Loop-switch sequence
A loop-switch sequence[1] (also known as the for-case paradigm[2] or Anti-Duff's Device) is a programming antipattern where a clear set of steps is implemented as a switch-within-a-loop. The loop-switch sequence is a specific derivative of spaghetti code.
It is not necessarily an antipattern to use a switch statement within a loop—it is only considered incorrect when used to model a known sequence of steps. The most common example of the correct use of a switch within a loop is an inversion of control such as an event handler. In event handler loops, the sequence of events is not known at compile-time, so the repeated switch is both necessary and correct (see event-driven programming, event loop and event-driven finite state machine).
This is not a performance antipattern, though it may lead to an inconsequential performance penalty due to the lack of an unrolled loop. Rather, it is a clarity antipattern, as in any non-trivial example it is much more difficult to decipher the intent and actual function of the code than the more straightforward refactored solution.
Example
[edit]An event-driven solution would implement a listener interface:
String key = null; String value = null; List<String> params = null; int column = 0; public void addToken(token) { // parse a key, a value, then three parameters switch (column) { case 0: params = new LinkedList<String>(); key = token; break; case 1: value = token; break; default: params.add(token); break; } if (++column >= 5) { column = 0; completeRow(key, value, params); } } But without the listener, it becomes an example of the antipattern:
// parse a key, a value, then three parameters String key = null; String value = null; List<String> params = new LinkedList<String>(); for (int i = 0; i < 5; i++) { switch (i) { case 0: key = stream.parse(); break; case 1: value = stream.parse(); break; default: params.add(stream.parse()); break; } } And here is the refactored solution:
// parse a key and value String key = stream.parse(); String value = stream.parse(); // parse 3 parameters List<String> params = new LinkedList<String>(); for (int i = 0; i < 3; i++) { params.add(stream.parse()); } References
[edit]- ^ "Loop-switch sequences". LEVEL UP CODE. 30 November 2013. Retrieved 11 April 2016.
- ^ The FOR-CASE paradigm and Switched on Loops at The Daily WTF