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Tuesday, January 31, 2006

 


about_Switch Documentation



Jeff Jones, just provided the "Missing" Switch documentation
(See also Last Post Switch -regEx )

An interesting option is the -File switch.

gr /\/\o\/\/
Tags :

as I can not Link to the Attachement in the NG I will post the Text here as reference.

about_Switch

SHORT DESCRIPTION
Using a switch to handle multiple if statements.

LONG DESCRIPTION

You use an If statement to make a decision in a script or program.
Essentially it says; “If this condition exists, do this action.
Otherwise do that action.” You can perform that operation as many
times as you want but if you have a long list of conditions, an If
statement quickly gets unwieldy. With a long list of conditions you
can combine them in a switch statement. As in all branching
statements, braces ({}) are required to enclose script blocks.

A switch statement is, in effect, a series of If statements. It matches
the expression with each of the conditions case by case. If a match
is found, the action associated with that condition is performed. The
switch statement, very basically, takes this form:

MSH> $a = 3
MSH> switch ($a) {
1 {"It’s one"}
2 {"It’s two"}
3 {"It’s three"}
4 {"It’s four"}
}
It’s three

This simple example takes a value and compares it with each condition
in the list. The action just echoes a string from the match. But you
could face a problem if you check all of the conditions.
For example:

MSH> $day = "day5"
MSH> switch ($day){
day1 {"They call it stormy Monday"; break}
day2 {"Tuesday's just as bad"; break}
day3 {"Wednesday's worse"; break}
day4 {"Thursday's oh so sad"; break}
day5 {"The eagle flies on Friday"; break}
day6 {"Saturday I go out to play"; break}
day7 {"Sunday I go out to play"; break}
day5 {"Wait, too many days."; break}
}

The eagle flies on Friday

There are 2 day5 conditions in the list. But the break at the end of
each condition tells the switch to stop looking further and do the
action it finds. If the break statements were not there, then both
day5 actions would take place.

If the value to switch against is an array, then each element in the
array will be evaluated in order, starting at element 0. At least
one element must be present that meets at least one condition or
an error will result. If there is more than one default clause, an
error will result.

The complete switch syntax is as follows:

switch [-regex-wildcard-exact][-casesensitive] ( pipeline )
or
switch [-regex-wildcard-exact][-casesensitive] -file filename
followed by
{
"string"numbervariable{ expression } { statementlist }
default { statementlist }
}

By default, if no options are used, switch behaves as if a case
insensitive exact match is in effect. If "pipeline" results in an
array, each element of the array shall be evaluated in ascending offset
order (starting at 0).

At least one conditional element must be present in the switch
codeblock and only one default clause may be present. If more than
one default clause is present, a ParseException shall be thrown.

Switch has the following options:

-regex Indicates that the match clause, if a string, is
treated as a regex string. Use of this parameter
disables -wildcard and -exact. If not a string,
this option is ignored.

-wildcard Indicates that the match clause, if a string, is
treated as a wildcard string. Use of this
parameter disables -regex and -exact. If not a
string, this option is ignored.

-exact Indicates that the match clause, if a string, must
match exactly. Use of this parameter disables
-wildcard and -regex.. If not a string,
this option is ignored.

-casesensitive Modify the match clause, if a string, to be case
sensitive. If not a string, this is ignored.

-file Take input from a file (or representative) rather
than statement. If multiple -file parameters are
used, the last one is be used. Each line of the
file is read and passed through the switch block.

Multiple uses of -regex, -wildcard or -exact are allowed, however only
the last parameter used governs the behavior.

The keyword "break" indicates that no more processing shall occur and
the switch statement shall exit.

The keyword "continue" indicates that no processing shall continue
against the current token and the next token in the conditional will
be evaluated. If no tokens are available, the switch statement will
exit.

The “{ expression }” block may be a code block that will be evaluated
at the time of the comparison. The object under scrutiny is bound to
the automatic variable "$_" and is available during the evaluation of
the expression. A comparison is considered a match if the expression
evaluations to "true". This expression is evaluated in a new scope.

The “default” keyword within the switch statement indicates that if
no matches are found, the code block that follows the keyword shall
be evaluated. Program flow shall not be allowed from stanza to
stanza (e.g., the closing “}” in the compound-list is an explicit
"break". )

If multiple matches are found, each match shall result in the
expression being executed. To avoid this, the break or continue
keywords may be used to halt further comparisons.

SEE ALSO
For information about break, enter the following command
at the MSH command prompt:


help about_break


For information about continue, enter the following
command at the MSH command prompt:


help about_continue


For information about the if conditional, enter the following
command at the MSH command prompt:


help about_if


For information about script blocks, enter the following command at
the MSH command prompt:


help about_Script_block



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