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Constants, Scope & Value Protection

A variable stores a value that may change. A constant names a value that does not change while the program runs. Naming a fixed price or limit makes its purpose clear and avoids repeating unexplained numbers.

CONSTANT TicketPrice = 7
DECLARE Tickets : INTEGER
INPUT Tickets
OUTPUT TicketPrice * Tickets

Worked example

For three tickets, the expression is 7 × 3 = 21. The number of tickets is input and may differ next time; TicketPrice stays fixed in this program. Changing the price in one declaration is easier to maintain than finding many occurrences of 7.

Do not write INPUT TicketPrice: input belongs in a variable. Scope—where an identifier can be used—is taught with procedures and functions, after you have seen those constructs.

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Exercise:
Task 1 Task 2 Task 3

Application: Check the reasoning

+30 XP

Application: Check the reasoning

  1. Identifier whose value stays fixed during execution: [Space 1]
  2. Should changing user input be stored in a constant or variable? [Space 2]
  3. Does a named constant help explain an otherwise unexplained number: yes or no? [Space 3]
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