{"id":796,"date":"2010-10-13T09:53:32","date_gmt":"2010-10-13T13:53:32","guid":{"rendered":"http:\/\/www.elblender.com\/wordpress\/?p=796"},"modified":"2010-10-14T08:04:13","modified_gmt":"2010-10-14T12:04:13","slug":"prolog-using-examples-part-4-continuation","status":"publish","type":"post","link":"http:\/\/blog.acarlstein.com\/?p=796","title":{"rendered":"Prolog using Examples – Part 5"},"content":{"rendered":"

The follow is a continuation of the previous posting “Prolog using Examples – Part 4”<\/p>\n

Example 1:<\/span> Check the prefix of a list.<\/p>\n

\/* Base case *\/\r\n\/* When the list of prefix checked is empty means that we stop checking *\/\r\nprefix([],_).\r\n\r\n\/* Recursive case *\/\r\n\/* Both head of the list must be the same *\/\r\nprefix([Head|Tail_A], [Head|Tail_B]):-\r\n\u00a0\u00a0\u00a0 prefix(Tail_A, Tail_B).\r\n\r\n?- prefix([1,b], [1,b,c,4]).\r\ntrue.\r\n\r\n?- prefix([1,b], [1,d,c,4]).\r\nfalse.\r\n\r\n<\/span>\r\n\/* prefix will search for each prefix everytime OR ';' is used *\/\r\n?- prefix(Lst, [1,2,3,4]).\r\nLst = [] ;\r\nLst = [1] ;\r\nLst = [1, 2] ;\r\nLst = [1, 2, 3] ;\r\nLst = [1, 2, 3, 4] ;\r\nfalse.\r\n\r\n\/* Generate a list where list [1,2,3,4] is the prefix and the tail is a dynamic variable *\/\r\n?- prefix([1,2,3,4],Lst).\r\nLst = [1, 2, 3, 4|_G3713].\u00a0<\/span>\r\n<\/span><\/pre>\n
Example 2:<\/span> Check the suffix of a list.<\/p>\n
\/* Base case *\/\r\n\/* When both list are the same, we found the suffix *\/\r\nsuffix(Lst_tail, Lst_tail).\r\n\r\n\/* Recursive case *\/\r\n\/* We want to check always the tail of the list, so the base case can check if they are the same *\/\r\nsuffix([_|Tail], Lst) :-\r\n\u00a0\u00a0\u00a0 suffix(Tail, Lst).\r\n\r\n?- suffix([a,2,c,4],[c,4]).\r\ntrue.\r\n\r\n?- suffix([a,2,c,d],[c,4]).\r\nfalse.\r\n\r\n\/* Obtain the next tail everytime OR ';' is used *\/\r\n?- suffix([1,2,3,4], Lst).\r\nLst = [1, 2, 3, 4] ;\r\nLst = [2, 3, 4] ;\r\nLst = [3, 4] ;\r\nLst = [4] ;\r\nLst = [].\r\n\r\n\/* Be careful, in this kind of cases prolog keep generating dynamic variables in from of the list *\/\r\n?- suffix(Lst, [1,2,3,4]).\r\nLst = [1, 2, 3, 4] ;\r\nLst = [_G3704, 1, 2, 3, 4] ;\r\nLst = [_G3704, _G3707, 1, 2, 3, 4] ;\r\nLst = [_G3704, _G3707, _G3710, 1, 2, 3, 4] ;\r\nLst = [_G3704, _G3707, _G3710, _G3713, 1, 2, 3, 4] ;\r\nLst = [_G3704, _G3707, _G3710, _G3713, _G3716, 1, 2, 3, 4] ;\r\nLst = [_G3704, _G3707, _G3710, _G3713, _G3716, _G3719, 1, 2, 3|...] ;\r\n... \/* Keep generating dynamic variables *\/\r\n\r\n<\/span>\u00a0<\/span><\/pre>\n
Example 3:<\/span> In this example, we are introducing append<\/em>. append<\/em> is the unification of two lists, List_A, List_B, into one list that we are going to call list AB, List_AB.<\/p>\n
\"\"<\/a><\/p>\n
The code of append<\/em> is the follow:<\/p>\n
\/* Base case *\/\r\n\/* Will stop recursion when List_A is empty indicating that all the elements are in List_AB *\/\r\nappend([], Lst, Lst).\r\n\r\n\/* Recursive case *\/\r\nappend([Head|Tail_A], List_B, [Head|Tail_AB]):-\r\n\u00a0\u00a0\u00a0 append(Tail_A, List_B, Tail_AB).<\/span><\/pre>\n
First append<\/em> will move 1, 2,\u00a0 and 3 atoms from List_A to List_AB using recursion:<\/p>\n
\/* Recursive case *\/\r\nappend([Head|Tail_A], List_B, [Head|Tail_AB]):-\r\n\u00a0\u00a0\u00a0 append(Tail_A, List_B, Tail_AB).<\/span><\/pre>\n
append([1,2,3], [4,5,6], []) \u2192 append([2,3], [4,5,6], [1])\u00a0 \u2192 append([3], [4,5,6], [1,2])\u00a0 \u2192 append([], [4,5,6], [1,2,3]) <\/span><\/em><\/p>\n
When List_A is empty, then it will append List_B to List_AB as a tail of List_AB using the base case:<\/p>\n
\/* Base case *\/\r\n\/* Will stop recursion when List_A is empty indicating that all the elements are in List_AB *\/\r\nappend([], Lst, Lst).\r\n\r\n<\/span><\/pre>\n

\nappend([], [4,5,6], [1,2,3]) \u2192 append([], [], [1,2,3,4,5,6])\u00a0 \u2192 List_AB = [1,2,3,4,5,6]<\/span><\/em><\/p>\n
Example 4:<\/span> Even do append<\/em> seems to create a list by appending List_A with List_B, append<\/em> can behave in a different ways depending of how it is used:<\/p>\n
Let see the code for append<\/em>:<\/span><\/p>\n
append([], Lst, Lst). \/*\u00a0\u2190 Base case *\/\r\nappend([Head|Tail_A], List_B, [Head|Tail_AB]):-\u00a0 \/* \u2190 Recursive case *\/\r\n\u00a0\u00a0\u00a0 append(Tail_A, List_B, Tail_AB).\r\n\r\n<\/span>\u00a0<\/span><\/pre>\n
Lets say we do\u00a0 append(List_A, [5,6], [1,2,3,4,5,6])<\/span>. What would be the result?<\/p>\n
\n
\"\"<\/a><\/p>\n
As you can see List_AB have 1,2,3,4,5, and 6 while List_B have 5 and 6, Prolog will unify List_A with 1,2,3, and 4.<\/p>\n
?- append(List_A, [5,6], [1,2,3,4,5,6]).\r\nList_A = [1, 2, 3, 4] .<\/span><\/pre>\n
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