+= 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

+= 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 https://gist.github.com/1563290