Possible Duplicate:
Get difference from two lists in Python
What is a simplified way of doing this? I have been trying on my own, and I can't figure it out. list a and list b, the new list should have items that are only in list a. So:
a = apple, carrot, lemon
b = pineapple, apple, tomato
new_list = carrot, lemon
I tried writing code, but every time it always returns the whole list a to me.
asked Jul 11, 2012 at 14:14
0
You can write this using a list comprehension which tells us quite literally which elements need to end up in new_list:
a = ['apple', 'carrot', 'lemon']
b = ['pineapple', 'apple', 'tomato']
# This gives us: new_list = ['carrot' , 'lemon']
new_list = [fruit for fruit in a if fruit not in b]
Or, using a for loop:
new_list = []
for fruit in a:
if fruit not in b:
new_list.append[fruit]
As you can see these approaches are quite similar which is why Python also has list comprehensions to easily construct lists.
answered Jul 11, 2012 at 14:18
Simeon VisserSimeon Visser
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You can use a set:
# Assume a, b are Python lists
# Create sets of a,b
setA = set[a]
setB = set[b]
# Get new set with elements that are only in a but not in b
onlyInA = setA.difference[b]
UPDATE
As iurisilvio and mgilson pointed out, this approach only works if a and b do not contain duplicates, and if the order of the elements does not matter.
Zulu
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answered Jul 11, 2012 at 14:22
5
You may want this:
a = ["apple", "carrot", "lemon"]
b = ["pineapple", "apple", "tomato"]
new_list = [x for x in a if [x not in b]]
print new_list
answered Jul 11, 2012 at 14:21
martinmartin
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Would this work for you?
a = ["apple", "carrot", "lemon"]
b = ["pineapple", "apple", "tomato"]
new_list = []
for v in a:
if v not in b:
new_list.append[v]
print new_list
Or, more concisely:
new_list = filter[lambda v: v not in b, a]
answered Jul 11, 2012 at 14:17
Alex WilsonAlex Wilson
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How about using
sets [or the built in set since Sets was deprecated in 2.6]?
from sets import Set
a = Set[['apple', 'carrot', 'lemon']]
b = Set[['pineapple','apple','tomato']]
new_set = a.difference[b]
print new_set
gives the output
Set[['carrot', 'lemon']]
istruble
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answered Jul 11, 2012 at 14:23
StuGreyStuGrey
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