Selection: CASE

CASE OF picks one branch

• A CASE runs the branch whose value matches the variable.
• Branches are tested in order. The first match runs, and the rest are skipped.
• OTHERWISE is the branch for every other value. It comes last.

DECLARE Move : CHAR
Move ← 'W'
CASE OF Move
  'W' : OUTPUT "west"
  'E' : OUTPUT "east"
  OTHERWISE : OUTPUT "other"
ENDCASE

A range of values, and longer branches

• In 9618, one clause can cover a range: <value1> TO <value2> :. Both ends count.
• A clause can run several statements. They run until the next value, OTHERWISE or ENDCASE.
• The order still matters. If two ranges overlap, only the first one that matches runs.

DECLARE Mark, Passes : INTEGER
Mark ← 64
Passes ← 0
CASE OF Mark
  70 TO 100 : OUTPUT "Distinction"
              Passes ← Passes + 1
  50 TO 69 : OUTPUT "Pass"
             Passes ← Passes + 1
  OTHERWISE : OUTPUT "Not yet"
ENDCASE
OUTPUT "Passes: ", Passes

The output is Pass, then Passes: 1. A mark of 70 gives Distinction, because 70 is an end of the first range.

Watch out

• A character value uses single quotes: 'W'.
• Many short branches read well as a CASE. Conditions such as Mark > 50 AND Absent = FALSE need an IF.
• IGCSE 0478 pseudocode has no ranges and keeps one statement per branch. In 9618 both are allowed.

Common mistakes

• Close the block with ENDCASE.
• Put a colon after the value, and after OTHERWISE.
• Write a range low to high: 50 TO 69, not 69 TO 50. A backwards range matches nothing.
• Leave no gaps between ranges. With 70 TO 100 and 50 TO 68, a mark of 69 falls to OTHERWISE.

Now you try

• Read a character and choose a word with CASE OF.

Independent transfer

The final task has no supplied algorithm. Plan its inputs, processing and outputs, then solve it before opening support. Check more than the sample case.