x = [0, 1, -2, 3, 4, 5]
all ([i for i in range (1, len(x)) if x [i-1] < x[i]]) ?
Why does this code print True?
1 > -2 so it should print False I think.
2 Answers
Your code doesn't do what you think it does. It first filters out some elements, and then evaluates a bunch of indices for truthiness. Since all of the indices are strictly positive, they are all truthy and your code always evaluates to True.
From your description, what you're actually trying to do is this:
>>> all(x[i-1] < x[i] for i in range (1, len(x)))
False
This iterates over all pairs of consecutive elements and checks whether the first element is less than the second.
Another way to write this is:
>>> all(a < b for (a, b) in zip(x, x[1::]))
False
1 Comment
Jean-François Fabre
shouldn't
zip(x, x[1::]) be faster ? (no need to generate an extra copy of the first list). And you removed the [] without explaining why it was useless.All values of i come from range(1, len(x)), so they're all positive integers. Positive integers are true-ish, so all() will return True.
5 Comments
Maroun
It's
True because the list is not empty, regardless of the sign of the numbers inside it.
Moses Koledoye
@MarounMaroun Wrong!. A list containing 0 (or something falsy) will not return
True with allMaroun
@MosesKoledoye Try
if [0]: print True
khelwood
@MarounMaroun Try
if all([0]): print TrueMaroun
My mistake, changing my vote. Thanks for the correction everyone.
[1, 3, 4, 5]all the elements in this list evaluates toTrue