Flow-control structures: Difference between revisions

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In this task, we document common flow-control structures. One common example of a flow-control structure is the <code>goto</code> construct. Note that [[Conditional Structures]] and [[Loop Structures]] have their own articles.

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In this task, we document common flow-control structures. One common example of a flow-control structure is the <code>goto</code> construct. Note that [[Conditional Structures]] and [[Loop Structures]] have their own articles.

=={{header|Ada}}==

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===goto===

Goto is typically looked down upon by most ~~perl~~ programmers

Goto is typically looked down upon by most Perl programmers

FORK:

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===goto===

goto l transfers control to the label l. goto may be used to exit from

nested loops:

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</pre>

Pop11 goto is a nonlocal one, so "jump out" from a chain of procedure

Pop11 goto is a nonlocal one, so "jump out" from a chain of procedure calls:

calls:

<pre>

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</pre>

This is useful to exit early from ~~succesful~~ recursive search, and for

This is useful to exit early from successful recursive search, and for exception handling.

exception handling.

===go_on===

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</pre>

If expression has value K the above will jump to label labK, if

If expression has value K the above will jump to label labK, if expression is not an integer, or if it outside range from 1 to N, then control passes to label elselab. The else part may be omitted (then out of range values of expression cause an exception).

expression is not an ingeger, or if it ouside range from 1 to N,

then control passes to label elselab. The else part may be

omitted (then out of range values of expression cause an exception).

There is a more structured variant of go_on:

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endgo_on;

where lab is a prefix ~~choosen~~ by the user.

where lab is a prefix chosen by the user.

===return===

return ends execution of current function. In simplest form

it is just:

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===chain===

chain has effect of "tail call" but is not ~~nececcarly~~ in tail position

chain has effect of "tail call" but is not necessarily in tail position. More precisely inside proc1.

More precisely inside proc1

<pre>

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</pre>

finishes execution of proc1 and transfers control to

finishes execution of proc1 and transfers control to the proc2 passing it x1, x2, and x3 as arguments. On return from proc2 control passes to caller of proc1.

the proc2 passing it x1, x2, and x3 as arguments.

On return from proc2 control passes to caller of proc1.

Remark: Pop11 does not perform "tail call optimization", one has to

Remark: Pop11 does not perform "tail call optimization", one has to explicitly use chain.

explicitely use chain.

=={{header|Python}}==

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===The "with" statement===

'''Interpreter:''' [[Python]] 2.6

[[Category:Python]]

See [[http://www.python.org/peps/pep-0343.html PEP 0343, The "with" statement]]

>>> with open("some_file"): # file ``some_file`` is closed after ``with`` block in any case whether an exception is raised or not

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===Yield expressions===

'''Interpreter:''' [[Python]] 2.5

[[Category:Python]]

See [[http://www.python.org/peps/pep-0342.html PEP 0342, Coroutines via Enhanced Generators]]

>>> value = 1