CODEWITHSUNDEEP
pythonpython-2.7python-2.x
user1823
user1823

Reputation: 1111

How do I use all() built-in function?

I am attempting to use all() but it is not working for me:

>>> names = ["Rhonda", "Ryan", "Red Rackham", "Paul"]
>>> all([name for name in names if name[0] == "R"])
True
>>>

I am trying to check if all the names begin with "R", and even though I added "Paul" to names, all() still returns True. How do I fix this so that all() returns False until "Paul" is removed?

Upvotes: 2

Views: 71

Answers (4)

user2555451
user2555451

Reputation:

You misunderstand how all works. From the docs:

all(iterable)

Return True if all elements of the iterable are true (or if the iterable is empty).

In your code, you are first collecting all names that start with R into a list and then passing this list to all. Doing this will always return True because non-empty strings evaluate to True.

Instead, you should write:

all(name[0] == "R" for name in names)

This will pass an iterable of booleans to all. If all of them are True, the function will return True; otherwise, it will return False.

As an added bonus, the result will now be computed lazily because we used a generator expression instead of a list comprehension. With the list comprehension, the code needed to test all strings before determining a result. The new code however will only check as many as necessary.

Upvotes: 8

ZdaR
ZdaR

Reputation: 22964

The reason why you were getting wrong result was because you are already creating a new list using list comprehension by applying the desired condition, So if we do a little breakdown then:

>>> print [name for name in names if name[0] == "R"]
>>> ['Rhonda', 'Ryan', 'Red Rackham']
>>> print all(['Rhonda', 'Ryan', 'Red Rackham'])
>>> True

So the correct way may be:

names = ["Rhonda", "Ryan", "Red Rackham", "Paul"]

res = all(map(lambda x : x[0]=="R", names))
# map() returns: [True, True, True, False] 
# all([True, True, True, False]) == False
print res

Upvotes: 0

Pedro Lobito
Pedro Lobito

Reputation: 99001

names = ["Rhonda", "Ryan", "Red Rackham", "Paul"]
if all(c[0] == "R" for c in names):
    print "ALL MATCH"

Demo:

http://ideone.com/KenqJl

Upvotes: 0

EvenLisle
EvenLisle

Reputation: 4812

names = ["Rhonda", "Ryan", "Red Rackham", "Paul"]
print all(map(lambda name: name[0] == "R", names))
# prints False
names = ["Rhonda", "Ryan", "Red Rackham"]
print all(map(lambda name: name[0] == "R", names))
# prints True

Upvotes: 0

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