Loop-switch sequence

A loop-switch sequence (also known as the for-case paradigm) 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 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

Here is 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 value of i itself
    // is not actually used anywhere in the loop body below is what
    // proves that the loop-and-switch construct was superfluous
    // in the example above.
    params.add(stream.parse());
}