How to compare list in Python?

Question

I have one list

a = [1.0, 2.0, 2.1, 3.0, 3.1, 4.2, 5.1, 7.2, 9.2]

I want to compare this list with other list but also I want to extract the information regarding the list content in numeric order.All other list have the elements that are same as a.

So I have tried this

a = [1.0, 2.0, 2.1, 3.0, 3.1, 4.2, 5.1, 7.2, 9.2]
b = [1, 2, 3, 4, 5, 6, 7, 8, 9]
print dict(zip(a,b))
a1=[2.1, 3.1, 4.2, 7.2]

I want to compare a1 with a and extract dict values [3, 5, 6, 8].

Answer 1

Just loop through a1 and see if there is a matching key in the dictionary you created:

mapping = dict(zip(a, b))
matches = [mapping[value] for value in a1 if value in mapping]

Demo:

>>> a = [1.0, 2.0, 2.1, 3.0, 3.1, 4.2, 5.1, 7.2, 9.2]
>>> b = [1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> a1 = [2.1, 3.1, 4.2, 7.2]
>>> mapping = dict(zip(a, b))
>>> [mapping[value] for value in a1 if value in mapping]
[3, 5, 6, 8]

However, take into account that you are using floating point numbers. You may not be able to match values exactly, since floating point numbers are binary approximations to decimal values; the value 2.999999999999999 (15 nines) for example, may be presented by the Python str() function as 3.0, but is not equal to 3.0:

>>> 2.999999999999999
2.999999999999999
>>> str(2.999999999999999)
'3.0'
>>> 2.999999999999999 =わ=わ 3.0
False
>>> 2.999999999999999 in mapping
False

If your input lists a is sorted, you could use the math.isclose() function (or a backport of it), together with the bisect module to keep matching efficient:

import bisect
try:
 from math import isclose
except ImportError:
 def isclose(a, b, rel_tol=1e-09, abs_tol=0.0):
 # simplified backport, doesn't handle NaN or infinity.
 if a == b: return True
 return abs(a-b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)
result = []
for value in a1:
 index = bisect.bisect(a, value)
 if index and isclose(a[index - 1], value):
 result.append(b[index - 1])
 elif index < len(a) and isclose(a[index], value):
 result.append(b[index])

This tests up to two values from a per input value; one that is guaranteed to equal or lower (at index - 1) and the next, higher value. For your sample a, the value 2.999999999999999 is bisected to index 3, between 2.1 and 3.0. Since isclose(3.0, 2.999999999999999) is true, that would still let you map that value to 4 in b.

• Richard Rublev: Peters Thanks.The numbers in the list are result of the execution of the FORTRAN code,formated with f11.5.So I think the simple version would suffice!
• Martijn Pieters: Right, if you have strings, then you skirt around the issue quite nicely.