How to know if a list has an even or odd number of elements

Question

How can I find out if there is even, or odd, number of elements in an arbitrary list.

I tried list.index() to get all of the indices... but I still don\'t k

Answer 1

if you want to be bitwise about it you could also use.

if len(mylist)&1:
    print("odd")
else:
    print("even")

this is really Nice looking if you know your list is more likely to be odd.

if len(mylist)&1:
    print("list was odd")
elif CheckSomthingElse():
    print("list was even AND CheckSomthingElse was True")
else:
    print("list was even AND CheckSomthingElse was False")

Wow in python by the test below % 2 is faster. python must be optimized for it. in some other languages & 1 is faster. this is why it's important to always test.

import timeit

MyListOdd=[1,2,3,4,5,6,7]
MyListEven=[1,2,3,4,5,6]
print("MyListOdd  == ",MyListOdd)
print("MyListEven == ",MyListEven)
print("len(MyListOdd)  == ",len(MyListOdd))
print("len(MyListEven) == ",len(MyListEven))

def TestIfEvenBitwise(MyList):
    if len(MyList)&1:
        return False #Odd
    else:
        return True #Even

def TestIfEvenModulus(MyList):
    if len(MyList)%2:
        return False #Odd
    else:
        return True #Even

print("TestIfEvenBitwise(MyListOdd)  == ",TestIfEvenBitwise(MyListOdd))
print("TestIfEvenModulus(MyListOdd)  == ",TestIfEvenModulus(MyListOdd))
print("TestIfEvenBitwise(MyListEven) == ",TestIfEvenBitwise(MyListEven))
print("TestIfEvenModulus(MyListEven) == ",TestIfEvenModulus(MyListEven))

mysetup = """
MyListOdd=[1,2,3,4,5,6,7]
MyListEven=[1,2,3,4,5,6]

def TestIfEvenBitwise(MyList):
    if len(MyList)&1:
        return False #Odd
    else:
        return True #Even

def TestIfEvenModulus(MyList):
    if len(MyList)%2:
        return False #Odd
    else:
        return True #Even
"""

print("timeit.timeit(setup = mysetup,stmt ='TestIfEvenBitwise(MyListOdd)', number=100000) == ",
      timeit.timeit(setup = mysetup,stmt ='TestIfEvenBitwise(MyListOdd)', number=100000))
print("timeit.timeit(setup = mysetup,stmt ='TestIfEvenModulus(MyListOdd)', number=100000) == ",
      timeit.timeit(setup = mysetup,stmt ='TestIfEvenModulus(MyListOdd)', number=100000))
print("timeit.timeit(setup = mysetup,stmt ='TestIfEvenBitwise(MyListEven)', number=100000) == ",
      timeit.timeit(setup = mysetup,stmt ='TestIfEvenBitwise(MyListEven)', number=100000))
print("timeit.timeit(setup = mysetup,stmt ='TestIfEvenModulus(MyListEven)', number=100000) == ",
      timeit.timeit(setup = mysetup,stmt ='TestIfEvenModulus(MyListEven)', number=100000))

results of test.

MyListOdd  ==  [1, 2, 3, 4, 5, 6, 7]
MyListEven ==  [1, 2, 3, 4, 5, 6]
len(MyListOdd)  ==  7
len(MyListEven) ==  6
TestIfEvenBitwise(MyListOdd)  ==  False
TestIfEvenModulus(MyListOdd)  ==  False
TestIfEvenBitwise(MyListEven) ==  True
TestIfEvenModulus(MyListEven) ==  True
timeit.timeit(setup = mysetup,stmt ='TestIfEvenBitwise(MyListOdd)', number=100000) ==  0.02574796500000004
timeit.timeit(setup = mysetup,stmt ='TestIfEvenModulus(MyListOdd)', number=100000) ==  0.022446242000000005
timeit.timeit(setup = mysetup,stmt ='TestIfEvenBitwise(MyListEven)', number=100000) ==  0.026081517000000054
timeit.timeit(setup = mysetup,stmt ='TestIfEvenModulus(MyListEven)', number=100000) ==  0.025758655999999935