A list is either empty or it is composed of a first element head and a tail, which is a list itself. In Prolog we represent the empty list by the atom [] and a non-empty list by a term [H T] where H denotes the head and T denotes the tail.

prolog lists

Prolog Site. Search this site. A First Glimpse. Syntax and Meaning. Prolog Lists. Logic and Codes. Binary Trees. Multiway Trees. Solutions can be found here. Subpages 1 : Solutions Home Author Prolog Course 1. A First Glimpse 2. Syntax and Meaning Prolog Problems 1.

Prolog Lists 2. Arithmetic 3. Logic and Codes 4. Binary Trees 5. Multiway Trees 6. Graphs 7. Miscellaneous Sitemap. Prolog Lists Solutions can be found here.

prolog lists

The first element in the list is number 1. A palindrome can be read forward or backward; e. Transform a list, possibly holding lists as elements into a 'flat' list by replacing each list with its elements recursively. If a list contains repeated elements they should be replaced with a single copy of the element.

The order of the elements should not be changed. If a list contains repeated elements they should be placed in separate sublists. Use the result of problem 1. Consecutive duplicates of elements are encoded as terms [N,E] where N is the number of duplicates of the element E.

Modify the result of problem 1. Only elements with duplicates are transferred as [N,E] terms. Given a run-length code list generated as specified in problem 1.

Construct its uncompressed version. Implement the so-called run-length encoding data compression method directly.The second line can be read, "X is mortal if X is a man;'' in other words, "All men are mortal. We can test the program by asking the question:. A variable is written as a sequence of letters and digits, beginning with a capital letter. An atom is any sequence of letters and digits, beginning with a lowercase letter. Alternatively, an atom is any sequence of characters, enclosed by single quotes ' ; an internal single quote must be doubled.

Comments are not restricted to a single line, but may not be nested. Prolog variables are similar to "unknowns'' in algebra: Prolog tries to find values for the variables such that the entire clause can succeed. Once a value has been chosen for a variable, it cannot be altered by subsequent code; however, if the remainder of the clause cannot be satisfied, Prolog may backtrack and try another value for that variable.

The anonymous variable consists of a single underscore. Each occurrence of the anonymous variable is considered to be a new, distinct variable i. The scope of a variable name is the clause in which it occurs. There are no "global'' variables. Approximate procedural reading: To show that X is the mother of Yfirst show that X is a parent of Ythen show that X is female. Typically Prolog predicates work regardless of which arguments are instantiated, and may instantiate the others.

Neither is predefined. Definitions are:. A double-quoted character string is syntactic sugar for a list of the ASCII codes for those characters. If one of the files contains a predicate that already occurs in the database, it replaces the old definition. To type in predicates directly, use reconsult user. The prompt will change from "? Enter predicates as you would on a file.

You may call any predicate with any arguments, and you may make multiple calls in one inquiry by separating them with commas. Use a period at the end of each inquiry. There is no "main'' program. When Prolog does not give you a new prompt after it answers an inquiry, that means there may be other answers.Any language needs a way to handle collections of objects and prolog is no exception.

A list in prolog can look like this:. The brackets are the beginning and the end of the list, and the commas separate the various elements. Here all the elements are atoms, but a list can contain all sorts of elements, and different types of element can occur in the same list.

The following are examples of valid lists:. Most prolog code will not define a list explicitly, like these examples, but rather handle lists of any length, with many possible elements. To handle lists without knowing what's inside them or how long they are, you use the bar notation:.

In this notation the variable Head represents the leftmost element of the list and the variable Tail represents the rest of the list represented as another list. A head can be anything, from a predicate to another list, but the tail is always another list. Some default library rules are provided, e. To see how these work, try the following codes:. Note that the empty list [ ] cannot be split up and therefore will not unify with [H T].

This will split a list up into the first three elements, and the rest of the list. Note, however that this will fail if the list has fewer than three elements. This relation defines the last element of a list. It can be used as a program to find a list's last element:.

First, there is the stop predicate, that says that the last element of a list with one element, is that element. Normally, you would use built in predicates for these list operations, instead of writing them yourself. Built in predicates are defined by your prolog implementation, but can be used in any program. These implementations are shown here to illustrate how to modify lists. Member is a standard prolog built-in predicate. You use it like this:. Where List is any prolog list, and Element is any element in that list.

The following, for instance, succeeds returns 'Yes' :. It's used like this:.

prolog lists

You can even use it with three variables. Try this for yourself to see what the result looks like. The first line simply unifies the last two lists, it succeeds for queries like: append [], [a,b,c], [a,b,c]. The second line the recursive clause declares that, given append A,B,C the head of A is equal to the head of C, and appending the tail of A, with B, gives the tail of C.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

prolog lists

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That is, take the first element of the first list in your input list and continue recursively with the remaining lists. As a second chance, skip that element and redo with the remaining elements. Thus we can write. Learn more. Working with list of lists in Prolog Ask Question. Asked 8 years, 2 months ago.

Prolog Tutorial

Active 1 year, 5 months ago. Viewed 16k times. Please help me to solve this problem: I have a list of lists [[1,2],[3,4]] How do I get: [1,3] [1,4] [2,3] [2,4] Or if I have a list of lists [[1,2],[3,4],[6,7]] How do I get: [1,3,6] [1,3,7] [1,4,6] [1,4,7] [2,3,6] [2,3,7] [2,4,6] [2,4,7]. Welcome Welcome 2 2 gold badges 4 4 silver badges 11 11 bronze badges.

Active Oldest Votes. You can do something like this: lists [], []. Thus we can write combine Ls, Rs :- maplist get1, Ls, Rs. Test output:? CapelliC CapelliC Why this command: findall X, maplist member, [[a,b,c], [1,2,3]], XL. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name.

Email Required, but never shown. The Overflow Blog. Featured on Meta. Feedback on Q2 Community Roadmap. Question Close Updates: Phase 1. Technical site integration observational experiment live on Stack Overflow. Linked 1.List processing — handling sequences of elements — is a powerful technique in Prolog. In this tutorial, we explain what lists are and how to declare them, and then give several examples that show how you might use list processing in your own applications.

We also define two well known Prolog predicates — member and append — while looking at list processing from both a recursive and a procedural standpoint.

After that, we introduce findall, a Visual Prolog standard predicate that enables you to find and collect all solutions to a single goal. We round out this tutorial with a discussion of compound lists — combinations of different types of elements — and an example of parsing by difference lists.

In Prolog, a list is an object that contains an arbitrary number of other objects within it. Lists correspond roughly to arrays in other languages, but, unlike an array, a list does not require you to declare how big it will be before you use it. A list that contains the numbers 12and 3 is written as. Each item contained in the list is known as an element. To form a list data structure, you separate the elements of a list with commas and then enclose them in square brackets.

Here are some examples:. The elements in a list can be anything, including other lists. However, all elements in a list must belong to the same domain.

A list is really a recursive compound object. It consists of two parts: the head, of a list, which is the first element, and the tail, which is a list comprising all the subsequent elements. If you take the first element from the tail of a list enough times, you will eventually get down to an empty list [ ]. This means that, conceptually, lists have a tree structure just like other compound objects.

The tree structure of [a, b, c, d] is:.

Prolog/Lists

Further, a one-element list such as [a] is not the same as the element that it contains, because [a] is really the compound data structure shown here:. Prolog provides a way to make a head and a tail of a list explicit. Instead of separating elements with commas, you can separate the head and tail with a vertical bar.

For instance. You can even use both kinds of separators in the same list, provided the vertical bar is the last separator. So, if you really want to, you can write [a, b, c, d] as [a, b [c, d]]. Because a list is really a recursive compound data structure, you need recursive algorithms to process it. The most basic way to process a list is to work through it, doing something to each element until you reach the end.

An algorithm of this kind usually needs two clauses. One of them says what to do with an ordinary list one that can be divided into a head and a tail. The other says what to do with an empty list.

Now, which clause will this goal match? Recall that, even though the list [3] has only one element; it does have a head and tail; the head is 3 and the tail is []. Now you see why this program needs the first clause. The second clause will not match this goal because [] cannot be divided into head and tail. So, if the first clause were not there, the goal would fail. As it is, the first clause matches and the goal succeeds without doing anything further.

Now consider how you might find out how many elements are in a list.Learn Prolog Now!

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The online version has been available sinceand now there is also a throughly revised version available in book form. We wanted to do two things with this course. First, we wanted to provide a text that was relatively self contained, a text that would permit someone with little or no knowledge of computing to pick up the basics of Prolog with the minimum of fuss.

We also wanted the text to be clear enough to make it useful for self study. We believe that if you read the text, and do the associated exercises, you will gain a useful partial entry to the world of Prolog. But only a partial entry, and this brings us to our second point. We want to emphasize the practical aspects of Prolog. Prolog is something you do.

You can't learn a programming language simply by reading about it, and if you really want to get the most out of this course, we strongly advise you to get hold of a Prolog interpreter you'll find pointers to some nice ones on this website and work through all the Practical Sessions that we provide. And of course, don't stop with what we provide. The more you program, the better you'll get We hope you enjoy the course. And whether you're using this book to teach yourself Prolog, or you're using it as the basis for teaching others, we would like to hear from you.

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