#!/usr/bin/env python
#                    Simple python examples
# python/centrallimit/randomwalk4w.py
#                    David MacKay
#
# Make a walk with a fair coin, printing to file
# the points reached at times 0, period,
# 2*period, 3*period...
#
# Usage:
# $ randomwalk4w.py [R [T [period [filename]]]]
# Optional arguments:
#    R      = number of walks
#    T      = duration of walk
#    period = period between plots
#
# Example:
# $ randomwalk4w.py 1 100 1  1walk.txt
# $ randomwalk4w.py 8 10>000 10 8walks.txt
import random
import sys
import os

def walk(T, period, outputfile):
    """random walk with a fair coin"""
    x=0;    outputfile.write( "0 \t"+str(x)+"\n" ) # start
    for t in xrange(T+1):
        u = random.random()
        if (u<= 0.5): x+=1 
        else:         x-=1 
        if (t%period == 0 ) :
            outputfile.write( "%d \t %d\n" % ( t+1 , x ) )
    pass

def makewalks(R=10, T=100,period=1, filename="tmp.txt"):
    outputfile = open( filename , "w" )
    for r in xrange(R):
        walk(T,period,  outputfile)
        outputfile.write("\n\n") ## two blank lines
        ## between runs - useful for gnuplot
    outputfile.close();

if __name__ == '__main__':
    R=2
    T=100
    period=1
    filename="tmp.txt"
    if(len(sys.argv)>1):
        R=int(sys.argv[1])
    if(len(sys.argv)>2):
        T=int(sys.argv[2])
    if(len(sys.argv)>3):
        period=int(sys.argv[3])
    if(len(sys.argv)>4):
        filename=str(sys.argv[4])
    makewalks(R, T, period , filename )
    pass

# This program illustrates
#  - how to read the command line
#  - how to print stuff to file ready for plotting
#  - how to use random.random()
#      (there are many other random functions
#       available in python; random.random()
#       makes numbers between 0 and 1)
#  - how to write a package that contains
#       functions and that has a default behaviour
#       if run directly from a terminal
#       or from a python development environment