Randomly chose an element of one list that's NOT in a second list
Question:
Say I have a list2 of randomly chosen elements from a large list1. Is there a clever way of choosing an element from list1 that’s NOT already in list2?
For example:
list1 = range(20,100)
list2 = [37,49,22,35,72] # could be much longer
while True:
n = random.choice(list1)
if n not in list2:
break
# now n is an element of list1 that's not in list2
I feel like there must be a more efficient way of doing this than a guess-and-check while-loop.
Answers:
In case that there are no repeating elements in list1, this is a pythonic way, working with set and -:
import random
list1 = range(20,100)
list2 = [37,49,22,35,72] # could be much longer
n = random.choice(tuple(set(list1)-set(list2)))
# now n is an element of list1 that's not in list2
The tuple call is needed to avoid a NotIndexable exception.
You can subtract list2 of list1:
list3 = list(set(list1)-set(list2))
and choose from it randomly:
random.choice(list3)
Note: you need to reconvert the set to a list.
You could use:
import random
list1 = range(20,100)
list2 = [37,49,22,35,72]
not_in_list2 = [item for item in list1 if item not in list2]
n = random.choice(not_in_list2)
This uses a list comprehension to create a list of all elements in list1 that aren’t inlist2. It then selects randomly from this list. Unlike when working with sets, this technique does not change the probability of items being selected, because it does not remove duplicate elements from list1.
If you want to randomly select more than one item from a list, or select an item from a set, it’s better to use random.sample instead of choice
import random
diff = set(list1)-set(list2)
num_to_select = 1 # set the number to select here.
list_of_random_items = random.sample(diff, num_to_select)
If you do not want the overhead of creating a new list (or a new list and two sets) which can become quite costly if list1 is very large, there is another option.
import random
list1 = range(20,100)
list2 = [37,49,22,35,72]
for i in list2:
while i in list1:
list1.remove(i)
random.choice(list1)
Just iterate through the items in list2 and remove them from list1. Since list.remove() only removes the first occurrence of an item, I added a while-loop to ensure that all occurences are removed.
Say I have a list2 of randomly chosen elements from a large list1. Is there a clever way of choosing an element from list1 that’s NOT already in list2?
For example:
list1 = range(20,100)
list2 = [37,49,22,35,72] # could be much longer
while True:
n = random.choice(list1)
if n not in list2:
break
# now n is an element of list1 that's not in list2
I feel like there must be a more efficient way of doing this than a guess-and-check while-loop.
In case that there are no repeating elements in list1, this is a pythonic way, working with set and -:
import random
list1 = range(20,100)
list2 = [37,49,22,35,72] # could be much longer
n = random.choice(tuple(set(list1)-set(list2)))
# now n is an element of list1 that's not in list2
The tuple call is needed to avoid a NotIndexable exception.
You can subtract list2 of list1:
list3 = list(set(list1)-set(list2))
and choose from it randomly:
random.choice(list3)
Note: you need to reconvert the set to a list.
You could use:
import random
list1 = range(20,100)
list2 = [37,49,22,35,72]
not_in_list2 = [item for item in list1 if item not in list2]
n = random.choice(not_in_list2)
This uses a list comprehension to create a list of all elements in list1 that aren’t inlist2. It then selects randomly from this list. Unlike when working with sets, this technique does not change the probability of items being selected, because it does not remove duplicate elements from list1.
If you want to randomly select more than one item from a list, or select an item from a set, it’s better to use random.sample instead of choice
import random
diff = set(list1)-set(list2)
num_to_select = 1 # set the number to select here.
list_of_random_items = random.sample(diff, num_to_select)
If you do not want the overhead of creating a new list (or a new list and two sets) which can become quite costly if list1 is very large, there is another option.
import random
list1 = range(20,100)
list2 = [37,49,22,35,72]
for i in list2:
while i in list1:
list1.remove(i)
random.choice(list1)
Just iterate through the items in list2 and remove them from list1. Since list.remove() only removes the first occurrence of an item, I added a while-loop to ensure that all occurences are removed.