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Welcome to Practicum 4!

Let's start with a recap of for loops. Complete the quizes below (they are anonymous and not graded), and we'll discuss the answers immediately in class. Please do one at a time (you have 5 minutes for each), wait for the discussion and only do the next one when everybody is ready!

Functions

During the lecture you were introduced to the concept of functions. The correct use of functions will make your code much easier to:

• write because (1) it will "force" you to break down the big problem into smaller tasks and (2) if you find a mistake in a calculation that you do in different places, you only need to fix it once, rather than all over the code (you will see in the examples in a second).
• test because it will be possible to verify that each task separately is done correctly.
• read because if you name the functions intuitively, whoever reads your code will know immediately what the code does, without having to fully understand how it's doing it.

Have a look at these code samples. They both calcuate and print the GPA of two students, given their grades and the subject weights.

Example 1:
student_1_grades = [10, 12, 9]
student_2_grades = [11, 13, 8]
weights = [0.2, 0.3, 0.5]

student_1_gpa = 0
for idx in range(len(weights)):
print(student_1_gpa)

student_2_gpa = 0
for idx in range(len(weights)):
print(student_2_gpa)
Example 2:
student_1_grades = [10, 12, 9]
student_2_grades = [11, 13, 8]
weights = [0.2, 0.3, 0.5]

result = 0
for idx in range(len(weights)):
result += weights[idx] * grades[idx]
return result

In Example 1, the code for calculating the GPA is written twice, even though it's basically the same. In Example 2, we created the function for calculating the GPA and then we can use it as many times as we want.

We can test that calc_gpa works by giving it different values without having to run all the rest of the code.

When somebody reads this code they can skip the actual implementation of the GPA calculation and go directly to what the code is doing and read it in plain English: print the calculated gpa of student_1 and then of student_2.

What to always remember about functions

• Functions are defined with the def keyword. When we define a function it does not run yet (see Example 2 above). For it to run, we need to call it by it's name and arguments (see Example 2 again).
• Functions communicate their results using the return statement. The very first time a function encounters a return statement, it communicates the requested value and then does not run anymore!.
• Any variables created inside a function disappear when the function returns. The code outside the function will only know whatever the function returned.
• Conversely, functions should only rely on the variables that they receive as the arguments.
• A function should always return something but sometimes you will forget the return statement, or you'll put it somewhere in an if without an else. Then, the function runs its course are returns None.

Time for Quiz 4!

Excercise 1

Download pr04.py file. There are a few functions whose definitions are empty and you'll need to implement them. Do not modify anything in the main() function. Do not modify the argument lists of the functions, only implement their bodies.