Count Duplicate Values in a List

Question

"Write a function named count_duplicates that accepts a list of integers as a parameter and that returns the number of duplicate values in the list. A duplicate value is a value that also occurs earlier in the list. For example, if a list named a contains [1, 4, 2, 4, 7, 1, 1, 9, 2, 3, 4, 1], then the call of count_duplicates(a) should return 6 because there are three duplicates of the value 1, one duplicate of the value 2, and two duplicates of the value 4.

Constraints: The list could be empty or could contain only a single element in such cases, your function should return 0. Do not modify the contents of the list."

This is what I have...

def count_duplicates(a):
count = 0
for i in range(1, len(a), 1):
first = str(a[i])
second = str(a[i + 1])
if first == second:
count += 1
return count

however I keep getting an "IndexError: list index out of range" after the "second = str(a[i + 1])" function.

Any ideas how to fix this?

Answer 1

"""
Rather than fixing your code (which have also logical mistake: you only compare contiguous items in the list), I will suggest another way to reach your goal:
"""

def count_duplicates(a):
return len(a) - len(set(a))

Answer 2

i suggest 2* for loops, the first take each element in the list and the other for loop compare it with each element in the list