Programming Constructs — IF, CASE, and loop structures

By the end of this lesson

By the end you can write pseudocode for all selection and iteration constructs: IF/ELSE (including nested), CASE, FOR loops (count-controlled), WHILE loops (pre-condition), and REPEAT...UNTIL loops (post-condition). You can justify which loop type fits a problem.

Selection — IF statements

Selection means choosing what to do, based on a condition.

A condition is a question that is either true or false, such as Mark >= 50.

Simple IF runs some statements only when the condition is true:

IF condition THEN
  statement(s)
ENDIF

IF...ELSE picks one of two routes. The ELSE part runs when the condition is false:

IF condition THEN
  statement(s)
ELSE
  statement(s)
ENDIF

Three or more routes need a nested IF: an IF written inside another IF.

Cambridge pseudocode has no separate keyword for a second test. Instead, put a new IF inside the ELSE part, and give that inner IF its own ENDIF:

IF condition1 THEN
  statement(s)
ELSE
  IF condition2 THEN
    statement(s)
  ELSE
    statement(s)
  ENDIF
ENDIF

Count the words: two IFs, so two ENDIFs. Every IF needs its own ENDIF.

Nesting works in the THEN part too:

IF Score >= 50 THEN
  IF Score >= 75 THEN
    OUTPUT "Distinction"
  ELSE
    OUTPUT "Pass"
  ENDIF
ELSE
  OUTPUT "Fail"
ENDIF

Conditions use these comparison operators:

= equal to
<> not equal to
<, >, <=, >= less than, greater than, less than or equal to, greater than or equal to

Conditions can also use the logic operators AND, OR and NOT.

Worked example: Nested IF — grade calculator

Input a mark (0-100). Output grade: A (80+), B (70-79), C (60-69), D (50-59), F (below 50).

Have a go first?

Try it — finish the grade calculator

This grade program has a bug. It gives an F to every mark under 70, because the C and D tests are missing.

Press Run first. The default input is 65, so it prints F. A mark of 65 should get a C.
Replace the comment with the missing tests. Use nested IFs, as in the worked example, so each ELSE only runs when the test above it failed.
Run it again. Then press Check my code.

Keep the test for 70 and the OUTPUT Grade line as they are.

DECLARE Mark : INTEGER
DECLARE Grade : CHAR
OUTPUT "Enter a mark (0-100)"
INPUT Mark
IF Mark >= 80 THEN
  Grade ← 'A'
ELSE
  IF Mark >= 70 THEN
    Grade ← 'B'
  ELSE
    // write the C, D and F tests here
    Grade ← 'F'
  ENDIF
ENDIF
OUTPUT Grade

Open this lesson on a computer to edit and run this code and check your answer.

Selection — CASE structure

CASE checks one variable against a list of fixed values. It is neater than a long run of nested IFs when every test compares the same variable with a value.

CASE OF Variable
  value1 : statement(s)
  value2 : statement(s)
  OTHERWISE : statement(s)
ENDCASE

The variable is looked at once.
The branches are checked from the top down. The first branch that matches runs, and the rest are skipped.
OTHERWISE runs when no branch matches. It is optional, but good practice to include.
There is no fall-through: a branch never runs on into the next one.

A branch can also cover a range of values with TO. The range includes both ends:

CASE OF Mark
  80 TO 100 : Grade ← 'A'
  70 TO 79 : Grade ← 'B'
  OTHERWISE : Grade ← 'F'
ENDCASE

A mark of 85 matches the first branch, so it gets A. A mark of 75 skips the first branch and matches the second.

Worked example: CASE — menu selection

Input a menu choice (1-4). Output the action: 1 = Add, 2 = Delete, 3 = Search, 4 = Exit. Any other number gives Invalid.

Have a go first?

Quick check — IF or CASE

In your own wordsQuestion 1 · Apply

Answer the question above to continue.