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Built-in Functions Common to Most Data Structures
7. count()


Good job! Let's move on to count(), which counts the number of occurrences of a given element in a structure. How many times did we process 18 requests in one day? Let's find out:

requests_processed = [12, 18, 15, 23, 24, 8, 0, 18, 14, 13]

The code above returns 2 because the number 18 appears twice in the list. Note that the syntax is a bit different than before: We use structure_name.count(element) to find the answer.

Internally, count() iterates over the list and keeps a temporary variable with the current count of the given element. After the iteration process, that temporary value is returned. You could easily write such a function yourself, but using count() makes your code shorter and easier to read. Other developers will also be able to quickly understand your code.


Write a function named add_unique(initial_list, new_elements_list) that accepts two arguments:

  1. initial_list – a list with any number of elements.
  2. new_elements_list – a list with new elements that should be added to initial_list.

However, any element from new_elements_list that already occurs more than once in initial_list should NOT be added. Also, two identical elements should appear in the list at most twice.

For example, for the following input:

initial_list = [1, 5, 5, 3]
new_elements_list = [4, 5, 6, 3, 3]

The function should return:

initial_list = [1, 5, 5, 3, 4, 6, 3]

You can use the template code to test your solution.

Stuck? Here's a hint!

To add an element to a list, use: