CODEWITHSUNDEEP
python
ca9163d9
ca9163d9

Reputation: 29159

Iterate filter object multiple times doesn't get correct value

The following code gets correct answer because list() is called before assigning to b. max(b) may get wrong value if removing list.

a = map(lambda x: (x[0]*10 + x[1], x[2]*10 + x[3]), aLongList)
b = list(filter(lambda x: x[0] < 24 and x[1] < 60, a)) # convert to a list
if not any(b):
    #.... omitted
max(b)

Is it a way to get the right result without creating a list (in case the list is huge)?

Test:

The following code

def perm(A):
    for i,x in enumerate(A):
        for j,y in enumerate(A):
            if i == j: continue
            for k,z in enumerate(A):
                if i == k or j == k: continue
                for l,a in enumerate(A):
                    if l == i or l == j or l == k: continue
                    yield [A[i], A[j], A[k], A[l]]

a = map(lambda x: (x[0]*10 + x[1], x[2]*10 + x[3]), perm([1,9,6,0]))
b = (filter(lambda x: x[0] < 24 and x[1] < 60, a)) # Removed list, so b is a filter obj instead of list

if not any(b):
    pass
max(b)

returns (16, 9) instead of (19, 6).

Upvotes: 0

Views: 150

Answers (1)

Chris
Chris

Reputation: 29742

Use itertools.tee.

Return n independent iterators from a single iterable.

from itertools import tee

b1, b2 = tee(filter(lambda x: x[0] < 24 and x[1] < 60, a), 2)

if not any(b1):
    pass
max(b2)

Output:

(19, 6)

Upvotes: 1

Related Questions