在Python中线性化列表的最短方法

Question

I want to make a list with linearly increasing values from a list with non-linearly increasing values in Python. For example

input =[10,10,10,6,6,4,1,1,1,10,10]

should be transformed to:

output=[0,0,0,1,1,2,3,3,3,0,0]

My code uses a python dictionary

def linearize(input):
    """
    Remap a input list containing values in non linear-indices list
    i.e.
    input = [10,10,10,6,6,3,1,1]
    output= [0,0,0,1,1,2,3,3] 
    """
    remap={}
    i=0
    output=[0]*len(input)
    for x in input:
        if x not in remap.keys():
            remap[x]=i
            i=i+1
    for i in range(0,len(input)):
        output[i]=remap[input[i]]
    return output

but I know this code can be more efficient. Some ideas to do this task better and in a more pythonic way, Numpy is an option? This function has to be called very frequently on big lists.

Answer 1

As per your comment in the question , you are looking for something like this

data = [8,8,6,6,3,8]
from itertools import count
from collections import defaultdict
counter = defaultdict(lambda x=count(): next(x))
print([counter[item] for item in data])
# [0, 0, 1, 1, 2, 0]

Thanks to poke ,

list(map(lambda i, c=defaultdict(lambda c=count(): next(c)): c[i], data))

Its just a one liner now :)

Answer 2

Use collections.OrderedDict :

In [802]: from collections import OrderedDict
     ...: odk=OrderedDict.fromkeys(l).keys()
     ...: odk={k:i for i, k in enumerate(odk)}
     ...: [odk[i] for i in l]
Out[802]: [0, 0, 0, 1, 1, 2, 3, 3, 3]

Answer 3

A simpler solution without imports:

input =[10,10,10,6,6,4,1,1,1,10,10]
d = {}
result = [d.setdefault(x, len(d)) for x in input]

Answer 4

I came up with this function using numpy which in my tests worked faster than yours when input list was very big like 2,000,000 elements.

import numpy as np
def linearize(input):
    unique, inverse = np.unique(input, return_inverse=True)
    output = (len(unique)-1) - inverse
    return output

Also, this function only works if your input is in Descending order like your example. Let me know if it helps.