Are LabVIEW Strings Immutable?

[mje]

Easy question: Are LabVIEW strings immutable?

If I'm working with large strings, and changing individual characters in the string, does each operation force a new string to be allocated? Or does the compiler internally operate on the string like an array if the operation does not force a frame shift, length change, or what have you?

[asbo]

It looks that way. I checked the buffer allocations on two different string functions I think shouldn't need to re-allocate the string, but both showed allocations.

[mje]

Good point, although keep in mind the allocation dots only say a buffer might be allocated. Of course I can't think of a single case where reversing a string would ever not be able to operate in place, so I expect that it tells the compiler it might allocate, in reality it always does...

[Aristos Queue]

LabVIEW strings are not immutable. The compiler does consolidate and modify. What have I repeatedly said? "Buffer allocation dot is not the same as a copy". A buffer allocation is a place where we can *hold onto* a string. Two string buffer allocations would mean that there are two strings, but we can modify one of them and just swap it into the next position, without making a copy.

All LV data types are mutable and subject to this inplaceness analysis.

For the special case of "reverse a string", we just set a flag on the string that says "this string should be traversed in reverse", so it is a constant time operation.

[GregR]

Before someone times this and realizes it is not true. Although we have some internal support that could someday allow reverse string to be constant time, it is not. Reverse string will actually swap all the characters in the string. Reverse array on the other hand is a constant time operation.

[asbo]

LabVIEW strings are not immutable. The compiler does consolidate and modify. What have I repeatedly said? "Buffer allocation dot is not the same as a copy". A buffer allocation is a place where we can *hold onto* a string. Two string buffer allocations would mean that there are two strings, but we can modify one of them and just swap it into the next position, without making a copy.

I hadn't heard that before, thanks for the clarification.

[Aristos Queue]

Read all the documentation about "inplaceness"

Before someone times this and realizes it is not true. Although we have some internal support that could someday allow reverse string to be constant time, it is not. Reverse string will actually swap all the characters in the string. Reverse array on the other hand is a constant time operation.

My mistake. I thought they were both that way. Thanks, Greg.

[BrentonLang]

Before someone times this and realizes it is not true. Although we have some internal support that could someday allow reverse string to be constant time, it is not. Reverse string will actually swap all the characters in the string. Reverse array on the other hand is a constant time operation.

Is this also the case for ARM Development? As LabVIEW generates C code?

[Wire Warrior]

Okay what is the "allocation dot" I have not heard of that before. Sounds like a good thing to know.

Thanks

jason

[mje]

The little dots that appear on the diagram when you use the Tools > Profile > Show Buffer Allocations... option.

Just be aware, that a buffer allocation doesn't mean that a copy will happen. It's ambiguous, I know, but it's all we got.

Thanks for the info, AQ.