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 know how I can tell the program what is an even and what is an odd number.

Solution 1

You can use the built in function len() for this.

Python Doc -- len()

Gets the length (# of elements) of any arbitrary list.

myList = [0,1,2,3,4,5]

if len(myList) % 2 == 0:
    print ("even")
else
    print ("odd")

Define function that returns a bool (true or false).

def is_even(myList):

    if len(myList) % 2 == 0:
        return true
    else:
        return false

main():

    myList = [0,1,2,3]
    theListIsEven = is_even(myList)  # will be true in this example
                                     # because 4 items in myList

    if theListIsEven(myList) == True:
        # do something
    else:
        # do something else

    return 0

The modulus operator % gives the remainder.

EX: 7 % 2 = 1

• Closest number to 7 that 2 will divide evenly is 6
• Which is 1 away from 7.
• Thus, remainder of 1 for 7 % 2.

EX: 4 % 2 = 0

• Any even number n will give 0 as the remainder when n % 2
• Because n has divided evenly by 2

Solution 2

your_list = [1,2,3,(4,5)]

# modulo operation finds the remainder of division of one number by another.
if len(your_list) % 2 == 0:
    print "Even Number"
else:
    print"number is odd"

Solution 3

All you need is

len(listName)

Which will give you the length.

I guess you could also do this then

if len(listName) % 2 == 0:
    return True  # the number is even!
else:
    return False # the number is odd!

Solution 4

if len(mylist)%2==0:
     #even
else:
     #odd

Solution 5

def has_even_length(some_sequence):
    return not len(some_sequence)%2

def has_odd_length(some_sequence):
    return bool(len(some_sequence)%2)

View more solutions

Comments

[loopbackbee]

you can get the number of elements of a list using the len() builtin function

[user3029969]

@goncalopp... but how would I tell it is odd number of even

[dm03514]

len(your_list) % 2 == 0

[user3029969]

@dm03514 ... what does your code mean

[datenwolf]

@user3029969: It does what you asked for.

[user3029969]

but how would I tell it is odd number of even for any arbitrary list

[datenwolf]

By applying modulus 2 on the returned length of the list. Which is exactly what codehorse did in his answer.

[jonhopkins]

@user3029969, the code len(listName) % 2 is dividing the number of elements in the array by 2 and returning the remainder. If the remainder is 0, then the number of elements is divisible by 2, which means it is even. If it returns 1, then it is odd.

[Ed S.]

return len(listName) % 2 == 0