C＃中的字符串操作优化

Question

The following C# code takes 5 minutes to run:

int i = 1;
string fraction = "";
while (fraction.Length < 1000000)
{
    fraction += i.ToString();
    i++;
}

"Optimising it" like this causes it to run in 1.5 seconds:

int i = 1;
string fraction = "";
while (fraction.Length < 1000000)
{
    // concatenating strings is much faster for small strings
    string tmp = "";
    for (int j = 0; j < 1000; j++)
    {
        tmp += i.ToString();
        i++;
    }
    fraction += tmp;
}

EDIT: Some people suggested using StringBuilder , which is an excellent suggestion also, and this comes out at 0.06s:

int i = 1;
StringBuilder fraction = new StringBuilder();
while (fraction.Length < 1000000)
{
    fraction.Append(i);
    i++;
}

Playing around to find the optimum value of j is a topic for another time, but why exactly does this non-obvious optimisation work so well? Also, on a related topic, I've heard it said that you should never use the + operator with strings, in favour of string.Format() , is this true?

Answer 1

I don't get your results at all. On my box StringBuilder wins hands down. Could you post your full test program? Here's mine, with three variants - your string concatenation optimisation, the "simple" StringBuilder one, and StringBuilder with an initial capacity. I've increased the limit as it was going too fast on my box to be usefully measurable.

using System;
using System.Diagnostics;
using System.Text;

public class Test
{
    const int Limit = 4000000;

    static void Main()
    {
        Time(Concatenation, "Concat");
        Time(SimpleStringBuilder, "StringBuilder as in post");
        Time(SimpleStringBuilderNoToString, "StringBuilder calling Append(i)");
        Time(CapacityStringBuilder, "StringBuilder with appropriate capacity");
    }

    static void Time(Action action, string name)
    {
        Stopwatch sw = Stopwatch.StartNew();
        action();
        sw.Stop();
        Console.WriteLine("{0}: {1}ms", name, sw.ElapsedMilliseconds);
        GC.Collect();
        GC.WaitForPendingFinalizers();
    }

    static void Concatenation()
    {
        int i = 1;
        string fraction = "";
        while (fraction.Length < Limit)
        {
            // concatenating strings is much faster for small strings
            string tmp = "";
            for (int j = 0; j < 1000; j++)
            {
                tmp += i.ToString();
                i++;
            }
            fraction += tmp;            
        }
    }

    static void SimpleStringBuilder()
    {
        int i = 1;
        StringBuilder fraction = new StringBuilder();
        while (fraction.Length < Limit)
        {
            fraction.Append(i.ToString());
            i++;
        }
    }

    static void SimpleStringBuilderNoToString()
    {
        int i = 1;
        StringBuilder fraction = new StringBuilder();
        while (fraction.Length < Limit)
        {
            fraction.Append(i);
            i++;
        }
    }

    static void CapacityStringBuilder()
    {
        int i = 1;
        StringBuilder fraction = new StringBuilder(Limit + 10);
        while (fraction.Length < Limit)
        {
            fraction.Append(i);
            i++;
        }
    }
}

And the results:

Concat: 5879ms
StringBuilder as in post: 206ms
StringBuilder calling Append(i): 196ms
StringBuilder with appropriate capacity: 184ms

The reason your concatenation is faster than the very first solution is simple though - you're doing several "cheap" concatenations (where relatively little data is being copied each time) and relatively few "large" concatenations (of the whole string so far). In the original, every step would copy all of the data obtained so far, which is obviously more expensive.

Answer 2

Use StringBuilder for concatenating more than (approximately) 5 strings (results may vary slightly). Also, give the StringBuilder's constructor a hint on the expected maximum size.

[Update]: just commenting on your edit to the question. You can also increase StringBuilder 's performance if you have an approximate (or exact) idea of the final size of the concatenated strings, because this will reduce the number of memory allocations it has to perform:

// e.g. Initialise to 10MB
StringBuilder fraction = new StringBuilder(10000000);

Answer 3

You will probably see that the first 1000 chars will take almost no time opposed to the last 1000 chars.

I would assume that the time-consuming part is the actual copying of the large string into a new memory-area every time you add a char that is the tough work for your computer.

Your optimization can easily be compared to what you usually do with streams, you use a buffer. Larger chunks will usually result in better performance until you hit the critical size where it no longer makes any difference, and starts to be a downside when your handling small amounts of data.

If you however would have defined a char-array with the appropriate size from the beginning, it would probably be blazing fast, because then it won't have to copy it over and over again.

Answer 4

Also, on a related topic, I've heard it said that you should never use the + operator with strings, in favour of string.Format(), is this true?

No, like all absolute statements it's nonsense. However, it is true that using Format usually makes formatting code more readable and it's often slightly faster than concatenation – but speed isn't the deciding factor here.

As for your code … it results in smaller strings being copied (namely, tmp ) in the concatenation. Of course, in fraction += tmp you copy a larger string but this happens less often.

Therefore, you've reduced many large copies to a few large and many small copies.

Hmm, I've just noticed that your outer loop has the same size in both cases. This shouldn't be faster, then.

Answer 5

I can't do tests now, but try to use StringBuilder.

int i = 1;
    StringBuilder fraction = new StringBuilder();
    while (fraction.Length < 1000000)
    {
        fraction.Append(i);
        i++;
    }
return sb.ToString();

Answer 6

Answer to the modified queston ("why does this non-obvious optimization work so well" and "is it true you shouldn't use + operator on strings"):

I'm not sure which non-obvious optimization you are talking about. But the answer to the second question, I think, covers all of the bases.

The way strings work in C# is that they are allocated as fixed-length, and cannot be changed. This means that any time you try to change the length of the string, an entire new string is created and the old string is copied in up to the proper length. This is obviously a slow process. When you use String.Format it internally uses a StringBuilder to create the string.

StringBuilders work by using a memory buffer which is more intelligently allocated than fixed-length strings, and thus performs significantly better in most situations. I'm not sure on the details of StringBuilder internally, so you'll have to ask a new question for that. I can speculate it either doesn't reallocate the old portions of the string (instead creating a linked list internally and only actually allocating the final output when needed by ToString) or it reallocates with exponential growth (when it runs out of memory, it allocates twice as much the next time, thus for a 2GB string it would only need to reallocate about 30 times).

Your example with the nested loops grows linearly. it takes a small string and grows that up to 1000, and then tacks that 1000 on to the larger string in one large operation. As the large string gets really large, the copy that results from creating a new string gets to take a long time. When you reduce the amount of times this is done (by instead resizing a smaller string more often instead) you increase the speed. Of course, StringBuilder is even smarter about allocating memory, and thus is much faster.

Answer 7

Adding a character to a string can have two consequences:

• if there is still space for the character it is just added at the end; (as a commenter noticed, this can not happen with c# strings, as thy are immutable).
• if there is no space at the end a new block of memory is allocated for the new string, the contents of the old string is copied there and the character is added.

To analyse your code, it is simpler to add 1000000 times a single character. Your exact example is a bit more complex to explain because for higher i's you add more characters at a time.

Then in the situation where no extra space is reserved, the first example has to do 1000000 allocations and copies, of an average of 0.5 * 1000000 characters. The second one has to do 1000 allocations and copies of an average 0.5 * 1000000 characters, and 1000000 allocations and copies of 0.5 * 1000 characters. If copying is lineair with the size of the copy and allocation free, the first situation takes 500 000 000 000 units of time and the second one 500 000 000 + 500 000 000 units of time.