How should I convert a slice to a string in golang?

Question

I'm using strings.Split to split a string.

I would like my program to retain one of the elements of the array and release the underlying array.

Unfortunately I can't figure out how to convert a slice of a string into a string that doesn't refer to the underlying string.

Am I supposed to do something like this:

func unslice(s string) (string) {
  return string([]byte(s))
}

The background is:

the underlying string is very large
the slice I want to retain is very small
the slice I want to retain will be retained for a long time
the program will run for a long time - weeks or more
during the lifetime of the program it will split many of these strings (millions)

Here is an example in response to the comment.

func takesBigStringOften(big string) {
    parts := strings.Split(big, " ")

    saveTinyStringForALongTime(parts[0])
}

@MuffinTop I am not sure if it does what I want. I'm also not sure if there is a standard way of doing this. I imagine that this is a very common thing so I was surprised I couldn't find anything by googling. — Coder
– Coder, Commented Mar 13, 2016 at 20:48
@XXXX I added an example, although I'm not sure what you're looking for. — Coder
– Coder, Commented Mar 13, 2016 at 20:52

WeakPointer · Accepted Answer · 2016-03-14 17:13:46Z

3

Just as some further information. Some benchmark code and memory profiling shows that as of go 1.5.3, both methods allocate the same amount of memory from the heap, i.e. a new copy is made either way. In building a string from a byte slice, the compiler calls a routine that makes a unique copy of the bytes - since strings are immutable and byte slices are not.

$ go tool pprof -alloc_space so002.test cprof0
Entering interactive mode (type "help" for commands)
(pprof) list copy
Total: 9.66MB
    9.62MB     9.62MB (flat, cum) 99.55% of Total
         .          .     15:
         .          .     16:var global string
         .          .     17:
         .          .     18:func benchmarkcopy(b *testing.B, c int) {
         .          .     19:   big := "This is a long string"
         .       240B     20:   parts := strings.Split(big, " ")
         .          .     21:   old := parts[0]
         .          .     22:   jlimit := 100
         .          .     23:   for i := 0; i < b.N; i++ {
         .          .     24:       for j := 0; j < jlimit; j++ {
    3.21MB     3.21MB     25:           global = string([]byte(old))
         .          .     26:       }
         .          .     27:       for j := 0; j < jlimit; j++ {
         .          .     28:           b := []byte(old)
    3.21MB     3.21MB     29:           global = string(b)
         .          .     30:       }
         .          .     31:       for j := 0; j < jlimit; j++ {
    3.21MB     3.21MB     32:           new := make([]byte, len(old))
         .          .     33:           copy(new, old)
         .          .     34:           global = string(old)
         .          .     35:       }
         .          .     36:   }
         .          .     37:}

answered Mar 14, 2016 at 17:13

WeakPointer

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2 Comments

Coder Over a year ago

Nice! That makes sense. Very thorough answer. I had not though about profiling thiss

WeakPointer Over a year ago

So many things to admire about go. The easy cpu and memory profiling are one of those. I'm still trying to streamline my own profiling setup so it feels easy to profile things that I care about. I see others who are better at it and seem to take it for granted. Still trying to get to that level of comfort myself so the question here seemed perfect for a little experimentation.

Abex · Accepted Answer · 2016-03-13 20:50:40Z

2

To ensure that Go doesn't keep the underlying string in memory you will have to explicitly copy it to a new location:

func unslice(old string) string {
    new := make([]byte,len(old))
    copy(new,old)
    return string(old)
}

SmallString := unslice(BigString[0:7])

answered Mar 13, 2016 at 20:50

Abex

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4 Comments

Coder Over a year ago

Thank you, does that mean in my attempted solution []byte(s) returns a slice?

evanmcdonnal Over a year ago

@Coder yes []byte(s) returns a byte slice where each byte is one character from the string s.

Abex Over a year ago

I Think it does, but I am not sure. Either way this shows very explicitly what you are trying to do, making it easier for future maintainers.

Gerry Gleason Over a year ago

The byte slice isn't exactly "each character", if there are multibyte characters in the string (runes ?). You can iterate over these insteade. It is "each byte" of the string.

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