Control flow: Difference between revisions

Revision as of 02:28, 12 February 2005

Revision as of 04:41, 12 February 2005

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using nested loops (see Missing Control Structures below).

== Structured non-local control flow ==

== Error recovery ==

Many programming languages, particularly those which favor more dynamic styles of programming, offer constructs for '''non-local control flow'''. These cause the flow of execution to jump out of a given context and resume at some predeclared point. ''[[Exception]]s'', ''[[condition]]s'', and ''[[continuation]]s'' are three common sorts of non-local control constructs.

Most programming languages have some way

in which a program can detect that end-of-file has been reached when reading data,

=== Conditions in PL/I ===

but very few programming languages have any systematic way of handling the situation

when something goes wrong or something unusual happens.

[[PL/I programming language|PL/1]] has some 22 standard conditions

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(especially SUBSCRIPTRANGE), so many programmers tried to avoid using conditions.

=== Exceptions in C++ and derived languages ===

[[C Plus Plus|C++]], [[D programming language|D]], [[Java programming language|Java]], and [[C sharp|C#]] have a special construct for exception handling:

'''try''' {

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}

Any number and variety of '''catch''' clauses can be used above. In D, Java and C#, a '''finally''' clause can be added to the '''try''' construct. No matter how control leaves the '''try''' the code inside the '''finally''' clause is guaranteed to execute. This is useful when writing code that must relinquish an expensive resource (such as an opened file or a database connection) when finished processing:

clause can be added to the '''try''' construct. No matter how control leaves the '''try'''

the code inside the '''finally''' clause is guaranteed to execute. This is useful when

writing code that must relinquish an expensive resource (such as an opened file

or a database connection) when finished processing:

FileStream stm = null; // C# example

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Other languages such as [[Python programming language|Python]], [[Ruby programming language|Ruby]], and [[Objective C programming language|Objective C]] (and many others) support similar exception handling constructs.

''Can anyone add something helpful about these or other programming languages?''

''OK:''

The [[AppleScript]] [[script language]] provides several pieces of information to a "try" block:

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== Missing control structures ==

In his 1974 [http://pplab.snu.ac.kr/courses/PL2001/papers/p261-knuth.pdf article], [[Donald Knuth]] identified two situations which were not covered

by the control structures listed above, and gave examples of control structures which could handle these situations. Despite their utility, these constructions have not yet found their way into main-stream programming languages.

[[Donald Knuth]] identified two situations which were not covered

by the control structures listed above,

and gave examples of control structures which could handle these situations.

Despite their utility, these constructions have not yet found their way

into main-stream programming languages.

=== Loop with test in the middle ===