From 0331b2ff089f33a33f66470cec305ff08f829a30 Mon Sep 17 00:00:00 2001
From: Aleksander Sadikov <aleksander.sadikov@fri.uni-lj.si>
Date: Tue, 30 Aug 2016 02:33:37 +0200
Subject: English translation for last_elem/2 added.

---
 prolog/problems/lists/last_elem_2/en.py | 84 ++++++++++++++++++++++++++++++++-
 prolog/problems/lists/last_elem_2/sl.py |  2 +-
 2 files changed, 84 insertions(+), 2 deletions(-)

(limited to 'prolog/problems/lists/last_elem_2')

diff --git a/prolog/problems/lists/last_elem_2/en.py b/prolog/problems/lists/last_elem_2/en.py
index 93c7c05..5eec3b8 100644
--- a/prolog/problems/lists/last_elem_2/en.py
+++ b/prolog/problems/lists/last_elem_2/en.py
@@ -10,4 +10,86 @@ description = '''\
   X = 1.
 </pre>'''
 
-hint = {}
+plan = ['''\
+<p>It's easy to access the first element in a list, but to get to the last element one needs to
+recursively go through the whole list.</p>
+''', '''\
+<p>The list can be divided into its head and tail, and the search can proceed with the tail.
+The problem is now smaller (the tail is shorter than the whole list), so we can use recursion.</p>
+''', '''\
+<p>If <code>X</code> is the last element of tail <code>T</code>, then <code>X</code> is also
+the last element of the whole list that looks like <code>[H|T]</code>.</p>
+''']
+
+hint = {
+    'eq_instead_of_equ': '''\
+<p>The operator <code>==</code> is "stricter" than operator <code>=</code> in the sense that
+for the latter it is enough to be able to make the two operands equal (unification). Perhaps by using <code>=</code>
+you can make the predicate <code>last_elem/2</code> more general (e.g. able to work with output arguments becoming inputs).</p>
+<p>Of course, you can also solve the exercise without explicit use of either of these two operators, just
+remember that unification is implicitly performed with the predicate's arguments (head of clause).</p>
+''',
+
+    'eq_instead_of_equ_markup': '''\
+<p>Perhaps the operator for unification (=) would be better?</p>
+''',
+
+    'base_case': '''\
+<p>Did you think of a base case? What's the simplest possible case? What if the list contains only one element?</p>
+''',
+
+    '[]_should_not_succeed': '''\
+<p>How did you succeed to find a last element in an empty list? You likely need a different base case.</p>
+''',
+
+    'list_returned': '''\
+<p>You are returning a list instead of an element.</p>
+''',
+
+    'clumsy_conc_use': '''\
+<p>Are you using <code>conc/3</code>? An interesting idea. Don't forget that the second list you're
+concatenating must be <em>of length one</em> if you want to achieve the desired effect.
+So a pattern of the form <code>[X]</code>, right?</p>
+''',
+
+    'unsuccessful_conc_use': '''\
+<p>Are you using <code>conc/3</code>? An interesting idea; it is possible to solve in this way.
+However, a bit of tweaking is still needed. Don't forget that <code>conc/3</code> has three arguments,
+and all three are lists. Think about what kind of a pattern do you need...</p>
+''',
+
+    'recursive_case': '''\
+<p>The base case is ok. However, what about the general recursive case?</p>
+''',
+
+    'predicate_always_false': '''\
+<p>It seems your predicate is <emph>always</emph> "false". Did you give it the correct name,
+or is it perhaps misspelled?</p>
+<p>If the name is correct, check whether something else is misspelled, perhaps there is a full stop instead of
+a comma or vice versa, or maybe you typed a variable name in lowercase?</p>
+<p>It is, of course, also possible that your conditions are too restrictive, or even impossible to satisfy
+(as would be, for example, the condition that an empty list <code>[]</code> is equal to a list with
+exactly three elements <code>[A,B,C]</code>,
+or something similarly impossible).</p>
+''',
+
+    'timeout': '''\
+<p>Is there an infinite recursion at work here? How will it ever stop?</p>
+<p>Or perhaps is there a missing, faulty, or simply incompatible (with the general recursive case) base case?</p>
+''',
+
+    'final_hint': '''\
+<p>Interesting fact: predicate <code>conc/3</code> can be used to search for patterns in lists. The last
+element in a list is also a kind of pattern. What happens if we concatenate an arbitrary list <code>_</code>
+and a list <em>of length one</em> (in this order)? A list of length one is of course written as
+<code>[Element]</code>.</p>
+<p>Try asking the following query:</p>
+<p><code>?- conc(_, [Element], [a,b,c,d,e,f,q]).</code></p>
+<p>So, can you now fetch the list's last element using <code>conc/3</code>? This will be very useful in
+further exercises. On the other hand, of course, accessing the last element of a list is still quite
+expensive, it's done in O(n) time. Therefore, if it's not important which element of a list is to be used,
+or where in a list a new element is to be added, always work with the head.</p>
+<p>And what does this query do? ;)</p>
+<p><code>?- conc([a,b,c], [q], L).</code></p>
+''',
+}
diff --git a/prolog/problems/lists/last_elem_2/sl.py b/prolog/problems/lists/last_elem_2/sl.py
index afba57a..961963c 100644
--- a/prolog/problems/lists/last_elem_2/sl.py
+++ b/prolog/problems/lists/last_elem_2/sl.py
@@ -47,7 +47,7 @@ implicitno že kar v argumentih predikata (glavi stavka).</p>
 
     'clumsy_conc_use': '''\
 <p>Uporabljaš <code>conc/3</code>? Zanimiva ideja. Ne pozabi, da mora drugi seznam, ki ga konkateniraš
-biti dolžine ena, če hočeš doseči to kar želiš. Torej vzorec oblike <code>[X]</code>, kajne?</p>
+biti <em>dolžine ena</em>, če hočeš doseči to kar želiš. Torej vzorec oblike <code>[X]</code>, kajne?</p>
 ''',
 
     'unsuccessful_conc_use': '''\
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