Lecture-04-Prolog-Backtracking control. Heterogeneous lists.pdf

🖋 Notes

Lists

• a list is an object that contains an arbitrary number of other objects
• lists in SWI-Prolog are heterogeneous (the component elements can have different types)
• lists are built using square brackets
• their elements are separated by a comma

[1, 2, 3]
[dog, cat, canary]
[“valerie ann”,“jennifer caitlin”,“benjamin thomas”]

• if we were to declare the type of a list (homogeneous) with integer elements, we would use a domain declaration of the following type

% element = integer
% list = element*

Head & Tail of a list

• a list is a recursive object
• consists of two parts:
  • the head:
    • the first element of the list
    • is an element
  • the tail
    • the rest of the list
    • is a list

The head of the list [a, b, c] is a 
The tail of [a, b, c] is [b, c]

The head of the list [c] is c 
The tail of [c] is []

The empty list [] cannot be split into head and tail.

List processing

• instead of separating the elements of a comma list, we will separate the head of the queue with the character '|’

[a bc]
[a|[b, c]]
[a|[b|[c]]]
[a|[b|[c|[]]]]

• ,before the '|' sign several elements may be written, not just the first

[a|[b,c]]
[a,b|[c]]
[a,b,c|[]]