teaspoon on Never create Ruby strings longer than 23 characters

Comment on Never create Ruby strings longer than 23 characters parent

I was curious, so I benchmarked this with Ruby MRI. join('') beat reduce(:+) even on two strings, but is twice as fast only on four or more strings.

   $ ruby -v
   ruby 1.8.7 (2010-01-10 patchlevel 249) [universal-darwin11.0]
   $ ruby benchmark.rb
         user     system      total        real
   add  2  0.390000   0.000000   0.390000 (  0.385910)
   join 2  0.300000   0.000000   0.300000 (  0.305812)

   add  3  0.530000   0.000000   0.530000 (  0.527388)
   join 3  0.320000   0.010000   0.330000 (  0.329390)

   add  4  0.720000   0.000000   0.720000 (  0.720500)
   join 4  0.350000   0.000000   0.350000 (  0.353237)

Code: https://gist.github.com/1562617

bodhi 14y

In case you were wondering why, it is most likely* because `#join` is appending to a single string, whereas the `#reduce` call is creating intermediate strings for each step of the fold.

* I'd have to check the implementation of `#join` to be sure.

teaspoon 14y

I thought so too, but appending to a single string does even worse than reduce(:+):

  add 2  0.390000   0.000000   0.390000 (  0.390529)
  +=  2  0.540000   0.000000   0.540000 (  0.537450)

  add 3  0.530000   0.000000   0.530000 (  0.534131)
  +=  3  0.760000   0.000000   0.760000 (  0.752280)

  add 4  0.660000   0.000000   0.660000 (  0.668154)
  +=  4  0.960000   0.000000   0.960000 (  0.954727)

Code: https://gist.github.com/1562994

subwindow 14y

+= is not append

  str += append_str

is the equivalent to:

  str.dup << append_str

That is, it copies the string and then appends to the copy. Benchmarking the speed of a true append is difficult because in order to preserve the original string in the benchmark you must dup it anyhow (done here outside of the bench.report block). However, the bigger the strings get, the more pronounced the advantage of appending is.

                user     system      total        real
  add  2      0.260000   0.000000   0.260000 (  0.263414)
  join 2      0.320000   0.000000   0.320000 (  0.325341)
  append 2    0.230000   0.010000   0.240000 (  0.235669)

  add  3      0.500000   0.000000   0.500000 (  0.497219)
  join 3      0.840000   0.020000   0.860000 (  0.866401)
  append 3    0.260000   0.010000   0.270000 (  0.268676)

  add  4      0.360000   0.040000   0.400000 (  0.397778)
  join 4      0.970000   0.030000   1.000000 (  0.997321)
  append 4    0.280000   0.000000   0.280000 (  0.281020)

  add  5      0.460000   0.000000   0.460000 (  0.454780)
  join 5      0.910000   0.030000   0.940000 (  0.946600)
  append 5    0.330000   0.000000   0.330000 (  0.336214)

Code: https://gist.github.com/1563290

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