Calculate difference between adjacent items in a python list

Question

I have a list of millions of numbers. I want to find out if the difference between each number in the ordered list is the same for the entire list.

list_example = [ 0, 5, 10, 15, 20, 25, 30, 35, 40, ..etc etc etc]

What's the best way to do this?

My try:

import collections

list_example = [ 0, 5, 10, 15, 20, 25, 30, 35, 40 ]

count = collections.Counter()

for x,y in zip(list_example[0::],list_example[1::]):
    print x,y,y-x
    count[y-x] +=1

if len( count ) == 1:
    print 'Differences all the same'

Result:

0 5 5
5 10 5
10 15 5
15 20 5
20 25 5
25 30 5
30 35 5
35 40 5
Differences all the same

Answer 1

Using pure Python:

>>> x = [0,5,10,15,20]
>>> xdiff = [x[n]-x[n-1] for n in range(1,len(x))]
>>> xdiff
[5, 5, 5, 5]
>>> all([xdiff[0] == xdiff[n] for n in range(1,len(xdiff))])
True

It's a little easier, and probably faster, if you use NumPy:

>>> import numpy as np
>>> xdiff = np.diff(x)
>>> np.all(xdiff[0] == xdiff)
True

But both of these create two extra lists (or arrays, in the case of NumPy) which may gobble up your available memory if you have millions of numbers.

Answer 2

The straight approach here is the best:

x = s[1] - s[0]
for i in range(2, len(s)):
    if s[i] - s[i-1] != x: break
else:
    #do some work here...