Semiconductor & Computer Engineering
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goto label | goto label | ||
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| − | </syntaxhighlight>The first /goto label statement gets mIRC to parse the rest as token, probably just words, and :label is found, %test is increased to 1 and then to 2, proving that the label is found. | + | </syntaxhighlight>The first /goto label statement gets mIRC to parse the rest as token, probably just words, and :label is found despite the if statement being false, %test is increased to 1 and then to 2, proving that the label is found from the first time with the first goto label (forward jump) and then backward since the label was found the first time. |
== Conditional transfer of control == | == Conditional transfer of control == | ||
Revision as of 16:36, 8 April 2014
Template:mIRC Guide Goto is a command that allows you to jump unconditionally to a specific location within a procedure. Gotos can 'jump' forward or backward within a script but they may not leave the alias or event itself (they cannot jump to any calling routine as well). The goto command tells mIRC to jump to another line which matches a label.
Although in many cases the use of gotos can often lead to spaghetti-code. and can usually be replaced with easier to read and follow while statements and if statements, it is still important to understand this command and have it in your toolbox.
Contents
Syntax
The goto command has the following syntax:
goto label
A line is labeled using the following syntax:
:label
The label has to start with the colon symbol (:) and must be a single word.</p>
Jumping
There is a neat difference in the way mIRC jumps backward and the way it jumps forward. To get mIRC to reach a goto label that is before the current position, mIRC must has seen that goto label, for example:
alias test { if (a == b) { :label } goto label }
displays "* /goto: 'label' not found (line N, script.mrc)" since the condition does not get executed, the goto label is not seen by mIRC.
Change the code to:
alias test { if (a == a) { :label inc -us %test if (%test == 2) return } goto label }
Now the goto is seen by mIRC and you should see %test being increased to 1 and then to 2.
To get mIRC to jump to a goto label that is after the current position, this is less strict: mIRC parse the code without executing it, which means it is basically parsing the code according to major rules defining the language, the '{', '}', '|' and newlines tokens. so considering the following:
alias test { goto label if (a == b) { :label inc -us %test if (%test == 2) return } goto label }The first /goto label statement gets mIRC to parse the rest as token, probably just words, and :label is found despite the if statement being false, %test is increased to 1 and then to 2, proving that the label is found from the first time with the first goto label (forward jump) and then backward since the label was found the first time.
Conditional transfer of control
Many times you only want a script to go to a specific location according to a certain condition. You can use an if statement to achieve this. A simple syntax looks like this:
if (<condition>) { goto label }
Goto Loops
Goto statements can also be used to create a loop by jumping back to a previous location in the script. The example below is a simple loop counting from 1 to 10. If for any reason you mistakenly caused your script to execute endlessly, you manually break the loop using the Ctrl+Break keys.
Alias Count { :loop var %c $calc(%c + 1) echo -a %c if (%c <= 9) goto loop }
Random Name Example
Sometimes a goto can result in smaller code if we need a do-while style loop like other languages. A practical example of this is a random nickname generator which excludes us from the list. Such alias might look like this:
; precondition: $nick($chan, 0) > 1 alias rnd { :retry if ($nick($chan, $rand(1, $nick($chan, 0))) == $me) goto retry return $v1 }
Error Handling
Although it is rare to use this feature, the "error" label is a designated goto section for error checking.
If there is an error in the script, mIRC try to find an :error label in the current routine. If no :error label is found in the current routine, mIRC will propagate the error backward to any calling routine and look for an :error label in those routines.
This gives you the ability to continue with the script regardless of the error if you want.
After catching an error with :error, you must check for an error by checking $error with a typical /if statement to make sure you reached the :error part because of an error and not because the script is just reaching that point.
If you want to keep going with the code, use /reseterror as soon as possible, this reset the error (and therefore $error's value), mIRC leaves the error state and you can safely execute anything you want. If you try to execute anything else than /reseterror after reaching an :error label because of an error, mIRC can broke (see examples). Indeed, here mIRC is in an error state, yet it must still process your code somehow to allow you to *at least* check for error and reset it, so it's unclear what you can do and what you can't do besides an if statement on $error followed by /reseterror. You would think that since mIRC is somehow forced to allow you to use the scripting engine to check for error and reset it, you could be able to do anything even without calling /reseterror first (still after checking for $error), this is not true, the exact list of features that are working in this situation are unknown, it's recommended not to do anything before using /reseterror in this situation.
Alias Example { /* Second parameters of the $rand identifier is missing. Deliberate Error */ echo -a $rand(1,) return :error echo -a Error: $error } ;In this case, we try to execute something else than /reseterror after reaching :error, for some reasons, that $regsubex gets mIRC to silentely halts while inside a routine ;You should get the first echo 'ok' and an echo '> $str(ab,2000)' is expected, but you shouldn't see it. alias testgoto { if :error echo -a ok | echo -a > $regsubex($str(ab,2000),/(a*)b*(a*)*b/,) } /* Although this example might look a little confusing, bear with me: Calling alias /Foo will echo what alias FooBar returns. Alias foobar will return what alias bar returns. Because alias bar errors out (since $mid is missing a few parameters), mIRC will look for an :error label in alias Bar but won't find any. So it will look in the previous routine, the alias FooBar. an :error label is found in the alias FooBar. It is executed, echoing Error followed by the error message and returning 1000 to alias foo. The /reseterror command effectively prevents mIRC from halting the script and allows it to finish executing. */ Alias Foo { echo -a $FooBar } Alias Bar { return $mid($1) } Alias FooBar { return $Bar(Example) :error echo -a Error Using Value 1000 instead! ( $+ $error $+ ) reseterror return 1000 }