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    <p>If you're using SWI-Prolog, you can achieve this for individual cases using <code>include/3</code>:</p> <pre><code>?- include(integer, [1,a,2,z,3,b,b], S). S = [1, 2, 3]. </code></pre> <p><code>include/3</code> calls the predicate <code>integer/1</code> on each element of the list and collects in S the elements X for which <code>call(integer, X)</code> is true (you could write this predicate yourself using <code>call/1</code> and the skeleton of the solution dasblinkenlight supplied).</p> <p>With this predicate in hand, we can easily write a predicate that lets us call <code>include/3</code> on the list multiple times, using different types each time:</p> <pre><code>select_type([], _, []). select_type([Type|Types], List, [S|Selected]) :- include(Type, List, S), select_type(Types, List, Selected). </code></pre> <p>This predicate takes a list of types as its first argument and gives a list of lists as its third argument. Each list in the latter contains the elements in <code>List</code> of the given type. Can be used thus:</p> <pre><code>?- select_type([integer, atom, var, float, is_list], [2.112,A,3.111,A,B,a,2,3,s,d], X). X = [[2, 3], [a, s, d], [A, A, B], [2.112, 3.111], []]. </code></pre> <p>Of course, this kind of predicate needn't be confined to built-in types, but will work with a list of any predicates which have room for an extra argument. E.g.,</p> <pre><code>?- select_type([plus(2,2)], [1,2,3,4,5,6], X). X = [[4]]. </code></pre> <p>because <code>call(plus(2,2), 4)</code> is true.</p> <hr> <p><strong>Edit:</strong></p> <p>To answer your question more directly, and in accordance with your clarification in the comments, you can write a predicate like your proposed <code>select/3</code> thus:</p> <pre><code>select_int_atom(List, Int, Atom) :- include(integer, List, Int), include(atom, List, Atom). </code></pre> <p>As you can see, this is equivalent to two consecutive calls to <code>include/3</code>. Note as remarked by mbratch, you don't want to use <code>select</code> for your predicate, since there is already a <code>select/3</code> in the core library.</p>
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    1. POHow to create a predicate that picks up a specific sort of atoms from a list?
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    1. POHow to create a predicate that picks up a specific sort of atoms from a list?
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    1. COthanks a lot! another question: how can I define e.g. an atom with the variable "Y"? with call(atom,Y)?
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    2. CO@user3058876 Sure! Please ask if you have any other questions. Could you elaborate on your question about defining an atom with the variable "Y"? I don't quite understand. Perhaps an example?
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    3. COI want to define every integer with an "X" and every atom with an "Y", so that the program gives e.g. the solution X= 1,5,3,4 and Y=a,bg,d. My problem is that in your answer the different sorts of elements are not separated, but put in the same list. (was it understandable this time?sry)
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