Loop-switch sequence: Difference between revisions

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A '''loop-switch sequence''' (also known as the '''for-case paradigm'''<ref>[http://thedailywtf.com/Articles/The_FOR-CASE_paradigm.aspx The FOR-CASE paradigm] and [http://thedailywtf.com/Articles/Switched_on_Loops.aspx Switched on Loops] at [[The Daily WTF]]</ref> 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]].

A '''loop-switch sequence'''<ref name="LEVEL ">{{cite web | url=https://levelupcode.wordpress.com/avoid/loop-switch-sequences/ | title=Loop-switch sequences | date=30 November 2013 | publisher=LEVEL UP CODE | accessdate=11 April 2016}}</ref> (also known as the '''for-case paradigm'''<ref>[http://thedailywtf.com/Articles/The_FOR-CASE_paradigm.aspx The FOR-CASE paradigm] and [http://thedailywtf.com/Articles/Switched_on_Loops.aspx Switched on Loops] at [[The Daily WTF]]</ref> 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 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]]).

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 [[Loop unwinding|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.

Note that thisThis is not a performance antipattern, though it may lead to an inconsequential performance penalty due to the lack of an [[Loop unwinding|unrolled loop]]. Rather, thisit 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:

An event-driven solution would implement a [[Observer pattern|listener interface]]:

<sourcesyntaxhighlight lang="java">

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);

}

}

</syntaxhighlight>

HereBut iswithout the listener, it becomes an example of the antipattern:

<sourcesyntaxhighlight lang="java">

// parse a key, a value, then three parameters

String key = null;

Line 27 ⟶ 58:

}

}

</syntaxhighlight>

</source>

And here is the refactored solution:

<syntaxhighlight lang="java">

<source lang="java">

// parse a key and value

String key = stream.parse();

Line 38 ⟶ 68:

// parse 3 parameters

List<String> params = new LinkedList<String>();

for (int i = 20; i < 53; 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());

}

</syntaxhighlight>

</source>

==References==

<references />

[[Category:Anti-patternsComputer programming]]