hints.md

Hints for Exercise 9

Contents

• General hints for Exercise 9
• Problem 1 hints
• Problem 2 hints

General hints

Problem 1

Calculating summations

There are several ways in which you can calculate a summation in Python, including using the sum() function. However, it is often better to use a for loop for summing values, particularly if you are calculating values and don't want to store their results separately from the summation. Perhaps an example will make this clear:

>>> numbers = [1,2,3,4,5,6,7,8,9]
>>> squaredSum = 0    # Set sum equal to zero before starting for loop
>>> for i in range(len(numbers)):
...     squaredSum = squaredSum + numbers[i]**2.0
...
>>> print(squaredSum)
285.0
>>> sum(numbers)    # Sum of the list values
45
>>> sum(numbers)**2.0    # Square of the sum of the list values, not equal to squaredSum
2025.0

The point here is that in order to calculate the sum of each value squared in a Python list, you need to calculate the square of each value in the list separately, then add those together. You could save the squared values in a separate list and use the sum() function, but in this case it may be more clear and logical to use a for loop. You may want to do something similar for calculating your chi squared values.

Shorthand Python notation for adding to a variable

It is extremely common to need to add to an existing variable in computer programs. Because of this, there is a shorthand notation in Python for just this kind of operation.

>>> number = 34
>>> number = number + 5
>>> print(number)
39
>>> number += 5
>>> print(number)
44

As you can see, number += 5 is exactly the same as number = number + 5, just written a bit more compactly. As you might imagine, there are similar shortcuts for subtracting (-=), multiplying (*=), and dividing (/=).

Problem 2