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).
Note that this is not a performance antipattern, though it may lead to an inconsequential performance penalty due to the lack of an unrolled loop. Rather, this 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
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 = 2; i < 5; i++) {
// in practice, that might be written as (i = 0; i < 3; i++),
// but for illustration we use the same range of indices here
// as in the switch above; the fact that the loop body does
// not actually use the value of i proves that the
// loop-and-switch construct was superfluous in the example above.
params.add(stream.parse());
}
References
- ↑ "Loop-switch sequences". LEVEL UP CODE. Retrieved 11 April 2016.
- ↑ The FOR-CASE paradigm and Switched on Loops at The Daily WTF