重新编号一维数组的元素

Question

I am new here, I want to share with you, a problem that I am facing

Basically I have to renumber an array in accordance to the elements of an another array.

More in details:

I have an int array a

a=array([    1,     2,     3, ..., 21041, 21042, 21043])

with

len(a)=21043

Then, applying a mask to "a" it obtains b

b=a[mask]
b=array[1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 28 32 33 34 47 54 55 62....21043]

with

len(b)=15717

At this point I have sorted easly, this array in such way:

b_renumber=array[1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23....15717]

So the number 28, in "b", becames 21 in "b_renumber", 32 becames 22 and so on.

Now the problem is the third array c

since c has a lenght greater then b.

len(c)=46153

and is given by:

c=array[ 3 4 142 633 12 19 564 497 513 54 308 177 254 532 155 3 273 28 ...21037]

So basically c is an array of length 46153 (unsorted) with the elements that go from 1 to 21043, as well as the array b, and like b some numbers doesn't exist (eg 21 22 23...etc) ie the same elements that do not exist in b also in c are not present.

What I want to do is renumber the elements of c in such that there is a linking between the value of elements of c and the b_renumber. Example in b we have the number 28, that in b_renumber becames 21 (from b to b_renumber, 28 -> 21), and what I want to do is the follow: each time that I meet the value 28 in c, this must becames 21, same for the other numbers.

Just a last stuff: the array are still small, but I have to work with array with greater dimension, so I have to do this in efficient way, maybe exist a function o some package that allow me do this. Please I am open to every advises

Sorry for the length of the post I hope someone can and want help me.

Thank you in advance.

Matteo

Answer 1

Assuming b_renum just contains the index+1 of each element in b:

Input:

b = [0,1,2,100]
c = [20,50,60,100]

Note that dict.get(key, default) returns the key's value if there's a match, or the default otherwise.

b_renum = list(range(1,len(b)+1))
renum_dict = dict(zip(b, b_renum))
# slightly slower
# renum_dict = {elem:ind for ind,elem in enumerate(b)}

c_renum = [renum_dict.get(key,key) for key in c]

>>> print(b_renum)
[1, 2, 3, 4]

>>> print(c_renum)
[20, 50, 60, 3]

Note how 100 gets mapped to 3 in c_renum . I think the useful things for you are 1) using zip() since you know b and b_renum are the same length, and 2) using a dictionary to lookup values + the dict.get() trick. The list comprehension is also quite fast.