I've just had to write a string reverse function in C# 2.0 (i.e. LINQ not available) and came up with this:
public string Reverse(string text)
{
char[] cArray = text.ToCharArray();
string reverse = String.Empty;
for (int i = cArray.Length - 1; i > -1; i--)
{
reverse += cArray[i];
}
return reverse;
}
Personally I'm not crazy about the function and am convinced that there's a better way to do it. Is there?
here's my approach:
private string Reverse(string input)
{
Stack<char> resultStack = new Stack<char>();
foreach (char c in input)
{
resultStack.Push(c);
}
StringBuilder sb = new StringBuilder();
while (resultStack.Count > 0)
{
sb.Append(resultStack.Pop());
}
return sb.ToString();
}
Edit: Ok, I ran the various approaches to the profiler, using the string "2fd4e1c67a2d28fced849ee1bb76e7391b93eb12" and 10.000 Reversals, counting "Time With Children (ms)". And yes, i know that there is no Reverse4, because it was a duplicate. Ran the test 3 times, results did not fluctuate too much for me. So yes, my approach sucks by a factor of 10, so i'll change my function in my code now :-)
Linq approach: 64,325ms (Changed Target Framework from 2.0 to 3.5 for this)
Have a look at the wikipedia entry here. They implement the String.Reverse extension method. This allows you to write code like this:
string s = "olleh";
s.Reverse();
They also use the ToCharArray/Reverse combination that other answers to this question suggest. The source code looks like this:
public static string Reverse(this string input)
{
char[] chars = input.ToCharArray();
Array.Reverse(chars);
return new String(chars);
}
Firstly you don't need to call ToCharArray as a string can already be indexed as a char array, so this will save you an allocation.
The next optimisation is to use a StringBuilder to prevent unnecessary allocations (as strings are immutable, concatenating them makes a copy of the string each time). To further optimise this we pre-set the length of the StringBuilder so it won't need to expand its buffer.
public string Reverse(string text)
{
if (string.IsNullOrEmpty(text))
{
return text;
}
StringBuilder builder = new StringBuilder(text.Length);
for (int i = text.Length - 1; i >= 0; i--)
{
builder.Append(text[i]);
}
return builder.ToString();
}
Edit: Performance Data
I tested this function and the function using Array.Reverse with the following simple program, where Reverse1 is one function and Reverse2 is the other:
static void Main(string[] args)
{
var text = "abcdefghijklmnopqrstuvwxyz";
// pre-jit
text = Reverse1(text);
text = Reverse2(text);
// test
var timer1 = Stopwatch.StartNew();
for (var i = 0; i < 10000000; i++)
{
text = Reverse1(text);
}
timer1.Stop();
Console.WriteLine("First: {0}", timer1.ElapsedMilliseconds);
var timer2 = Stopwatch.StartNew();
for (var i = 0; i < 10000000; i++)
{
text = Reverse2(text);
}
timer2.Stop();
Console.WriteLine("Second: {0}", timer2.ElapsedMilliseconds);
Console.ReadLine();
}
It turns out that for short strings the Array.Reverse method is around twice as quick as the one above, and for longer strings the difference is even more pronounced. So given that the Array.Reverse method is both simpler and faster I'd recommend you use that rather than this one. I leave this one up here just to show that it isn't the way you should do it (much to my surprise!)
public static string Reverse( string s )
{
char[] charArray = s.ToCharArray();
Array.Reverse( charArray );
return new string( charArray );
}
I think the above works not tested, although the stringbuilder class may also have a reverse function I haven't checked that though.
This is turning out to be a surprisingly tricky question.
I would recommend using Array.Reverse for most cases as it is coded natively and it is very simple to maintain and understand.
It seems to outperform StringBuilder in all the cases I tested.
public string Reverse(string text)
{
if (text == null) return null;
// this was posted by petebob as well
char[] array = text.ToCharArray();
Array.Reverse(array);
return array;
}
There is a second approach that can be faster for certain string lengths which uses Xor.
public static string ReverseXor(string s)
{
if (s == null) return null;
char[] charArray = s.ToCharArray();
int len = s.Length - 1;
for (int i = 0; i < len; i++, len--)
{
charArray[i] ^= charArray[len];
charArray[len] ^= charArray[i];
charArray[i] ^= charArray[len];
}
return new string(charArray);
}
Note If you want to support the full Unicode UTF16 charset read this. And use the implementation there instead. It can be further optimized by using one of the above algorithms and running through the string to clean it up after the chars are reversed.
Here is a performance comparison between the StringBuilder, Array.Reverse and Xor method.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
namespace ConsoleApplication4
{
class Program
{
delegate string StringDelegate(string s);
static void Benchmark(string description, StringDelegate d, int times, string text)
{
Stopwatch sw = new Stopwatch();
sw.Start();
for (int j = 0; j < times; j++)
{
d(text);
}
sw.Stop();
Console.WriteLine("{0} Ticks {1} : called {2} times.", sw.ElapsedTicks, description, times);
}
public static string ReverseXor(string s)
{
char[] charArray = s.ToCharArray();
int len = s.Length - 1;
for (int i = 0; i < len; i++, len--)
{
charArray[i] ^= charArray[len];
charArray[len] ^= charArray[i];
charArray[i] ^= charArray[len];
}
return new string(charArray);
}
public static string ReverseSB(string text)
{
StringBuilder builder = new StringBuilder(text.Length);
for (int i = text.Length - 1; i >= 0; i--)
{
builder.Append(text[i]);
}
return builder.ToString();
}
public static string ReverseArray(string text)
{
char[] array = text.ToCharArray();
Array.Reverse(array);
return (new string(array));
}
public static string StringOfLength(int length)
{
Random random = new Random();
StringBuilder sb = new StringBuilder();
for (int i = 0; i < length; i++)
{
sb.Append(Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65))));
}
return sb.ToString();
}
static void Main(string[] args)
{
int[] lengths = new int[] {1,10,15,25,50,75,100,1000,100000};
foreach (int l in lengths)
{
int iterations = 10000;
string text = StringOfLength(l);
Benchmark(String.Format("String Builder (Length: {0})", l), ReverseSB, iterations, text);
Benchmark(String.Format("Array.Reverse (Length: {0})", l), ReverseArray, iterations, text);
Benchmark(String.Format("Xor (Length: {0})", l), ReverseXor, iterations, text);
Console.WriteLine();
}
Console.Read();
}
}
}
Here are the results:
26251 Ticks String Builder (Length: 1) : called 10000 times.
33373 Ticks Array.Reverse (Length: 1) : called 10000 times.
20162 Ticks Xor (Length: 1) : called 10000 times.
51321 Ticks String Builder (Length: 10) : called 10000 times.
37105 Ticks Array.Reverse (Length: 10) : called 10000 times.
23974 Ticks Xor (Length: 10) : called 10000 times.
66570 Ticks String Builder (Length: 15) : called 10000 times.
26027 Ticks Array.Reverse (Length: 15) : called 10000 times.
24017 Ticks Xor (Length: 15) : called 10000 times.
101609 Ticks String Builder (Length: 25) : called 10000 times.
28472 Ticks Array.Reverse (Length: 25) : called 10000 times.
35355 Ticks Xor (Length: 25) : called 10000 times.
161601 Ticks String Builder (Length: 50) : called 10000 times.
35839 Ticks Array.Reverse (Length: 50) : called 10000 times.
51185 Ticks Xor (Length: 50) : called 10000 times.
230898 Ticks String Builder (Length: 75) : called 10000 times.
40628 Ticks Array.Reverse (Length: 75) : called 10000 times.
78906 Ticks Xor (Length: 75) : called 10000 times.
312017 Ticks String Builder (Length: 100) : called 10000 times.
52225 Ticks Array.Reverse (Length: 100) : called 10000 times.
110195 Ticks Xor (Length: 100) : called 10000 times.
2970691 Ticks String Builder (Length: 1000) : called 10000 times.
292094 Ticks Array.Reverse (Length: 1000) : called 10000 times.
846585 Ticks Xor (Length: 1000) : called 10000 times.
305564115 Ticks String Builder (Length: 100000) : called 10000 times.
74884495 Ticks Array.Reverse (Length: 100000) : called 10000 times.
125409674 Ticks Xor (Length: 100000) : called 10000 times.
It seems that Xor can be faster for short strings.
"Better way" depends on what is more important to you in your situation, performance, elegance, maintainability etc.
Anyway, here's an approach using Array.Reverse:
string inputString="The quick brown fox jumps over the lazy dog.";
char[] charArray = inputString.ToCharArray();
Array.Reverse(charArray);
string reversed = new string(charArray);
Try using Array.Reverse
public string Reverse(string str)
{
char[] array = str.ToCharArray();
Array.Reverse(array);
return new string(array);
}
Had to submit a recursive example:
private static string Reverse(string str)
{
if (str.Length == 1)
return str;
else
return str[str.Length - 1] + Reverse(str.Substring(0, str.Length - 1));
}
Sorry for long post, but this might be interesting
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;
namespace ConsoleApplication1
{
class Program
{
public static string ReverseUsingArrayClass(string text)
{
char[] chars = text.ToCharArray();
Array.Reverse(chars);
return new string(chars);
}
public static string ReverseUsingCharacterBuffer(string text)
{
char[] charArray = new char[text.Length];
int inputStrLength = text.Length - 1;
for (int idx = 0; idx <= inputStrLength; idx++)
{
charArray[idx] = text[inputStrLength - idx];
}
return new string(charArray);
}
public static string ReverseUsingStringBuilder(string text)
{
if (string.IsNullOrEmpty(text))
{
return text;
}
StringBuilder builder = new StringBuilder(text.Length);
for (int i = text.Length - 1; i >= 0; i--)
{
builder.Append(text[i]);
}
return builder.ToString();
}
private static string ReverseUsingStack(string input)
{
Stack<char> resultStack = new Stack<char>();
foreach (char c in input)
{
resultStack.Push(c);
}
StringBuilder sb = new StringBuilder();
while (resultStack.Count > 0)
{
sb.Append(resultStack.Pop());
}
return sb.ToString();
}
public static string ReverseUsingXOR(string text)
{
char[] charArray = text.ToCharArray();
int length = text.Length - 1;
for (int i = 0; i < length; i++, length--)
{
charArray[i] ^= charArray[length];
charArray[length] ^= charArray[i];
charArray[i] ^= charArray[length];
}
return new string(charArray);
}
static void Main(string[] args)
{
string testString = string.Join(";", new string[] {
new string('a', 100),
new string('b', 101),
new string('c', 102),
new string('d', 103),
});
int cycleCount = 100000;
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
for (int i = 0; i < cycleCount; i++)
{
ReverseUsingCharacterBuffer(testString);
}
stopwatch.Stop();
Console.WriteLine("ReverseUsingCharacterBuffer: " + stopwatch.ElapsedMilliseconds + "ms");
stopwatch.Reset();
stopwatch.Start();
for (int i = 0; i < cycleCount; i++)
{
ReverseUsingArrayClass(testString);
}
stopwatch.Stop();
Console.WriteLine("ReverseUsingArrayClass: " + stopwatch.ElapsedMilliseconds + "ms");
stopwatch.Reset();
stopwatch.Start();
for (int i = 0; i < cycleCount; i++)
{
ReverseUsingStringBuilder(testString);
}
stopwatch.Stop();
Console.WriteLine("ReverseUsingStringBuilder: " + stopwatch.ElapsedMilliseconds + "ms");
stopwatch.Reset();
stopwatch.Start();
for (int i = 0; i < cycleCount; i++)
{
ReverseUsingStack(testString);
}
stopwatch.Stop();
Console.WriteLine("ReverseUsingStack: " + stopwatch.ElapsedMilliseconds + "ms");
stopwatch.Reset();
stopwatch.Start();
for (int i = 0; i < cycleCount; i++)
{
ReverseUsingXOR(testString);
}
stopwatch.Stop();
Console.WriteLine("ReverseUsingXOR: " + stopwatch.ElapsedMilliseconds + "ms");
}
}
}
Results:
If you want to play a really dangerous game, then this is by far the fastest way there is (around four times faster than the Array.Reverse method). It's an in-place reverse using pointers.
Note that I really do not recommend this for any use, ever (have a look here for some reasons why you should not use this method), but it's just interesting to see that it can be done, and that strings aren't really immutable once you turn on unsafe code.
public static unsafe string Reverse(string text)
{
if (string.IsNullOrEmpty(text))
{
return text;
}
fixed (char* pText = text)
{
char* pStart = pText;
char* pEnd = pText + text.Length - 1;
for (int i = text.Length / 2; i >= 0; i--)
{
char temp = *pStart;
*pStart++ = *pEnd;
*pEnd-- = temp;
}
return text;
}
}
If the string contains Unicode data (strictly speaking, non-BMP characters) the other methods that have been posted will corrupt it, because you cannot swap the order of high and low surrogate code units when reversing the string. (More information about this can be found on my blog.)
The following code sample will correctly reverse a string that contains non-BMP characters, e.g., "\U00010380\U00010381" (Ugaritic Letter Alpa, Ugaritic Letter Beta).
public static string Reverse(this string input)
{
if (input == null)
throw new ArgumentNullException("input");
// allocate a buffer to hold the output
char[] output = new char[input.Length];
for (int outputIndex = 0, inputIndex = input.Length - 1; outputIndex < input.Length; outputIndex++, inputIndex--)
{
// check for surrogate pair
if (input[inputIndex] >= 0xDC00 && input[inputIndex] <= 0xDFFF &&
inputIndex > 0 && input[inputIndex - 1] >= 0xD800 && input[inputIndex - 1] <= 0xDBFF)
{
// preserve the order of the surrogate pair code units
output[outputIndex + 1] = input[inputIndex];
output[outputIndex] = input[inputIndex - 1];
outputIndex++;
inputIndex--;
}
else
{
output[outputIndex] = input[inputIndex];
}
}
return new string(output);
}
Before I discovered that everyone was posting their test results to this thread I wrote this up on my blog. Essentially the results of my tests on 10 and 500 character strings show that the Array.Reverse() function wins the game and that's why I've marked this one as the correct answer.
Much obliged to everyone for helping me with this. You guys rock!
public string rev(string str) { if (str.Length <= 0) return string.Empty; else return str[str.Length-1]+ rev(str.Substring(0,str.Length-1));
}
Greg Beech posted an unsafe option that is indeed as fast as it gets (it's an in-place reversal); but, as he indicated in his answer, it's a completely disastrous idea.
That said, I'm surprised there is so much of a consensus that Array.Reverse is the fastest method. There's still an unsafe approach that returns a reversed copy of a string (no in-place reversal shenanigans) significantly faster than the Array.Reverse method for small strings:
public static unsafe string Reverse(string text)
{
int len = text.Length;
// Why allocate a char[] array on the heap when you won't use it
// outside of this method? Use the stack.
char* reversed = stackalloc char[len];
// Avoid bounds-checking performance penalties.
fixed (char* str = text)
{
int i = 0;
int j = i + len - 1;
while (i < len)
{
reversed[i++] = str[j--];
}
}
// Need to use this overload for the System.String constructor
// as providing just the char* pointer could result in garbage
// at the end of the string (no guarantee of null terminator).
return new string(reversed, 0, len);
}
Some benchmark results:
1000000 strings, 5 characters each: 946.8337 ms (Array.Reverse method) 792.3596 ms (safe variation of above method) 327.5075 ms (unsafe version posted bove) 1000000 strings, 10 characters each: 1077.9051 ms 1026.4824 ms 452.2965 ms 1000000 strings, 25 characters each: 1345.3698 ms 1178.9666 ms 849.9485 ms 500000 strings, 100 characters each: 1089.5289 ms 1516.1195 ms 924.4814 ms 250000 strings, 200 characters each: 740.3678 ms 1000.0229 ms 841.6740 ms 125000 strings, 400 characters each: 568.6292 ms 828.5899 ms 761.3883 ms
You can see that the performance gain shrinks and then disappears against the Array.Reverse method as the strings get larger. For small- to medium-sized strings, though, it's tough to beat this method.