CODEWITHSUNDEEP
python
Zack Maril
Zack Maril

Reputation: 145

How to do "if-for" statement in python?

With python, I would like to run a test over an entire list, and, if all the statements are true for each item in the list, take a certain action.

Pseudo-code: If "test involving x" is true for every x in "list", then do "this".

It seems like there should be a simple way to do this.

What syntax should I use in python?

Upvotes: 13

Views: 10896

Answers (6)

delimy
delimy

Reputation: 23

all() alone doesn't work well if you need an extra map() phase.

see below:

all((x==0 for x in xrange(1000))

and:

all([x==0 for x in xrange(1000)])

the 2nd example will perform 1000 compare even the 2nd compare render the whole result false.

Upvotes: 0

dr jimbob
dr jimbob

Reputation: 17741

Example (test all elements are greater than 0)

if all(x > 0 for x in list_of_xs):
    do_something()

Above originally used a list comprehension (if all([x > 0 for x in list_of_xs]): ) which as pointed out by delnan (Thanks) a generator expression would be faster as the generator expression terminates at the first False, while this expression applies the comparison to all elements of the list.

However, be careful with generator expression like:

all(x > 0 for x in list_of_xs)

If you are using pylab (launch ipython as 'ipython -pylab'), the all function is replaced with numpy.all which doesn't process generator expressions properly. 

all([x>0 for x in [3,-1,5]]) ## False
numpy.all([x>0 for x in [3,-1,5]]) ## False
all(x>0 for x in [3,-1,5]) ## False
numpy.all(x>0 for x in [3,-1,5]) ## True

Upvotes: 4

Sven Marnach
Sven Marnach

Reputation: 601789

Use all(). It takes an iterable as an argument and return True if all entries evaluate to True. Example:

if all((3, True, "abc")):
    print "Yes!"

You will probably need some kind of generator expression, like

if all(x > 3 for x in lst):
    do_stuff()

Upvotes: 28

khachik
khachik

Reputation: 28693

if reduce(lambda x, y: x and involve(y), yourlist, True):
   certain_action()

involve is the action you want to involve for each element in the list, yourlist is your original list, certain_action is the action you want to perform if all the statements are true.

Upvotes: 3

user225312
user225312

Reputation: 131687

>>> x = [True, False, True, False]
>>> all(x)
False

all() returns True if all the elements in the list are True

Similarly, any() will return True if any element is true.

Upvotes: 6

Herms
Herms

Reputation: 38798

I believe you want the all() method:

$ python
>>> help(all)
Help on built-in function all in module __builtin__:

all(...)
    all(iterable) -> bool

    Return True if bool(x) is True for all values x in the iterable.

Upvotes: 3

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