coding/patterns/HashTable.md

+<!-- --- title: OpenFOAM C++ Coding Patterns (HashTable) -->
+
+[Back to _coding patterns_](/coding/patterns/patterns)
+***We are happy to incorporate content from volunteers!!***
+
+
+## Coding Patterns
+
+### HashSet
+
+Use range-for to dereference the HashSet iterators directly:
+```
+labelHashSet ids ...;
+
+for (const label id : ids)
+{
+    os << id << nl;
+}
+```
+
+**Anti-pattern**: using iterators:
+```
+labelHashSet ids ...;
+
+forAllConstIters(ids, iter)
+{
+    os <<iter.key() << nl;
+}
+
+// Even worse - using old macros!
+forAllConstIter(labelHashSet, ids, iter)
+{
+    os <<iter.key() << nl;
+}
+```
+
+### HashTable, Map
+
+Use `lookup()` to provide default values (const-access) and the `operator()`
+to create new zero-initialized entries if required:
+```
+Map<label> counter;
+...
+
+os << "hits: " << counter.lookup(i, 0) << nl;
+
+++counter(j);
+```
+
+**Anti-pattern**: the manual equivalents:
+```
+Map<label> counter;
+...
+
+os << "hits: " << (counter.found(i) ? *(counter.cfind(i)) : 0) << nl;
+
+
+Map<label>::iterator iter = counter.find(j);
+
+if (!iter.found())
+{
+    counter.insert(j, 0);
+}
+else
+{
+    ++(*iter);
+}
+```
+
+----
+
+Copyright (C) 2019 OpenCFD Ltd.