Variant, string, and number

[Dana]

I'm trying to understand how AutoIt treats variables. I understand that everything is a variant, and that comparisons (< or >) treat a variable as a string even if the data is numeric (which is what was confusing me, as it has confused others before). In the case of my script, I learned I can solve the problem either by using Number() or multiplying the variable by 1. I understand that using either technique to convert the string into a number makes the comparison work properly.

I can see that using Number($var) or $var*1 as input to the comparison should work. What I don't understand is why it also works if I do the conversion earlier and store that result in a new variable (in this case an element of an array). Once the numeric data is stored in a new variable, which again is a variant, how does AutoIt know later that it's still a number and not a string?

I understand why this doesn't work (returns false):

$line1 = "<ele>99.511963</ele>"
$ele1 = StringMid($line1, (StringInStr($line1, '<ele>') + 5), StringInStr($line1, '</ele>') - (StringInStr($line1, '<ele>') + 5))
$line2 = "<ele>125.511963</ele>"
$ele2 = StringMid($line2, (StringInStr($line2, '<ele>') + 5), StringInStr($line2, '</ele>') - (StringInStr($line2, '<ele>') + 5))
ConsoleWrite($ele2 > $ele1)

I understand why this does work (returns true):

$line1 = "<ele>99.511963</ele>"
$ele1 = StringMid($line1, (StringInStr($line1, '<ele>') + 5), StringInStr($line1, '</ele>') - (StringInStr($line1, '<ele>') + 5))
$line2 = "<ele>125.511963</ele>"
$ele2 = StringMid($line2, (StringInStr($line2, '<ele>') + 5), StringInStr($line2, '</ele>') - (StringInStr($line2, '<ele>') + 5))
ConsoleWrite(Number($ele2) > Number($ele1))

I understand [i think] why this works (returns true):

$line1 = "<ele>99.511963</ele>"
$ele1 = StringMid($line1, (StringInStr($line1, '<ele>') + 5), StringInStr($line1, '</ele>') - (StringInStr($line1, '<ele>') + 5))
$line2 = "<ele>125.511963</ele>"
$ele2 = StringMid($line2, (StringInStr($line2, '<ele>') + 5), StringInStr($line2, '</ele>') - (StringInStr($line2, '<ele>') + 5))
ConsoleWrite($ele2*1 > $ele1*1)

I'm not sure why this works (returns true)... once one of the values being compared is a number does AutoIt treat both as numbers?:

$line1 = "<ele>99.511963</ele>"
$ele1 = StringMid($line1, (StringInStr($line1, '<ele>') + 5), StringInStr($line1, '</ele>') - (StringInStr($line1, '<ele>') + 5))
$line2 = "<ele>125.511963</ele>"
$ele2 = StringMid($line2, (StringInStr($line2, '<ele>') + 5), StringInStr($line2, '</ele>') - (StringInStr($line2, '<ele>') + 5))
ConsoleWrite(Number($ele2) > $ele1)

Using "*1" instead of Number() on just one of the numbers also works.

I don't understand why this works:

$line1 = "<ele>99.511963</ele>"
$ele1 = Number(StringMid($line1, (StringInStr($line1, '<ele>') + 5), StringInStr($line1, '</ele>') - (StringInStr($line1, '<ele>') + 5)))
$line2 = "<ele>125.511963</ele>"
$ele2 = StringMid($line2, (StringInStr($line2, '<ele>') + 5), StringInStr($line2, '</ele>') - (StringInStr($line2, '<ele>') + 5))
ConsoleWrite($ele2 > $ele1)

In this one, the Number() function makes the string into a number, but then it's stuffed into $ele1, which is again a variant... so why does the last line still treat it as a number and not a string?

[water]

... and that comparisons (< or >) treat a variable as a string even if the data is numeric ...

I think this assumption is wrong. AutoIt treats a string as a string when it's a string.

So "a" < "b" is a string comparison but 3 < 4 is a numeric comparison. You only run into problems when a function you call returns a string (like FileRead) but you assume it's numeric. When a variable has been transformed from string to numeric it stays numeric until it is transformed to string again.

That's how I understand it - but I could be wrong

Edited December 6, 2011 by water

[Shaggi]

Variant is a term that describes a containers ability to hold multiple types (string, integer etc.). If this should work without undefined behaviour, the container needs to keep track of its own type - which it does, ofcourse.

An autoit variable is a variant, that knows its own type. it can be changed on the fly, and comparison + math operations differs in behaviour depending on the type used. You can read this: http://www.autoitscript.com/autoit3/docs/intro/lang_datatypes.htm

[water]

That describes it much better than I could ever have done (pitty not being a native speaker)

[Dana]

... the container needs to keep track of its own type..... it can be changed on the fly...

Thanks, that makes sense... there's more information in a variant than the visible data itself.

So ConsoleWrite("2" + "2") returns 4 even though it's being fed two strings, changing them on the fly because of the "+" operator, and ConsoleWrite(2 & 2) returns 22 because of the "&" operator... and only the greater than / less than operators are ambiguous (not really, but the supplied value needs to be unambiguous):

> Tests if the first value is greater than the second. Strings are compared lexicographically even if the contents of the string happen to be numeric.

As water said, I ran into trouble because my input string $line came from a FileRead operation and stayed a string through the string manipulations in the second line of my examples. I didn't have this problem elsewhere because other numbers I read in from the file had strictly numeric math operations performed on them rather than comparisons.

I guess it's good practice to use the Number() function on any numeric data read in from a file, or subject to string manipulation.

[czardas]

Although using Number() should always work, I still think it's best to check what happens with different types of function. Sometimes you can omit Number() , since the conversion will take place within the function you are calling. You want to avoid unnecessary duplication of arguments.

water, I never would have guessed your native langage was not English.

Edited December 6, 2011 by czardas

[Attckdog]

water, I never would have guessed your native langage was not English.

Read tons of his comments and never once would have thought. Makes me wonder what his natural tongue is.

[Jos]

Read tons of his comments and never once would have thought. Makes me wonder what his natural tongue is.

Mmm.. His sig says Austria so my educated guess would be a sort of German variation.

[water]

"Sort of German variation" hits the nail on the head

[water]

water, I never would have guessed your native langage was not English.

Hi Czardas,

thanks for this complement! I'm glad that this is not a french forum