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

An event-driven solution would implement a listener interface:

Stringkey=null;Stringvalue=null;List<String>params=null;intcolumn=0;publicvoidaddToken(token){// parse a key, a value, then three parameters switch(column){case0:params=newLinkedList<String>();key=token;break;case1: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 Stringkey=null;Stringvalue=null;List<String>params=newLinkedList<String>();for(inti=0;i<5;i++){switch(i){case0:key=stream.parse();break;case1:value=stream.parse();break;default:params.add(stream.parse());break;}}

And here is the refactored solution:

// parse a key and valueStringkey=stream.parse();Stringvalue=stream.parse();// parse 3 parametersList<String>params=newLinkedList<String>();for(inti=0;i<3;i++){params.add(stream.parse());}

References

1. "Loop-switch sequences". LEVEL UP CODE. 30 November 2013. Retrieved 11 April 2016.
2. The FOR-CASE paradigm and Switched on Loops at The Daily WTF