如何在不删除先前相同值的情况下 select 列表中具有重复项的特定数字？

Question

I have the following list in python3:

a = [1, 1, 1, 2, 2, 3, 3, 3, 1, 1, 1, 2, 2, 2, 4, 4, 4, 3, 3, 3, 3]

I want to have something like this as an output:

b = [1, 2, 3, 1, 2, 4, 3]

How can I do that? I have tried a = set(a) and other methods for getting rid of duplicates. But they remove all duplicates.

Answer 1

This would work:

a = [1, 1, 1, 2, 2, 3, 3, 3, 1, 1, 1, 2, 2, 2, 4, 4, 4, 3, 3, 3, 3]
b = [a[0]]
for k in a:
    if k!=b[-1]:
        b.append(k)
print(b)

Answer 2

If you are willing to use a module, you can use itertools.groupby :

from itertools import groupby

a = [1, 1, 1, 2, 2, 3, 3, 3, 1, 1, 1, 2, 2, 2, 4, 4, 4, 3, 3, 3, 3]

output = [k for k, _ in groupby(a)]
print(output) # [1, 2, 3, 1, 2, 4, 3]

Answer 3

If you comfortable with C, you may like this style:

a = [1, 1, 1, 2, 2, 3, 3, 3, 1, 1, 1, 2, 2, 2, 4, 4, 4, 3, 3, 3, 3]
b=len(a)
i=0
last_one=None
while i<b:
    if a[i]==last_one:
        del a[i]
        b-=1
        continue
    else:
        last_one=a[i]
    i+=1
print(a)