Merge branch 'master' of 192.168.15.97:codeq-problems

1 files changed, 85 insertions, 2 deletions

diff --git a/prolog/problems/lists_advanced/max_2/en.py b/prolog/problems/lists_advanced/max_2/en.py
index 59c00f0..fbdc612 100644
--- a/prolog/problems/lists_advanced/max_2/en.py
+++ b/prolog/problems/lists_advanced/max_2/en.py
@@ -2,7 +2,7 @@ name = 'max/2'
 slug = 'find the largest element in list'
 
 description = '''\
-<p><code>max(L, Max)</code>: <code>Max</code> is the largest value in the list <code>L</code>.</p>
+<p><code>max(L, Max)</code>: <code>Max</code> is the largest value in list <code>L</code>.</p>
 <pre>
 ?- max([5,4,1,6], M).
   M = 6.
@@ -10,4 +10,87 @@ description = '''\
   M = 3.
 </pre>'''
 
-hint = {}
+plan = ['''\
+<p>As usual, try to reduce the problem to a smaller one. Say you already have <em>the largest</em> element
+of the <em>tail</em>...</p>
+''', '''\
+<p>Compare the largest element in the tail (list without head <code>H</code>) with the value of head <code>H</code>,
+the larger of the two wins and gets returned.</p>
+''', '''\
+<p>If the given list <code>L</code> is composed of head <code>H</code> and tail <code>T</code>, and if we
+assume that some <code>MaxT</code> is the largest element in <code>T</code>, and if it's also true that
+the value of <code>H</code> is greater than <code>MaxT</code>, then <code>H</code> is the largest element
+in <code>L</code>. <em>Or</em> it is true that <code>H</code> is smaller than <code>MaxT</code>, and in this
+case <code>MaxT</code> is the largest element in <code>L</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>max/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 shortest list with an obvious largest element?</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 <code>N</code> is equal to <code>N + 1</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>
+''',
+
+    'empty_list_base_case': '''\
+<p>You'll be hard pressed to find the largest element of an empty list. What if you stop
+the recursion a bit earlier this time?</p>
+''',
+
+    'list_instead_of_elem_base_case': '''\
+<p>You should return an element, not a list!</p>
+''',
+
+    'duplicates_not_covered': '''\
+<p>The list can contain duplicate elements. Did you think of that?</p>
+''',
+
+    'args_not_instantiated': '''\
+<p>The error that prolog reported means that when it encountered an arithmetic operation, not all the
+values were known.</p>
+<p>Did you perhaps forget that conjunction has higher precedence that disjunction and that every prolog's
+sentence (branch, rule) is independent in terms of the scope of the variables? This could be the problem.
+Carefully inspect both blocks of code (before and after the semicolon) or both rules.</p>
+''',
+
+    'unprotected_branch': '''\
+<p>It seems you didn't "protect" one of the (conditional) branches. Both branches (of the disjunction) typically
+require a condition. Don't rely that if the execution reached the second branch that the first branch is false.
+The relation between them is OR and not XOR. It's your job to make them mutually exclusive. Try the following
+query and check <em>all possible</em> solutions and you'll notice the problem.</p>
+<p><code>?- max([6,9,3,8,1], Max).</code></p>
+''',
+
+    'one_branch_missing': '''\
+<p>Maybe you forgot one of the possibilities? The head can be <em>either</em> greater <em>or</em> smaller than
+the largest element in the tail.</p>
+''',
+}