Chapter Three: Lists,
Operators, Arithmetic
CS 461
3.2 Some operations on lists
1. Concatenation
2. Adding an item
3. Deleting an item
Concatenation
conc(L1,L2,L3)
L1 and L2 are two lists and L3 is their concatenation.
conc([a,b],[c,d],[a,b,c,d]) True
conc([a,b],[c,d],[a,b,a,c,d]) False
Concatenation (cont.)
Definition of conc:
(1) If the first argument is empty, then the second and
third arguments must be the same list
conc([],L,L).
(2) If the first argument of conc is non-empty list,
then it has a head and tail and must look like this
[X|L1]
the result of concatenation of L1 and L2 is the list
[X|L3] where L3 is the concatenation of L1 and L2
conc([X|L1],L2,[X|L3]):- conc(L1,L2,L3).
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Concatenation (Example)
conc([],L,L).
conc([X|L1],L2,[X|L3]):- conc(L1,L2,L3).
?- conc([a,b,c],[1,2,3],L)
L= [a,b,c,1,2,3]
?-conc([a,[b,c],d],[a,[],b],L)
L = [a, [b, c], d, a, [], b].
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Concatenation (Example)
conc([],L,L).
conc([X|L1],L2,[X|L3]):- conc(L1,L2,L3).
?- conc(L1,L2,[a,b,c])
L1 = [],
L2 = [a, b, c] ;
L1 = [a],
L2 = [b, c] ;
L1 = [a, b],
L2 = [c] ;
L1 = [a, b, c],
L2 = [] ;
(decompose)
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Concatenation (Example)
How can we find the months that precedes and
follows a given month in a list ?
conc([],L,L).
conc([X|L1],L2,[X|L3]):- conc(L1,L2,L3).
?-conc(Before,[may|After],
[jan,feb,mar,apr,may,jun,jul,aug,sep,oct,n
ov,dec]).
Before = [jan, feb, mar, apr],
After = [jun, jul, aug, sep, oct, nov, dec]
Class exercise (1)
Write a goal , using conc , to delete the last three
elements from the list L producing another list
L1.
HINT : L is a concatenating of L1 and another
three elements list
conc([],L,L).
conc([X|L1],L2,[X|L3]):- conc(L1,L2,L3).
Answer:
?- L = [1,2,3,4,5], conc(L1,[_,_,_],L).
L = [1, 2, 3, 4, 5],
L1 = [1, 2]
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Concatenation
Definition of member using conc:
We can program the membership relation using
conc:
member1(X,L):- conc(L1,[X|L2],L).
X is a member of list L if L can be decopmosed into two
lists so that the second one has X as its head.
member1(X,L):- conc(_,[X|_],L).
member1(X,L):- conc(L1,[X|L2],L).
member1(X,L):- conc(_,[X|_],L).
?- member1( b,[a,b,c]).
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Adding an item
Simply to put an item in front of a list:
[X|L]
Definition of add:
The procedure can be written explicitly as the
fact:
add(X,L,[X,L]).
Example
?-add(z,[a,b,c],NewList).
Delete an item
• Deleting an item X from list L, can be
programmed as a relation:
Del(X,L,L1).
Where L1 is equal to L with the item X removed.
Delete an item
• The del definition :
We have two cases :
1) If X is the head of the list, then the result after
deletion will be the tail of the list.
del(X,[X|Tail] , Tail).
2) If X is in the tail then it is deleted from there:
del(X,[X|Tail] , Tail) .
del(X, [Y|Tail] , Tail1) :- del(X, Tail, Tail1).
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Deletion (Example)
del(X,[X|Tail] , Tail) .
del(X, [Y|Tail] , Tail1) :- del(X, Tail, Tail1).
?- del(a,[a,b,a,a],L).
L = [b, a, a] ;
L = [a] ;
L = [] ;