Month: April 2009

Confidence interval in shell script

On By panoskrtIn coding, general, Linux1 Comment

An additional function for calculating the confidence interval of a sample for this script.

conf_inter() {
  avg=$(arith_mean); ntimes=$?
  conf=$1
  var1=$(echo "scale=$SC; ($avg - ($conf*1/sqrt($ntimes)))" | bc)
  var2=$(echo "scale=$SC; ($avg + ($conf*1/sqrt($ntimes)))" | bc)
  echo "$var1 <= m <= $var2"
}
conf=$(conf_inter $2)
echo "Confidence interval: $conf"
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Reading half file starting from random byte point in Java

On By panoskrtIn coding3 Comments

I had to write a method within a Java testing application to read exactly the half file (no matter the size) of a binary file full of integers and store the integers in an array of integers. The method should calculate the random byte point and the size of the array based on the file size which is unknown.

At first place a File object is created and its size is identified:

    File dataFile = new File("/tmp/datastream.data");
    long data_size = dataFile.length();

One integer consists of 4 bytes. Therefore a valid integer within the file starts from point 4, and that is going to be the minimum value for the random. The maximum value is determined as the half size of the file in integers. If the data in the file were written using raw byte data stream, that would be divided by 2, but because one integer is 4 bytes, it has to be divided by 8.

    int minRandom = 4;
    int maxRandom = data_size / 8;

Having set the boundaries of the random byte, it has now to be calculated. The computed value must be a valid integer within the file, that means that its modulus when divided by 4 should be zero.

int randomNum;
do {
    randomNum = minRandom + (int)(Math.random() * ((maxRandom - minRandom) + 1));
} while (randomNum % 4 != 0);

Once the number is calculated, it is assigned as the start seek point. There’s a need for two variables with the same value as one of them will be used to identify the next seek byte in the file in order to read the next integer.

    seekByte = randomNum;
    initialSeekByte = randomNum;

Before reading the data, the array that will store the data has to be created. That will be the half size of the file considering that integers need to be stored:

    int_array = new int[data_size / 8];

Finally, the data can be fetched. Every next integer to be read must be the previous seek byte plus 4 to ensure a valid integer is read:

RandomAccessFile rafIN = new RandomAccessFile("/tmp/datastream.data", "r");
for (int i = 0; i < int_array.length; i++) {
    rafIN.seek(seekByte);
    int_array[i] = rafIN.readInt();
    seekByte += 4;
}