Understand and use the three basic programming constructs (sequence, selection and iteration), definite and indefinite iteration, nested constructs, and the meaning of constants and variables.

What this dot point is asking

AQA wants you to describe and use the three basic programming constructs (sequence, selection and iteration), to distinguish definite from indefinite iteration, to nest one construct inside another, and to explain the difference between a constant and a variable.

The three basic constructs

• Sequence: read a number, double it, then print it. Order matters: changing it changes the result.
• Selection: IF mark >= 50 THEN print "Pass" ELSE print "Fail". A condition decides which branch runs. A CASE statement is a tidy way to select among many values of one variable.
• Iteration: repeating a block, for example printing the numbers 1 to 10.

These three are the building blocks from which every more complex structure is assembled, which is why they are introduced first. The conditions used in selection and iteration are Boolean expressions built from the relational and logical operators, linking this topic directly to arithmetic and logical operations.

Definite and indefinite iteration

FOR i = 1 TO 10        # definite: runs exactly 10 times
  OUTPUT i
ENDFOR

WHILE guess != answer  # indefinite: runs until the user is correct
  guess = USERINPUT
ENDWHILE

A WHILE loop is condition-controlled and tests before the body, so it may run zero times if the condition is false at the start. A REPEAT UNTIL loop tests after the body, so it always runs at least once. Choosing the right one matters: validating input that might already be acceptable suits a WHILE, while a menu that should always display once suits a REPEAT UNTIL. Every iteration must make progress towards ending, or the loop never terminates.

Nesting and variables versus constants

Constructs can be nested: a selection inside a loop, or a loop inside a loop. A nested loop is the standard way to process a two-dimensional array, with the outer loop stepping through the rows and the inner loop through the columns of each row, so the inner body runs once per cell.

A variable is a named store whose value can change during execution. A constant is a named value fixed once and never changed, for example VAT = 0.20. Constants make code clearer and safer because the value cannot be accidentally altered, and changing it in one place updates the whole program. Using a named constant rather than a bare literal also documents intent: VAT is more readable than the number 0.20 scattered through the code.