[ruby-core:103056] [Ruby master Misc#17751] Do these instructions (<<, +, [0..n]) modify the original string without creating copies?

Issue #17751 has been updated by mame (Yusuke Endoh).

Status changed from Open to Closed

> When you say **"may take a long time"** does it apply even if **'str'** is 3GB and **'header'** is only 5bits?

Yes, it does. `String#prepend` need to memmove the content of `str`, which may take time proportional to the length of `str`.

> Another solution to my problem would be to be able to write data to a file from any position I want (in bits) by replacing its content.

Well, do you really want to deal with bits, not bytes? If so, you cannot simply use Strings because they are byte-oriented (or character-oriented).


From only your statements, I think you need to implement a bit stream. `String#getbyte` and `String#setbyte` may be helpful to do that.
(BTW, your question looks like https://en.wikipedia.org/wiki/XY_problem to me. I recommend you to state what you want to do eventually.)

Anyway, this issue tracker is for bug reports and improvement proposals. If you want to ask a question, please use [e-mail] (See https://www.ruby-lang.org/en/community/mailing-lists/), or Q&A site like stackoverflow.

----------------------------------------
Misc #17751: Do these instructions (<<,+,[0..n]) modify the original string without creating copies?
https://bugs.ruby-lang.org/issues/17751#change-91117

* Author: stiuna (Juan Gregorio)
* Status: Closed
* Priority: Normal
----------------------------------------
In my program a string increases considerably in size inside a loop, at the end of that loop a header is created that will have to go to the beginning of that string.

During the whole loop:

``` ruby
str << "some data"
```


At the end:

``` ruby
header = "other data"
str = header + str
```

I understand that using (+) creates a copy to then modify the original variable, that is not desirable, I would like to do something similar to (<<), which I understand does not create copies.

If I do this:
``` ruby
header << str
```

I would have two variables with a very large size.

I also have this other code and I don't know if it is an "in place" modifier:
``` ruby
str = "12345"
str[0..2] = ""
#s => 45
```


In short, I want to know what instructions I should use to remove a given range from a string and how to concatenate to both the beginning and end of the target string without having to create copies.



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