Read random rows from the dataset

Question

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I have my dataset with 1000 records

This is list used to store the randomly selected records.

C#

private static List<string>[] BChrom = new List<string>[10];

How can I add 20% from the whole dataset RANDOMLY to that List of string

C#

try
            {
                using (sr = new StreamReader(@"C:\Users\*****\Documents\sub0000.data"))
                {
                    for (int i = 0; i < BChrom.Length; i++)
                    {
                        
                    }
                }
            }

Posted 22-Jul-13 2:01am

yamur

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Herman<T>.Instance 22-Jul-13 8:07am

Please read this

1 solution

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Kuthuparakkal · Answer 1 · 2013-07-22T04:06:00

Try this:

C#

ArrayList ReadRandom(string sourceFile, int sampleSize)
{
    ArrayList BChrom = new ArrayList(sampleSize);
    Random random = new Random();
    FileStream ifs = new FileStream(sourceFile, FileMode.Open);
    StreamReader sr = new StreamReader(ifs);
    string line = "";

    // determine extent of source file
    long lastPos = sr.BaseStream.Seek(0, SeekOrigin.End);

    for (int i = 0; i < sampleSize; ++i)
    {
        // generate a random position
        double pct = random.NextDouble(); // [0.0, 1.0)
        long randomPos = (long)(pct * lastPos);
        if (pct >= 0.99)
            randomPos -= 1024; // if near the end, back up a bit

        sr.BaseStream.Seek(randomPos, SeekOrigin.Begin);

        line = sr.ReadLine(); // consume curr partial line
        line = sr.ReadLine(); // this will be a full line
        sr.DiscardBufferedData(); // magic

        BChrom.Add(line);
    }

    sr.Close(); 
    ifs.Close();

    return BChrom;
}

There are some drawbacks(like last line is never read, if the file size is less than 1024 etc) but performance is guaranteed on large files...