11.3.108 is/2   ISO

Synopsis

-Term is +Expression

Evaluates Expression as an arithmetic expression, and unifies the resulting number with Term.

Arguments

Expression

expr, must be ground.

An expression made up of:

• functors representing arithmetic operations
• numbers
• variables bound to numbers or arithmetic expressions

Term

number

Exceptions

Arithmetic errors (see ref-ari-exc).

Examples

| ?- X is 2 * 3 + 4.

X = 10

| ?- Y = 32.1, X is Y * Y.

X = 1030.41
Y = 32.1

| ?- Arity is 3 * 8, X is 4 + Arity + (3 * Arity * Arity).

Arity = 24
X = 1756

| ?- X is 6/0.
! Domain error in argument 2 of is/2
! expected an integer not equal to 0, but found 0
! goal:  _98 is 6/0

| ?- X is "a".

X = 97

| ?- X is 4 * 5, Y is X * 4.

X = 20,
Y = 80

Comments

If a variable in an arithmetic expression is bound to another arithmetic expression (as opposed to a number) at runtime, then the cost of evaluating that expression is much greater. It is approximately equal to the cost of call/1 of an arithmetic goal.

See Also

ref-ari.