# Printing nth bell number

Bell number $B(n)$ is defined as the number of ways of splitting $n$ into any number of parts, also defined as the sum of previous $n$ Stirling numbers of second kind.

Here is a snippet of Python code that Wikipedia provides (slightly modified) to print bell numbers:

def bell_numbers(start, stop):

t = []                        ## Initialize the triangle as a two-dimensional array
c = 1                            ## Bell numbers count
while c <= stop:
if c >= start:
yield t[-1]           ## Yield the Bell number of the previous row
row = [t[-1][-1]]            ## Initialize a new row
for b in t[-1]:
row.append(row[-1] + b)  ## Populate the new row
c += 1                       ## We have found another Bell number
t.append(row)                ## Append the row to the triangle
for b in bell_numbers(1, 9):
print b


But I have to print the $n$th bell number, so what I did was I changed the second last line of code as follows:

for b in bell_numbers(n,n)


This does the job, but I was wondering of an even better way to print the $n$th bell number.

• Please check whether your code is indented correctly. – 200_success Nov 17 '16 at 22:31

You can use the Dobinski formula for calculating Bell numbers more efficiently.

Dobinski's formula is basically something like this line, although the actual implementation is a bit more complicated (this function neither returns precise value nor is efficient, see this blogpost for more on this):

import math
ITERATIONS = 1000
def bell_number(N):
return (1/math.e) * sum([(k**N)/(math.factorial(k)) for k in range(ITERATIONS)])


E.g. you can use the mpmath package to calculate the nth Bell number using this formula:

from mpmath import *
mp.dps = 30
print bell(100)


You can check the implementation here

• The first method is returning error values as follows, the 5th bell number is actually 52 but this returns 50.767... and same is for other higher bell polynomials as well, I dont understand why this error is showing up. – Tomarinator May 28 '12 at 14:54
• I know my bell_number function doesn't return precise value, it is just meant to display the general idea. The reason for the error is described in the referred blogpost, look for this part – bpgergo May 28 '12 at 15:26
• I see, thanks for this awesome answer, but i cant fully understand what mpmaths is, is it a pyhton library? and how to write the code that uses mpmaths in calculating Bell number,that code returns an error. – Tomarinator May 28 '12 at 15:36
• mpmath is a Python module (library). You'll need to install that before you can use it. mpmath install instructions – bpgergo May 28 '12 at 15:40

Change yield t[-1] to yield t[-1][-1] so the $n$th Bell number is on the $n$th line - that is, gives correct output, so the call:

for b in bell_numbers(1, 9):
print b


prints the correct bell numbers 1 to 9.

So, if you just want the $n$th Bell number only:

for b in bell_numbers(n, n):
print b


or change the code to take just one argument.