If then compare not working

[EmilyLove]

So what I want to happen is when I run the script on a nonNTFS file system it should fail to run and state an error message box. Instead, it fails at the comparing at the If then statement. No matter what the output is for DriveGetFileSystem (which is working properly), it always fails to compare it even when the strings are matching.

;ProblemDemoScript
Local $DriveLetter = StringLeft(@ScriptDir, 2) ;Get Drive Letter and Store it.
MsgBox(0,"Drive Letter",$DriveLetter) ;Echo Drive Letter
MsgBox(0,"Drive Letter",DriveGetFileSystem($DriveLetter)) ;Echo File system
If not DriveGetFileSystem($DriveLetter) = 'abcd' Then;intentional set wrong to show it failing. Even with not remove it still fails to properly compare. (problem is here)
  MsgBox(0, "Safety Engaged", 'Not using NTFS FileSystem. Currently using "'&DriveGetFileSystem($DriveLetter)&'"');should be called under normal conditions.
  Exit
Else
  MsgBox(0,"No Problems","Successful");should never be called since 'abcd' isn't a real file system and not a possible output.
EndIf

Edited February 13, 2015 by BetaLeaf

[Jos]

Use:

If not (DriveGetFileSystem($DriveLetter) = 'abcd') Then

Jos

[EmilyLove]

Use:

If not (DriveGetFileSystem($DriveLetter) = 'abcd') Then

Jos

Thanks you very much for your help. I implemented this and it's working properly. Now I have another question. Why is the () around the actual command necessary? I want to be able to understand it ya know?

Edited February 12, 2015 by BetaLeaf

[mikell]

It's because of the operators precedence

As the NOT has a higher precedence than "=" it occurs before the = operation

[Jos]

Thanks you very much for your help. I implemented this and it's working properly. Now I have another question. Why is the () around the actual command necessary? I want to be able to understand it ya know?

So translated to what is state: your initial statement does this:

If (not DriveGetFileSystem($DriveLetter)) = 'abcd' Then

Jos

[EmilyLove]

It's because of the operators precedence

As the NOT has a higher precedence than "=" it occurs before the = operation

I'm afraid I still don't understand. 

[EmilyLove]

So translated to what is state: your initial statement does this:

If (not DriveGetFileSystem($DriveLetter)) = 'abcd' Then

Jos

The first one you posted works fine. Should I be using this one instead?

[Jos]

The first one you posted works fine. Should I be using this one instead?

No because this is what happened with your initial If which wasn't working!

It was comparing not DriveGetFileSystem($DriveLetter) to 'abcd' which ofcourse fails.

Jos

[EmilyLove]

No because this is what happened with your initial If which wasn't working!

It was comparing not DriveGetFileSystem($DriveLetter) to 'abcd' which ofcourse fails.

Jos

I still don't get it. Let me rephrase my original question. When should I put () around the entire statement as in your first example? I want to be able to understand this but I have autism. Could you make a few examples of what does and doesn't work and explain why. Maybe that will help me understand it if I can see examples. To me, when I look at If not drivegetfilesystem($driveletter) = 'abcd' shouldn't it just know that I mean If this statement is not this, then do this?

I really want to be able to understand this. I don't like asking for help and perfer to be self reliant.

Edited February 12, 2015 by BetaLeaf

[Melba23]

BetaLeaf,

AutoIt, like all programming languages, has an "operator precedence" list - for AutoIt this is set out in the Help file:

 

When more than one operator is used in an expression the order in which things happen is controlled by operator precedence. The precedence used in AutoIt is given below. Where two operators have the same precedence the expression is evaluated left to right.

From highest precedence to lowest:

Not

^

* /

+ -

&

< > <= >= = <> ==

And Or

So let us run through what happened in your comparison:

; Original
not DriveGetFileSystem($DriveLetter) = 'abcd'

; DriveGetFileSystem always returns a value (either string or numeric)
; NOT is a Boolean operator and AutoIt interprets a value as TRUE, an empty string (") or 0 as FALSE
; So this line reads to AutoIt as
not (TRUE) = "abcd"

; Highest operator is "NOT" and the line becomes
FALSE = "abcd"

; Which is obviously not what you wanted to happen

Happy so far?

Now let us force operator precedence by adding parentheses which AutoIt actions before external operators

; Original
not (DriveGetFileSystem($DriveLetter) = 'abcd')

; AutoIt actions the code inside the parentheses first and so we get (assuming the comparison is TRUE)
not (TRUE)

; Which is what you wanted to happen

All clear?

M23

[EmilyLove]

BetaLeaf,

AutoIt, like all programming languages, has an "operator precedence" list - for AutoIt this is set out in the Help file:

 

When more than one operator is used in an expression the order in which things happen is controlled by operator precedence. The precedence used in AutoIt is given below. Where two operators have the same precedence the expression is evaluated left to right.

From highest precedence to lowest:

Not

^

* /

+ -

&

< > <= >= = <> ==

And Or

So let us run through what happened in your comparison:

; Original
not DriveGetFileSystem($DriveLetter) = 'abcd'

; DriveGetFileSystem always returns a value (either string or numeric)
; NOT is a Boolean operator and AutoIt interprets a value as TRUE, an empty string (") or 0 as FALSE
; So this line reads to AutoIt as
not (TRUE) = "abcd"

; Highest operator is "NOT" and the line becomes
FALSE = "abcd"

; Which is obviously not what you wanted to happen

Happy so far?

Now let us force operator precedence by adding parentheses which AutoIt actions before external operators

; Original
not (DriveGetFileSystem($DriveLetter) = 'abcd')

; AutoIt actions the code inside the parentheses first and so we get (assuming the comparison is TRUE)
not (TRUE)

; Which is what you wanted to happen

All clear?

M23

 

Yes, This explained it for me. Thank you for taking the time to explain it. I sincerely appreciate it. Have a great day. /Close_thread