Processing lists in Prolog - 2 - University of Birmingham

06-25433 ? Logic Programming

Processing lists in Prolog - 2

This lecture shows that techniques introduced before (analysing terminating conditions and recursive programming) can be used to develop more complex procedures.

06-25433 ? Logic Programming

This lecture is about:

? writing procedures with one or more terminating or recursive clauses;

? deleting one or all instances of an element from a list;

? The effects of matching v. unification; ? changing the order in which solutions are

presented by changing clause order.

8 - Processing lists in Prolog: 2

1

06-25433 ? Logic Programming

Last time: Terminating at the end of the list

For instance counting all elements:

Terminates at the

% 1 - terminating

end of the list.

count_elem([], Total, Total).

% 2 - recursive

count_elem([Hd|Tail], Sum, Total) :-

Sum1 is Sum + 1,

count_elem([Hd|Tail], Sum1, Total).

8 - Processing lists in Prolog: 2

2

06-25433 ? Logic Programming

Last time: Terminating when given element is found

For instance finding a given element:

% 1 - terminating elem(Elem, [Elem|_]). % 2 - recursive elem(Elem, [_|Tail]) :-

elem(Elem, Tail).

Terminates before the end of the list.

Notice, this can be run "backwards" to enumerate the

individual elements of a list.

Demo1

8 - Processing lists in Prolog: 2

3

06-25433 ? Logic Programming

Last time: Terminating when given number of elements have been scanned

% 1 ? recursive nth(Count, Item, [_|Tail]) :-

Count > 1, Count0 is Count - 1, nth(Count0, Item, Tail).

% 2 ? terminating nth(1, Head, [Head|_]).

8 - Processing lists in Prolog: 2

The code counts down from the given position to 1.

Demo2

4

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