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# update(+Key, +Struct, +Old, -New)

Incorporate Struct into a duplicate-free sorted list

Key
  Key argument position (non-negative integer, or list of those)
Struct
  Structure
Old
  Strictly ordered list of structures
New
  Strictly ordered list of structures, or variable

## Description

Inserts the compound term Struct into the ordered list Old, giving New. The lists are both in strictly ascending order (according to their elements' Key'th argument), i.e. without duplicates. If Old already contains an element with the same key as Struct, then Struct replaces this existing element. Otherwise, Struct is inserted at the correct position to maintain the list order.

The list New is either of the same length as Old, or one longer.

### Modes and Determinism

- update(+, +, +, -) is det

### Exceptions

(4) instantiation fault
  Arguments are insufficiently instantiated
(5) type error
  Some argument or its components are of the wrong type
(6) out of range
  Some structure does not have a Key'th argument

## Examples

        ?- update(1, f(2,b), [f(1,a),f(3,c)], New).
        New = [f(1,a), f(2,b), f(3,c)]
        Yes (0.00s cpu)

        ?- update(1, f(2,new), [f(1,a),f(2,b),f(3,c)], New).
        New = [f(1,a), f(2,new), f(3,c)]
        Yes (0.00s cpu)

## See Also

merge / 5, sort / 4