[poincare] In context, return 0 if constant has not been assigned

Change-Id: I3a338a7de54c55500982a8a0dd32a1193f8db504

poincare/include/poincare/context.h

@@ -3,6 +3,9 @@
 
 #include <poincare/expression.h>
 #include <poincare/symbol.h>
+#include <poincare/float.h>
+
+class Integer;
 
 //TODO: We should probably make a COPY of the expressions we store
 
@@ -16,6 +19,7 @@ class Context {
     int symbolIndex(const Symbol * symbol) const;
     static constexpr uint16_t k_maxNumberOfExpressions = 26;
     Expression * m_expressions[k_maxNumberOfExpressions];
+    static Float * defaultExpression();
 };
 
 #endif

poincare/src/context.cpp

@@ -8,14 +8,24 @@ Context::Context()
   }
 }
 
+Float * Context::defaultExpression() {
+  static Float * defaultExpression = new Float(0);
+  return defaultExpression;
+}
+
 int Context::symbolIndex(const Symbol * symbol) const {
   int index = symbol->name() - 'A';
-  assert(index < k_maxNumberOfExpressions);
   return index;
 }
 
 const Expression * Context::expressionForSymbol(const Symbol * symbol) {
   int index = symbolIndex(symbol);
+  if (index < 0 || index >= k_maxNumberOfExpressions) {
+    return nullptr;
+  }
+  if (m_expressions[index] == nullptr) {
+    return defaultExpression();
+  }
   return m_expressions[index];
 }