Cambridge pseudocode

How to Declare a Constant in Pseudocode

CONSTANT binds a name to a value for the rest of the program. CONSTANT Passing <- 50. Every later line may read Passing. No line may assign to it, including a line inside a procedure.

How it works

The name is locked after the first binding

DECLARE creates a variable: a slot you can overwrite. CONSTANT creates a name for a value the task has promised will not change, a pass mark, a rate, a size. The lock is checked when the program runs. Passing <- 40 later is an error, not a quiet second value.

CONSTANT Passing <- 50
DECLARE Mark : INTEGER
Mark <- 64
IF Mark >= Passing THEN
    OUTPUT "Pass"
ENDIF

Try it

Output

Pass

In Python

Capitals are a promise, not a lock

Python will not stop you. PASSING = 50 and then PASSING = 40 is a legal program. The capital letters are a convention other programmers recognise, which is weaker than a language rule. A tuple or a frozen object can hold a value you do not mean to edit, but that is a different tool.

Python

PASSING = 50
mark = 64
if mark >= PASSING:
    print("Pass")

PASSING = 40  # allowed. Python has no CONSTANT.

Beyond the exam

What the lock is for

A constant gives the reader a name instead of a bare 50, and it gives the language permission to assume the value never moves. In a compiled language that assumption can become a literal baked into the instructions. In an exam it is simpler than that: if the pass mark changes, it changes on one line, and a mistaken assignment is a mistake the interpreter will say out loud.

Run the example in the pseudocode compiler online, or read the matching section of the Cambridge pseudocode guide.