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Comment by trebor | Hacker News Reader

Why is

  "est".prepend('t')

more useful than

  "t" + "est"

? I don't see why a method for prepending to a string is necessary, when it is so simple anyway...

And random with a range parameter is useful shorthand.

Anyway, I'm more enthusiastic about performance improvements where file loading is concerned.

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First, let's say you subclassed String or have a delegate. obj.prepend(str) will adjust obj in place rather than create an all new String object (as str + obj would). This may be either desirable or critical, depending on what you're doing.

Second, prepend is a lot faster than using + in this case. As in, a simple prepend runs in a mere 1.9% of the time (in a benchmark I just ran). Why? Fewer new objects are being created. And that means easier and fewer GC runs too.

Update: I think there is something slightly bust since on 1.9.2 using + handily beats prepend on 1.9.3. But GC has changed a lot. I'll retest on RC1 rather than preview1 ;-)

Good point; I did not consider conserving objects and the overhead of allocation/initialization versus reallocation.

#prepend replaces in place? There's no #prepend! ?

No.

Conventionally, "bang" methods should only be created as 'dangerous' versions of non-bang methods. There was no non-bang method to make a 'dangerous' version of in this case so it's non-bang by default. This is a commonly misunderstood naming convention in Ruby (and I dare say I don't even understand all the intricacies!)

There are quite a few examples of this in Ruby. For example, Array#keep_if which changes an array in place.

I guess I misunderstood your definition of "in place". That jives with what I've understood about Ruby so far.

That would be redundant, since there would be no difference from String#+.

How does String#prepend compare to String#insert in your benchmarks? eg. "bc".insert(0, "a") # => "abc"

Sometime it's easier to chain with other functions and, depending of the situation, it might make the code easier to read.

More useful? It does a different thing, just like String#+ doesn't do the same thing as String <<.

They are not exchanges for each other. Rather - they are complements.

Not to mention more verb like - and thus descriptive.

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