I have a list like this:

 my_list=[False, False, True, True, False, False, True]

I want to change this list so that the first n occurrence of True remain in place and all others became False.

for example, if n=1, I want to create a new list same as this one:

my_new_list=[False, False, True, False, False, False, False]

and if n=2; this list should be created:

my_new_list=[False, False, True, True, False, False, False]

I can do this in a for loop easily, but what is the best way of doing this in Python?

Edit1

This is my code:

def f(l,n):
   c=1
   new_l=[False] * len(l)
   if n>= len(l):
       return new_l
   for i,v in enumerate(l):
      if v:
         if(c >= n):
            return new_l
         new_l[i]=True
         c  =1
        
   return new_l

This code goes over all the items of the list, but a portion of it. But it has 9 lines of code, is there any shorter version or a faster version?

CodePudding user response：

With a list comp:

my_new_list = [x and (n := n - 1) >= 0 for x in my_list]

Try it online!

CodePudding user response：

You could use accumulate (from itertools) to count how many True values you have so far and match it to the value of n:

my_list=[False, False, True, True, False, False, True]

from itertools import accumulate

n = 1
r = [isTrue and count<=n for isTrue,count in zip(my_list,accumulate(my_list))]
print(r)
[False, False, True, False, False, False, False]

n = 2
r = [isTrue and count<=n for isTrue,count in zip(my_list,accumulate(my_list))]
print(r)
[False, False, True, True, False, False, False]

using numpy

The same approach can be used more concisely with numpy:

import numpy as np

my_list=[False, False, True, True, False, False, True]

n=1
r = my_list & (np.cumsum(my_list)<=n)
print(r)
# [False False  True False False False False]

n=2
r = my_list & (np.cumsum(my_list)<=n)
print(r)
# [False False  True  True False False False]

CodePudding user response：

def function(l,n):
new_list = []
c = 0
for i in l:
    if not i:
        new_list.append(False)
    else:
        if c < n:
            new_list.append(False)
            c =1
        else:
            new_list.append(True)
return new_list

OR:

def function(l,n):
return [False if (not i) or (n := n-1) >= 0 else True for i in l]