import sys
import math

USAGE = "USAGE: compute-t.py <file>"

##############################################################################
# Turn a space-delimited string of numbers into a list of floats.
def makeArray (myString):

  # Break the string into words.
  words = myString.split()
  
  # Convert the words to floats.
  returnValue = []
  for word in words:
    returnValue.append(float(word))
    
  return(returnValue)

##############################################################################
# Compute the mean of a given list of numbers.
def computeMean (myList):
  sum = 0.0
  for value in myList:
    sum += value
  return(sum / len(myList))

##############################################################################
# Given the mean, compute the sum-of-squares of a list of numbers.
def computeSS (mean, myList):
  returnValue = 0.0
  for value in myList:
    difference = value - mean
    returnValue += difference * difference
  return(returnValue)

##############################################################################
# MAIN PROCEDURE
##############################################################################

# Parse the command line.
if (len(sys.argv) != 2):
  print USAGE
  sys.exit(1)
  
# Read each line of the file into an array.
myFile = open(sys.argv[1], "r")
lines = myFile.readlines()
if (len(lines) != 2):
  print "Too many lines in file."
  sys.exit(1)
firstSet = makeArray(lines[0])
secondSet = makeArray(lines[1])

# Compute the mean of each array.
firstMean = computeMean(firstSet)
secondMean = computeMean(secondSet)

# Compute the sum-of-squares for each array.
firstSS = computeSS(firstMean, firstSet)
secondSS = computeSS(secondMean, secondSet)

# Compute the pooled variance.
pooledVariance = (firstSS + secondSS) / (len(firstSet) + len(secondSet) - 2)

# Compute the t-statistic and print.
# N.B. I didn't include the absolute value, to allow a two-tailed statistic.
tStatistic = ((firstMean - secondMean) / 
              math.sqrt((pooledVariance / len(firstSet)) + 
                        (pooledVariance / len(secondSet))))
print "%g" % tStatistic