Programming Concepts

Logical operators combine Boolean values and return a Boolean. The three you must know are AND, OR and NOT.

Truth tables

The three operators are fully defined by their truth tables (these come up again in topics 26 and 27 on Boolean logic).

AND

OR

NOT

Combining comparisons and logical operators

A single comparison only tests one thing. To check more than one fact at once, link comparisons with AND, OR or NOT. Each part on either side of the operator must be its own complete comparison.

AND requires both sides to be true. A common pattern is a range check built from two comparisons:

IF (Mark >= 0) AND (Mark <= 100) THEN
   OUTPUT "Valid mark"
ENDIF

OR requires at least one side to be true. Useful when several different values should be treated the same way:

IF (Day = "Saturday") OR (Day = "Sunday") THEN
   OUTPUT "Weekend"
ENDIF

NOT flips the result of a single comparison, so it is true when the underlying condition is false:

IF NOT (Account = "locked") THEN
   OUTPUT "Login allowed"
ENDIF

Always bracket each comparison. The brackets are not strictly required by precedence, but they make the intent obvious and stop slips like writing Mark >= 0 AND <= 100, which is not valid pseudocode.

Operator precedence

When multiple operators appear in one expression:

1. Brackets first.
2. Then arithmetic (*, /, DIV, MOD before +, -).
3. Then comparisons (<, >, =, <>, <=, >=).
4. Then NOT.
5. Then AND.
6. Then OR.

When in doubt, use brackets generously. (A AND B) OR C is much clearer than relying on the reader to remember precedence.