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<title editable="nocomment">Example 11</title>
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<h1>Example 11: Random Shuffle</h1>
<h2>Overview</h2>
<p>The main structure of the program will be a) read lines, b)
random permutate lines, c) write lines. We can already define
the main program, without executing it:</p>
<pre class="code">
main :-
read_lines(L),
random_permutation(L, R),
write_lines(R).
</pre>
<h2>Random</h2>
<p>A simple method to produce a random permutation in Prolog,
is to first extend the given list by a uniform random number,
and then keysort and strip the list again. Dogelog Player provides
uniform random numbers in the interval [0,1) via library(util/math),
and a native implementation of keysort is found in library(compat).</p>
<p>Each time pressing the instrumentation button
will give a different result:</p>
<pre class="code">
:- ensure_loaded(library(compat)).
:- ensure_loaded(library(util/math)).
random_permutation(L, R) :-
add_key(L, H),
keysort(H, J),
remove_key(J, R).
add_key([], []).
add_key([X|L], [Y-X|R]) :- random(Y), add_key(L, R).
remove_key([], []).
remove_key([_-X|L], [X|R]) :- remove_key(L, R).
?- random_permutation([1,2,3,4,5], X).
</pre>
<h2>Input</h2>
<p>For input we rely on the Prolog term reading, whereas
we use the Prolog term end_of_file as a marker that the list
has been completely entered. We can explore reading directly in a
notebook, by adding the input after the reading query.
Because we read Prolog terms, we have to terminate each
input by a period. Feel free to edit the input and see what happens:</p>
<pre class="code">
read_lines(L) :- read(X), read_lines(X, L).
read_lines(end_of_file, []) :- !.
read_lines(X, [X|L]) :- read(Y), read_lines(Y, L).
?- read_lines(L).
foo.
bar.
baz.
end_of_file.
</pre>
<h2>Output</h2>
<p>For output we simple flesh out some Prolog terms in a
tail recursive loop. One might be tempted to use higher order
programming here and some maplist, but Dogelog Player doesn't
provide call/n for performance and didactical reasons, so we
have to do it more explicitly. The can explore writing
directly in a notebook, since it shows the output after the
writing query:</p>
<pre class="code">
write_lines([]).
write_lines([X|L]) :- write(X), nl, write_lines(L).
?- write_lines([foo,bar,baz]).
</pre>
<h2>Result</h2>
<p>We can now run the main program already sketched in the
first paragraph. We will provide a small list of Turing award
winners that were inclined with artificial intelligence.
The notebook allows combining reading and writing. Again each
time pressing the instrumention button will give a different result.
Feel also free to edit the input and see what happens:
</p>
<pre class="code">
?- main.
'Marvin Minsky'.
'John McCarthy'.
'Herbert A. Simon'.
'Allen Newell'.
end_of_file.
</pre>
<script type="module">
import {notebook_async
} from "../../../../dogelog/player/canned/dogelog.mjs";
await notebook_async();
</script>
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