How to print the number of times an element appears in a list in Python?

Question

I have two variables, x and y, where x is a number and y is a list of numbers.

I have to define a function count which determines how many times x appears in y, for instance,

count(1,[2,1,3,1,4])

should return 2.

Thank you in advance for your suggestions.

NB. I am not allowed to use the in-built count function, i.e. y.count(x).

Answer 1

You can use a generator expression and sum function like this

sum(item == x for item in y)

You can put that in a function, like this

def counter(my_list, key):
    return sum(item == key for item in my_list)

assert counter([2, 1, 3, 1, 4], 1) == 2
assert counter([2, 1, 3, 1, 4], 5) == 0
assert counter([2, 1, 3, 1, 4], 2) == 1

This works because, in Python booleans are subclasses of integers. So, True will be treated as 1 and False will be treated as 0. In our case, we take each element from my_list and check if it is equal to the key. If they are equal, it evaluates to 1 otherwise 0. Then we sum the entire result.

Answer 2

Its already built-in to Python:

>>> from collections import Counter
>>> Counter([2,1,3,1,4])[1]
2

If you want to do it the "long" way, use a dictionary:

d = {}
for i in [2,1,3,1,4]:
    d.setdefault(i, 0)
    d[i] += 1
print(d[1])  #  prints 2