Below is an example of a built-in function using strlen().
The phrase "I love to count!" contains 16 characters.

Below is an example of a user-defined function.
This function aims to print out full names of the Smith family
by passing the first name of family members into the function.
Jane Smith.
Frank Smith.
Paul Smith.
Tom Smith.
Mary Smith.

This is another example of a user-defined function which returns the result of adding two numbers.
5 + 10 = 15
7 + 13 = 20
2 + 4 = 6

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