHERWISE : statements
ENDCASE

• OTHERWISE runs when none of the values match. It is optional.

CASE can also test a range of values, written low TO high. This version uses the same grade boundaries as the IF example below:

CASE OF Mark
    70 TO 100 : Grade ← "A"
    60 TO 69 : Grade ← "B"
    50 TO 59 : Grade ← "C"
    OTHERWISE : Grade ← "U"
ENDCASE

• The clauses are tested in order, from top to bottom.
• The first clause that matches runs, and the rest are skipped.