Question

I had an interview question that asked me for my ‘feedback’ on a piece of code a junior programmer wrote. They hinted there may be a problem and said it will be used heavily on large strings.

public string ReverseString(string sz)
{
    string result = string.Empty;
    for(int i = sz.Length-1; i>=0; i--)
    {
      result += sz[i]
    }
    return result;
}

I couldn’t spot it. I saw no problems whatsoever.
In hindsight I could have said the user should resize but it looks like C# doesn’t have a resize (i am a C++ guy).

I ended up writing things like use an iterator if its possible, [x] in containers could not be random access so it may be slow. and misc things. But I definitely said I never had to optimize C# code so my thinking may have not failed me on the interview.

I wanted to know, what is the problem with this code, do you guys see it?

-edit-

I changed this into a wiki because there can be several right answers.
Also i am so glad i explicitly said i never had to optimize a C# program and mentioned the misc other things. Oops. I always thought C# didnt have any performance problems with these type of things. oops.

Answer 1

A few comments on the answers given so far:

• Every single one of them (so far!) will fail on surrogate pairs and combining characters. Oh the joys of Unicode. Reversing a string isn’t the same as reversing a sequence of chars.
• I like Marc’s optimisation for null, empty, and single character inputs. In particular, not only does this get the right answer quickly, but it also handles null (which none of the other answers do)
• I originally thought that ToCharArray followed by Array.Reverse would be the fastest, but it does create one “garbage” copy.
• The StringBuilder solution creates a single string (not char array) and manipulates that until you call ToString. There’s no extra copying involved… but there’s a lot more work maintaining lengths etc.

Which is the more efficient solution? Well, I’d have to benchmark it to have any idea at all – but even so that’s not going to tell the whole story. Are you using this in a situation with high memory pressure, where extra garbage is a real pain? How fast is your memory vs your CPU, etc?

As ever, readability is usually king – and it doesn’t get much better than Marc’s answer on that front. In particular, there’s no room for an off-by-one error, whereas I’d have to actually put some thought into validating the other answers. I don’t like thinking. It hurts my brain, so I try not to do it very often. Using the built-in Array.Reverse sounds much better to me. (Okay, so it still fails on surrogates etc, but hey…)