Editing Control flow (section)

=== Early exit from loops ===
When using a count-controlled loop to search through a table, it might be desirable to stop searching as soon as the required item is found. Some programming languages provide a statement such as <code>break</code> (most languages), <code>Exit</code> (Visual Basic), or <code>last</code> (Perl), which effect is to terminate the current loop immediately, and transfer control to the statement immediately after that loop. Another term for early-exit loops is [[loop-and-a-half]].

The following example is done in [[Ada (programming language)|Ada]] which supports both ''early exit from loops'' and ''[[Control flow#Loop with test in the middle|loops with test in the middle]]''. Both features are very similar and comparing both code snippets will show the difference: ''early exit'' must be combined with an '''if''' statement while a ''condition in the middle'' is a self-contained construct.

<syntaxhighlight lang="ada">
with Ada.Text IO;
with Ada.Integer Text IO;

procedure Print_Squares is 
    X : Integer;
begin
    Read_Data : loop
        Ada.Integer Text IO.Get(X);
    exit Read_Data when X = 0;
        Ada.Text IO.Put (X * X);
        Ada.Text IO.New_Line;
    end loop Read_Data;
end Print_Squares;
</syntaxhighlight>

[[Python (programming language)|Python]] supports conditional execution of code depending on whether a loop was exited early (with a <code>break</code> statement) or not by using an else-clause with the loop. For example,

<syntaxhighlight lang="python">
for n in set_of_numbers:
    if isprime(n):
        print("Set contains a prime number")
        break
else:
    print("Set did not contain any prime numbers")
</syntaxhighlight>

The <code>else</code> clause in the above example is linked to the <code>for</code> statement, and not the inner <code>if</code> statement. Both Python's <code>for</code> and <code>while</code> loops support such an else clause, which is executed only if early exit of the loop has not occurred.

Some languages support breaking out of nested loops; in theory circles, these are called multi-level breaks. One common use example is searching a multi-dimensional table. This can be done either via multilevel breaks (break out of ''N'' levels), as in bash<ref>Advanced Bash Scripting Guide: [http://tldp.org/LDP/abs/html/loopcontrol.html 11.3. Loop Control]</ref> and PHP,<ref>PHP Manual: "[http://php.net/manual/en/control-structures.break.php break]"</ref> or via labeled breaks (break out and continue at given label), as in Go, Java and Perl.<ref>perldoc: [http://perldoc.perl.org/functions/last.html last]</ref> Alternatives to multilevel breaks include single breaks, together with a state variable which is tested to break out another level; exceptions, which are caught at the level being broken out to; placing the nested loops in a function and using return to effect termination of the entire nested loop; or using a label and a goto statement. C does not include a multilevel break, and the usual alternative is to use a goto to implement a labeled break.<ref>comp.lang.c FAQ list · "[http://c-faq.com/misc/multibreak.html Question 20.20b]"</ref> Python does not have a multilevel break or continue – this was proposed in [https://www.python.org/dev/peps/pep-3136/ PEP 3136], and rejected on the basis that the added complexity was not worth the rare legitimate use.<ref>[http://mail.python.org/pipermail/python-3000/2007-July/008663.html <nowiki>[</nowiki>Python-3000<nowiki>]</nowiki> Announcing PEP 3136], Guido van Rossum</ref>

The notion of multi-level breaks is of some interest in [[theoretical computer science]], because it gives rise to what is today called the ''Kosaraju hierarchy''.<ref name=kozen>{{cite book |first=Dexter |last=Kozen |date=2008 |chapter=The Böhm–Jacopini Theorem is False, Propositionally |title=Mathematics of Program Construction |series=Lecture Notes in Computer Science |doi=10.1007/978-3-540-70594-9_11 |volume=5133 |pages=177–192 |isbn=978-3-540-70593-2 |url=http://www.cs.cornell.edu/~kozen/papers/BohmJacopini.pdf |citeseerx=10.1.1.218.9241}}</ref> In 1973 [[S. Rao Kosaraju]] refined the [[structured program theorem]] by proving that it is possible to avoid adding additional variables in structured programming, as long as arbitrary-depth, multi-level breaks from loops are allowed.<ref>Kosaraju, S. Rao. "Analysis of structured programs," Proc. Fifth Annual ACM Syrup.
Theory of Computing, (May 1973), 240-252; also in J. Computer and System Sciences, 9,
3 (December 1974). cited by {{cite journal
 |last=Knuth |first=Donald
 |author-link=Donald Knuth
 |year=1974
 |title=Structured Programming with go to Statements
 |journal=Computing Surveys
 |volume=6
 |issue=4
 |pages=261–301
 |doi=10.1145/356635.356640
 |citeseerx=10.1.1.103.6084
 |s2cid=207630080
 }}</ref> Furthermore, Kosaraju proved that a strict hierarchy of programs exists: for every integer ''n'', there exists a program containing a multi-level break of depth ''n'' that cannot be rewritten as a program with multi-level breaks of depth less than ''n'' without introducing added variables.<ref name="kozen"/>

One can also <code>return</code> out of a subroutine executing the looped statements, breaking out of both the nested loop and the subroutine. There are other [[#Proposed control structures|proposed control structures]] for multiple breaks, but these are generally implemented as exceptions instead.

In his 2004 textbook, [[David Watt (computer scientist)|David Watt]] uses Tennent's notion of [[S-algol|sequencer]] to explain the similarity between multi-level breaks and return statements. Watt notes that a class of sequencers known as ''escape sequencers'', defined as "sequencer that terminates execution of a textually enclosing command or procedure", encompasses both breaks from loops (including multi-level breaks) and return statements. As commonly implemented, however, return sequencers may also carry a (return) value, whereas the break sequencer as implemented in contemporary languages usually cannot.<ref name="WattFindlay2004b">{{cite book|author1=David Anthony Watt|author2=William Findlay| title=Programming language design concepts| year=2004| publisher=John Wiley & Sons|isbn=978-0-470-85320-7|pages=215–221}}</ref>