In C#, perform "static" copy into a substring of a StringBuilder object

Question

To build a sparsely populated fixed width record, I would like to copy a string field into a StringBuilder object, starting at a given position. A nice syntax for this would have been

StringBuilder sb = new StringBuilder(' ', 100);
string fieldValue = "12345";
int startPos = 16; 
int endPos = startPos + fieldValue.Length - 1;
sb[startPos..endPos] = fieldValue; // no such syntax

I could obviously do this C style, one character at a time:

for (int ii = 0; ii++; ii < fieldValue.Length)
    sb[startPos + ii] = fieldValue[ii];

But this seems way too cumbersome for c#, plus it uses a loop where the resulting machine code could more efficiently use a bulk copy, which can make a difference if the strings involved were long. Any ideas for a better way?

Anthony Pegram · Accepted Answer · 2011-08-07 05:36:54Z

2

Your original algorithm can be supported in the following way

var builder = new StringBuilder(new string(' ', 100));
string toInsert = "HELLO WORLD";
int atIndex = 10;
builder.Remove(atIndex, toInsert.Length);
builder.Insert(atIndex, toInsert);

Debug.Assert(builder.Length == 100);
Debug.Assert(builder.ToString().IndexOf(toInsert) == 10);

answered Aug 7, 2011 at 5:36

Anthony Pegram

128k28 gold badges229 silver badges252 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Ivan Danilov Over a year ago

It could be even slower than char-by-char replacing because of shifting tail back and forth. I'd not recommend that instead of a simple loop.

Anthony Pegram Over a year ago

For a single algorithmic operation such as this, I'd just use a string and forget the builder altogether, it's overkill. For something I would perform multiple times, say, in a loop, I'd want efficiency, certainly, but also something easy to reason about. char replacement operations in an array would rank lower on the reasonability meter, so the presumed performance gain would need to be worth it.

Ivan Danilov Over a year ago

It depends on the scenario heavily. If you read big file for example and replacing there several places in the beginning - this would be great loss of performance. And for readability for and Remote/Insert variants are almost equal here imo. So it is up to OP basically :)

Rick Sladkey · Accepted Answer · 2011-08-07 06:35:38Z

1

You can write your own specialized string builder class that uses the efficient machinery of char[] and string underneath the hood, in particular String.CopyTo:

public class FixedStringBuilder
{
    char[] buffer;

    public FixedStringBuilder(int length)
    {
        buffer = new string(' ', length).ToCharArray();
    }

    public FixedStringBuilder Replace(int index, string value)
    {
        value.CopyTo(0, buffer, index, value.Length);
        return this;
    }

    public override string ToString()
    {
        return new string(buffer);
    }
}

class Program
{
    static void Main(string[] args)
    {
        FixedStringBuilder sb = new FixedStringBuilder(100);
        string fieldValue = "12345";
        int startPos = 16; 
        sb.Replace(startPos, fieldValue);
        string buffer = sb.ToString();
    }
}

answered Aug 7, 2011 at 6:35

Rick Sladkey

34.3k6 gold badges73 silver badges95 bronze badges

1 Comment

Ran Ever-Hadani Over a year ago

Thanks - this is exactly I was looking for. Would never occur to me to look for it as a method of the immutable System.String :) Thanks!

Mario Vernari · Accepted Answer · 2011-08-07 05:34:46Z

0

The closest solution to your goal is to convert the source string in a char array, then substitute the cells. Any char array can be converted back to a string.

answered Aug 7, 2011 at 5:34

Mario Vernari

7,4241 gold badge37 silver badges57 bronze badges

Comments

NirMH · Accepted Answer · 2011-08-07 05:36:25Z

0

why are you pre-allocating memory in the string builder (it is only support performance).

i would append the known prefix, then the actual value and then the postfix to the string.

something like:

StringBuilder sb = new StringBuilder();
sb.Append(prefix).Append(value).Append(postfix);

answered Aug 7, 2011 at 5:36

NirMH

5,0255 gold badges53 silver badges77 bronze badges

1 Comment

Ran Ever-Hadani Over a year ago

This does not address the "seek" scenario, where you have a static character sequence of a known fixed length, and you want to overwrite an small area of it at a given "address".

Collectives™ on Stack Overflow

In C#, perform "static" copy into a substring of a StringBuilder object

4 Answers 4

3 Comments

1 Comment

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

1 Comment

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related