Fastest way to remove first char in a String

Question

Say we have the following string

string data= "/temp string";

If we want to remove the first character / we can do by a lot of ways such as :

data.Remove(0,1);
data.TrimStart('/');
data.Substring(1);

But, really I don't know which one has the best algorithm and doing that faster..
Is there a one that is the best or all are the same ?

Do you want to remove the first character anyway or do you need to check that this character is indeed a `/`? — SRKX, Jul 11 '10 at 06:43
`TrimStart` won't remove the first char, it will remove `n` chars from the beginning. `Substring` is the fastest. — Jaroslav Jandek, Jul 11 '10 at 06:44
If you're removing any first character, `TrimStart()` is completely out of the question. — BoltClock, Jul 11 '10 at 07:02

Jon Skeet · Accepted Answer · 2010-07-11T07:02:47.387

The second option really isn't the same as the others - if the string is "///foo" it will become "foo" instead of "//foo".

The first option needs a bit more work to understand than the third - I would view the Substring option as the most common and readable.

(Obviously each of them as an individual statement won't do anything useful - you'll need to assign the result to a variable, possibly data itself.)

I wouldn't take performance into consideration here unless it was actually becoming a problem for you - in which case the only way you'd know would be to have test cases, and then it's easy to just run those test cases for each option and compare the results. I'd expect Substring to probably be the fastest here, simply because Substring always ends up creating a string from a single chunk of the original input, whereas Remove has to at least potentially glue together a start chunk and an end chunk.

I do check now by call each one about 90000000 and I go the following result : Remove : 06.63 - TrimStart : 04.71 - subString : 03.09 so from result substring is the best — Amr Badawy, Jul 11 '10 at 07:25
Just remember that when you are testing performance this way, you are affected by CPU caching, so you need to do that on the random strings, that you pre-populated an array (list) with, and randomly select the element of that array (list). — ajeh, Sep 09 '16 at 19:51

Nicholas Petersen · Answer 2 · 2019-02-27T10:50:35.893

I know this is hyper-optimization land, but it seemed like a good excuse to kick the wheels of BenchmarkDotNet. The result of this test (on .NET Core even) is that Substring is ever so slightly faster than Remove, in this sample test: 19.37ns vs 22.52ns for Remove. So some ~16% faster.

using System;
using BenchmarkDotNet.Attributes;

namespace BenchmarkFun
{
    public class StringSubstringVsRemove
    {
        public readonly string SampleString = " My name is Daffy Duck.";

        [Benchmark]
        public string StringSubstring() => SampleString.Substring(1);

        [Benchmark]
        public string StringRemove() => SampleString.Remove(0, 1);

        public void AssertTestIsValid()
        {
            string subsRes = StringSubstring();
            string remvRes = StringRemove();

            if (subsRes == null
                || subsRes.Length != SampleString.Length - 1
                || subsRes != remvRes) {
                throw new Exception("INVALID TEST!");
            }
        }
    }

    class Program
    {
        static void Main()
        {
            // let's make sure test results are really equal / valid
            new StringSubstringVsRemove().AssertTestIsValid();

            var summary = BenchmarkRunner.Run<StringSubstringVsRemove>();
        }
    }
}

Results:

BenchmarkDotNet=v0.11.4, OS=Windows 10.0.17763.253 (1809/October2018Update/Redstone5)
Intel Core i7-6700HQ CPU 2.60GHz (Skylake), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.0.100-preview-010184
  [Host]     : .NET Core 3.0.0-preview-27324-5 (CoreCLR 4.6.27322.0, CoreFX 4.7.19.7311), 64bit RyuJIT
  DefaultJob : .NET Core 3.0.0-preview-27324-5 (CoreCLR 4.6.27322.0, CoreFX 4.7.19.7311), 64bit RyuJIT

|          Method |     Mean |     Error |    StdDev |
|---------------- |---------:|----------:|----------:|
| StringSubstring | 19.37 ns | 0.3940 ns | 0.3493 ns |
|    StringRemove | 22.52 ns | 0.4062 ns | 0.3601 ns |

score 15 · Answer 3 · answered Feb 21 '22 at 10:22

15

In .Net Core also this works:

data = data[1..];

answered Feb 21 '22 at 10:22

Vasilis Plavos

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score 10 · Answer 4 · answered Jul 11 '10 at 06:43

10

I'd guess that Remove and Substring would tie for first place, since they both slurp up a fixed-size portion of the string, whereas TrimStart does a scan from the left with a test on each character and then has to perform exactly the same work as the other two methods. Seriously, though, this is splitting hairs.

answered Jul 11 '10 at 06:43

Marcelo Cantos

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38
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Actually, `Substring` is faster than `Remove`, because `Remove` calls `Substring`. – Jaroslav Jandek Jul 11 '10 at 06:45
@Jaroslav: This is not true. Both `Substring` and `Remove` rely on a private method, `FillSubstring`. – Marcelo Cantos Jul 11 '10 at 06:54
Didn't verify it, but it very sounds plausible: `string Remove(this string source, int from, int to) { return source.SubString(0, from) + source.SubString(to); }` – Dykam Jul 11 '10 at 06:56
@Marcelo: maybe in `Singularity RDK`, but not in `.NET` (any version). `Substring` uses unsafe string copy (in memory). – Jaroslav Jandek Jul 11 '10 at 07:02
1

@Jaroslav: I'm staring at the Reflector disassembly of the two methods in mscorlib.dll on a fairly conventional Windows dev environment. They both call `System.PInvoke.EE.AllocateString` to allocate the destination string object and then call `FillSubstring` to copy characters across. Am I looking at the wrong thing? – Marcelo Cantos Jul 11 '10 at 07:20
@Jaroslav: That wasn't your original point at all. If A calls B then it is (almost) a truism that A will be slower than B. Your latest claim, however, relies on several assumptions, such as the assumption that the overhead of `Remove`'s extra call to FillSubstring, which has no work to do, will outweigh the cost of `Substring`'s more complex parameter verification code. Don't get me wrong. I'm not disagreeing with your conclusion. Substring probably is faster (a matter one would settle by profiling, not speculating). Just don't pretend that we've been debating the same point from the outset. – Marcelo Cantos Jul 11 '10 at 09:11
@Marcelo: `Remove` *esentially* calls `substring` (work-wise) ((A + B) > A always - it does the same checks btw.). And yes, it is WAY slower than `Substring`. Like 80%+ slower for this specific operation. If you wanted to do a regular `Remove` from the middle of a string, even then is `Remove` only marginally (10%+) faster than `Substring`. Also, I see no calls to `FillSubstring` in my methods. The bottom line is the `SubString` is the best method for that as I have pointed out in the first comment on the original post. – Jaroslav Jandek Jul 11 '10 at 10:00
1

@Marcelo: Anyway, your first comment originally said a totaly different thing. I should probably have used a better wording, the point is valid though (`Substring` > `Remove`). I am not going to comment further because the discussion took enough of my time. – Jaroslav Jandek Jul 11 '10 at 10:08

score 6 · Answer 5 · answered Jul 11 '10 at 06:43

6

You could profile it, if you really cared. Write a loop of many iterations and see what happens. Chances are, however, that this is not the bottleneck in your application, and TrimStart seems the most semantically correct. Strive to write code readably before optimizing.

answered Jul 11 '10 at 06:43

Stefan Kendall

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`TrimStart` is the least correct, since `"//temp string".TrimStart('/')` will *not* just remove the first `'/'`. – Marcelo Cantos Jul 11 '10 at 06:45
The function is poorly named then. I'm not a C# guy. – Stefan Kendall Jul 11 '10 at 07:20
@StefanKendall: Look at tags – Vijay Singh Rana Dec 29 '14 at 12:12

Fastest way to remove first char in a String

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