from random import *
from math import *
from pylab import *
import numpy as np


def exp_pdf(x,d,l):

	if x <= d:
		return 0
	else:
		return l * exp( - l * (x-d) )

def main():

	start = 0
	end = 20
	points = 1000


	X = np.linspace(start,end,points,endpoint=True)
	Y = exp_pdf(X,0,1)


	plot(X,Y)

	show()


main()

Traceback (most recent call last):
  File "tests.py", line 34, in <module>
    main()
  File "tests.py", line 26, in main
    Y = exp_pdf(X,0,1)
  File "tests.py", line 9, in exp_pdf
    if x <= d:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()