Statistics with Python

Question

Write a program to determine the mean, median, and standard deviation of a list of numbers
Count the duplicate numbers in the code, listing any that occur more than once with the number of times it occurs.

My implementation:

import math

def mean(input):
  return sum(input) / float(len(input))

def median(input):
  sorted_input = sorted(input)
  length = len(input)
  if length < 1:
    return None
  if length % 2 == 0:
    return (sorted_input[length / 2] + sorted_input[length / 2 - 1]) / float(2)
  else:
    return sorted_input[length / 2]

def duplicate_counts(input):
  distinct_input = set(input)
  for distinct in set(input):
    count = input.count(distinct)
    if (count > 1):
      print("{} occurs {} times".format(distinct, count))


def standard_deviation(input):
  return math.sqrt(mean([(x - mean(input)) ** 2 for x in input]))

def main():
  #sample run/test
  the_list = [3, 6, 7, 2, 8, 9, 2, 3, 7, 4, 5, 9, 2, 1, 6, 9, 6]
  print("The mean is: {}".format(mean(the_list)))
  print("The median is: {}".format(median(the_list)))
  print("The standard deviation is: {}".format(standard_deviation(the_list)))
  duplicate_counts(the_list)


if __name__ == "__main__":
  main()

Sample output:

The mean is: 5.23529411765  
The median is: 6  
The standard deviation is: 2.64640514805  
2 occurs 3 times  
3 occurs 2 times  
6 occurs 3 times  
7 occurs 2 times  
9 occurs 3 times  

Simple exercise I decided to do with Python for practice and to test/try all the excellent feedback I received on my previous two questions. There may be built in functions for these, and feel free to mention them, but for the sake of practice I implemented them this way.

Answer 1

• A variable name input is confusing. After all, input is a Python built-in.

• standard_deviation calls mean too many times (add a debug printout to mean to see). Better do

  def standard_deviation(input):
      input_mean = mean(input)
      return math.sqrt(mean([(x - input_mean) ** 2 for x in input]))
  

• Median can be calculated in \$ O(\log n)\$. Sorting a list in \$O(n \log n)\$ just for a median is an overkill.

Answer 2

In mean, you're wrapping the result of len in a float. I'm guessing this is because of Python 2's odd division that will always return an int and not a float.

>>> 12/5
2

In Python 3.x this is no longer an issue as it will return proper float values, and you can get this in Python 2 as well, by using from __future__ import division. from __future__ is a way of importing new additions in Python 3 that were added to 2 but not in a way that overrides the old functionality. By importing this, division returns floats in Python 2 as well:

>>> from __future__ import division
>>> 12/5
2.4

Also I notice you use float later on the literal value 2. But you could just use 2.0 and get the same effect even if you don't do the import.

>>> 12/5.0
2.4

Should median really return None for input of length 0? Shouldn't that raise ValueError rather than silently return a non result?

Also you should indent by 4 spaces rather than 2. That's the commonly accepted Python style, and since it affects flow people could easily misread your code this way.