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Simple bytearray comparing question.

Posted on 2003-12-02
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Last Modified: 2010-04-16
byte[] a = new byte[2];
byte[] b = new byte[2];
a[0] = 1;
a[1] = 2;
b[0] = 1;
b[1] = 2;
Console.WriteLine( a == b );
a[0] = 1;
a[1] = 2;
b[0] = 2;
b[1] = 2;
Console.WriteLine( a == b);

How can i effeciently compare if the values in a is equal to b ( in my base, the size of array should be more then 1M) ? I want the result to be true false, instead of false false that i get now.
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Question by:ho_alan
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6 Comments
 
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Assisted Solution

by:testn
testn earned 35 total points
Comment Utility
If you use == on reference type, it will compare the pointer instead of the value inside. That's why you got false in both cases
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Expert Comment

by:testn
Comment Utility
This means that to compare them, you have to compare it byte by byte....

    public bool CompareArrays (byte[] first, byte[] second)
    {
        if (first.Length != second.Length)
            return false;
        for (int i=0; i < first.Length; i++)
            if (first[i] != second[i])
                return false;
        return true;
    }
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Accepted Solution

by:
TheAvenger earned 65 total points
Comment Utility
As you will compare huge arrays, consider the following optimization of the code. Note that you must enable unsafe blocks in the project properties.

unsafe public bool CompareArrays (byte[] first, byte[] second)
{
      if (first.Length != second.Length)
            return false;

      fixed (byte* f = first)
            fixed (byte *s = second)
            {
                  long* p1 = (long*)f;
                  long* p2 = (long*)s;

                  // Compare as much as possible as longs
                  for (int i = 0; i < first.Length / sizeof (long); i++)
                        if (p1[i] != p2[i])
                              return false;

                  // Compare the rest as bytes
                  for (int i = 0; i < first.Length % sizeof (long); i++)
                        if (f[(first.Length / sizeof (long)) * sizeof (long) + i] !=
                              s[(first.Length / sizeof (long)) * sizeof (long) + i])
                              return false;

                  return true;
            }
}

An example usage:
private void button1_Click(object sender, System.EventArgs e)
{
      byte[] a = new byte[10000];
      byte[] b = new byte[10000];

      Random r = new Random();

      for (int i = 0; i < a.Length; i++)
            a[i] = b[i] = (byte)r.Next();

      MessageBox.Show (CompareArrays(a, b).ToString());
}
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by:ho_alan
Comment Utility
Hi TheAvenger

In my understanding, you are trying to convert the byte array into a long array first, then compare each block of the long array 1 by 1. So.. the looping will be 4 times less? So.. it still need to loop..

Is there a method which is similar to the function memcmp in c++?
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Expert Comment

by:TheAvenger
Comment Utility
Actually it's 8 times faster, because the long type in C# is 8 bytes, byte is 1 byte.

I don't know about such a method, but this does not mean it does not exist....
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Expert Comment

by:TheAvenger
Comment Utility
By the way this loop compares 10 Mb arrays for ~0.5 sec so I think it has a good performance...
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