Declaring variables: Local vs. Global

[tk1]

Apologies for the rudimentary question, I'm not a programmer by trade, but trying to learn AutoIt and automate a few things in the process.

Would someone please explain the difference between a local variable and a global one?

Back in the 90's when I took a VB6 class, I thought I knew what they were for VB.  Seems to me (if my foggy memory serves me correctly) local meant just for that section (Sub?) of code, whereas global was used for the whole code.  That was too many years ago so I may be all wrong on that.

The results from the forum searches I've done don't seem to give me the answer I'm looking for.

Thanks for the help!

tk

[mLipok]

start reading this:

http://www.autoitscript.com/wiki/Variables_-_using_Global,_Local,_Static_and_ByRef

http://www.autoitscript.com/wiki/Best_coding_practices

[BrewManNH]

I'm not sure I'd point to the second link as there are several issues that I can see with it that would lead to bad coding practices. Such as this line for example.

 

Always initialize a variable on it's declaration, and declare all the same type of variable on the same line.

If you follow this advice, you're apt to declare your variables in a loop, because the value you're assigning to it isn't valid until you're in the loop. Also the second part isn't good coding practice, it's just someone's opinion as to how you should do it.

[JohnOne]

local meant just for that section (Sub?) of code, whereas global was used for the whole code.

Correct.

[guinness]

If you follow this advice, you're apt to declare your variables in a loop, because the value you're assigning to it isn't valid until you're in the loop. Also the second part isn't good coding practice, it's just someone's opinion as to how you should do it.

Initialising variables is good practice for those who wish to branch out to languages such as C++. I have always seen AutoIt as just an intermediate language for those who are scared to work with the big dogs! Though in saying that there is not a single day I don't write a single line of AutoIt.

[junkew]

How is local treated in a module (so outside the function's in a UDF)

Documentation states this

 

But there is no point declaring any variables in the main body of a script as Local - they will be Global regardless. If you use the AutoItSetOption("MustDeclareVars", 1) directive you must declare the scope of your variables or you will get error messages.

so doing

local $NiceCodingPractice ;This is a global variable

will anyway be treated by AutoIT as Global

[BrewManNH]

How is local treated in a module (so outside the function's in a UDF)

Documentation states this

so doing

local $NiceCodingPractice ;This is a global variable

will anyway be treated by AutoIT as Global

That is correct, there is no file scope in AutoIt, anything declared outside of a function is a Global variable.

[tk1]

start reading this:

http://www.autoitscript.com/wiki/Variables_-_using_Global,_Local,_Static_and_ByRef

http://www.autoitscript.com/wiki/Best_coding_practices

 

Had to read it a few times, but that first link, along with a little common sense, I think gives me what I was looking for.

 

I'm not sure I'd point to the second link as there are several issues that I can see with it that would lead to bad coding practices...

If you follow this advice, you're apt to declare your variables in a loop, because the value you're assigning to it isn't valid until you're in the loop. Also the second part isn't good coding practice, it's just someone's opinion as to how you should do it.

 

Thank you for the input on the second link also!

 

...anything declared outside of a function is a Global variable.

So, (not trying to sound flipant here...) declaring the scope only really matters when inside a function --other than as a good practice, like guinness stated.

Thanks for all the replies, I'm sorry I haven't been in a position to reply until now.

 

tk

[BrewManNH]

Declaring it correctly only matters inside a function. What I mean by that is,

• Don't use Dim unless you do some reading up on it as it WILL bite you in the ass if you're not using it correctly.
• Don't declare globals inside Functions ever, no matter what arguments people make about this, they're going to be wrong
• Don't bother declaring variables as Locals outside of functions (because they're not local outside of a function), use Global instead.
• If you use the AutoItSetOption("MustDeclareVars", 1) then you'll be sure that you're declaring them before trying to use them.
• Also, another good check is to use "#AutoIt3Wrapper_AU3Check_Parameters=-d -w 1 -w 2 -w 3 -w- 4 -w 6" at the top of your script to make sure that all your variables are being used correctly.

[guinness]

I disagree with point 3, but have made my opinion known on this matter more than once.

[czardas]

I would reword point 3.

• Don't bother declaring variables.

Seriously though, former developer did know what he was talking about. Local refers to current scope, whatever that may be. This gives me an idea for a storyline in my new comic.

Edited December 11, 2013 by czardas

[jchd]

About point 3: my humble opinion is to use Global for variables used in more than one source file and always use Local for variables uses only within the current source. My point is that it introduces a file scope similar to what is common in other languages and -who knows- it's quite possible that AutoIt actually introduces file scoping in some future.

[BrewManNH]

Well, that's why I put 'don't bother', because there is no such thing as file scope in AutoIt as it stands at this point in time. Why confuse the issue?

[czardas]

I only ever use the Local keyword inside functions, unless the code is unfinished or posted here as an example - so as to imply intended use within a function and avoid inappropriate use within the final release. To me - using the Local keyword outside a function is like writing pseudo code.

This appears to be a contradiction.

Global $vReturn ; This variable belongs inside a function.

;

This appears to be unfinished

Local $vReturn ; This is an example.

;

This is correct usage.

Func _MyFunc()
    Local $vReturn ; The variable now has limited scope.
EndFunc

Edited December 11, 2013 by czardas

[jdelaney]

I'd emphasize the need to set Local in functions, to make sure the 'global' (or 'local' outside of the function...whatever) variables are not overwritten by resetting the value inside functions.

That bit me in the behind, when I first started...example with booleans being switched in a function, when outside the function I'd have liked to proceed...such as "If $bContinue Then" wrapper around code blocks.

Edited December 11, 2013 by jdelaney

[czardas]

I really don't see any need for confusion about this.

I'd emphasize the need to set Local in functions, to make sure the 'global' (or 'local' outside of the function...whatever) variables are not overwritten by resetting the value inside functions.

 

This is not practical. Nearly all users modify globals indide functions as a matter of course.

[jdelaney]

This is not practical. Nearly all users modify globals indide functions as a matter of course.

 Yes, and that's a set upon action to overwrite those variables (I would argue, that this can make for a debugging nightmare).  I'm calling into light those variables you don't expect to override globals, but will indeed do so, if you do not locally declare them.

Example, in all my functions, I have some variables that are independent, but are always the same name.  I don't want those to interact with each other, so I locally declare them.

It's also my practice to not declare globals in functions, but to instead set those globals through returns, byrefs, etc...if returns need more than one value set, then arrays work just fine.

Edited December 12, 2013 by jdelaney

[czardas]

There is no way to differentiate between erroneously undeclared local variables and globals you intentionally modify.

Edited December 12, 2013 by czardas

[jdelaney]

It's straight from the help file.  By adding the slight effort of declaring scope, you can save tons of debugging issues in the long run...this is the only point I'm trying to make:

By default when variables are declared using Dim or assigned in a function they have Local scope unless there is a global variable of the same name (in which case the global variable is reused).  This can be altered by using the Local and Global keywords to declare variables and force the scope you want.

Edited December 12, 2013 by jdelaney

[czardas]

By adding the slight effort of declaring scope, you can save tons of debugging issues in the long run...this is the only point I'm trying to make:

 

Ah okay. (Edit - I thought you meant something else).

By default when variables are declared using Dim or assigned in a function they have Local scope unless there is a global variable of the same name (in which case the global variable is reuse

 

I no longer use Dim and I wasn't aware it had this property.

Edited December 12, 2013 by czardas