diff options
Diffstat (limited to 'prolog/problems/denotational_semantics/prog_listswap_3/en.py')
| -rw-r--r-- | prolog/problems/denotational_semantics/prog_listswap_3/en.py | 18 |
1 files changed, 18 insertions, 0 deletions
diff --git a/prolog/problems/denotational_semantics/prog_listswap_3/en.py b/prolog/problems/denotational_semantics/prog_listswap_3/en.py new file mode 100644 index 0000000..1a7cb79 --- /dev/null +++ b/prolog/problems/denotational_semantics/prog_listswap_3/en.py @@ -0,0 +1,18 @@ +id = 174 +name = 'prog_listswap/3' +slug = 'list-manipulation language with semantics' + +description = '''\ +<p>Write a DCG for manipulating list elements. The first symbol in every word is <code>[begin]</code>, followed by any sequence of "instruction" symbols from the set {<code>left</code>, <code>right</code>, <code>swap</code>}, and finally <code>[end]</code>. The starting symbol should be named <code>prog_listswap</code>.</p> + +<p>The meaning of a word (program) in this language has the form <code>In-->Out</code>, mapping from input to output lists. Besides the list contents, internal states also hold the current cursor position. The <code>left</code> and <code>right</code> instructions move the cursor one step in the given direction, while the <code>swap</code> instruction swaps the element under the cursor with its left neighbor (and fails if cursor is currently pointing to the first element of the list).</p> + +<pre> + ?- prog_listswap([1,2,3,4]-->Out, [begin,right,swap,end], []). + Out = [2,1,3,4]. +</pre> + +<p>Helper predicate (already defined):<br /> + <code>swap(List,I,NewList)</code> creates <code>NewList</code> by swapping the <code>I</code>th element with its left neighbor in <code>List</code></p>''' + +hint = {} |