np.mean() for average in NumPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does np.mean() do in numpy?

np.mean() calculates the average value of numbers in an array. It adds all numbers and divides by how many there are.

beginner

How do you calculate the average of a list [2, 4, 6, 8] using np.mean()?

Use np.mean([2, 4, 6, 8]). It returns 5.0 because (2+4+6+8)/4 = 20/4 = 5.

intermediate

What is the difference between np.mean() and Python's built-in sum()/len() for average?

np.mean() works directly on numpy arrays and is faster for large data. sum()/len() works on lists but is slower and less flexible.

intermediate

How can you calculate the average along a specific axis in a 2D numpy array using np.mean()?

Use the axis parameter. For example, np.mean(arr, axis=0) averages columns, np.mean(arr, axis=1) averages rows.

advanced

What happens if you use np.mean() on an empty array?

It returns nan (not a number) and usually shows a warning because you cannot find an average of no numbers.

What does np.mean() calculate?

A The average of numbers

B The sum of numbers

C The maximum number

D The minimum number

np.mean() calculates the average (mean) of the numbers in an array.

How do you calculate the average of each row in a 2D numpy array arr?

A np.mean(arr, axis=0)

B np.mean(arr, axis=1)

C np.mean(arr)

D np.mean(arr, axis=2)

Using axis=1 calculates the mean across each row.

What is the output of np.mean([10, 20, 30])?

A 10

B 30

C 20

D 60

The average is (10+20+30)/3 = 60/3 = 20.

Which data type does np.mean() work best with?

A Python lists only

B Dictionaries

C Strings

D Numpy arrays

np.mean() is designed for numpy arrays but can also work with lists by converting them internally.

What does np.mean([]) return?

A nan

B 0

C Error

D 1

It returns nan because the mean of an empty array is undefined.

Explain how to use np.mean() to find the average of numbers in a numpy array and how to specify the axis parameter.

Think about how you find average in a list and how axis changes that in 2D arrays.

Describe what happens when you use np.mean() on an empty array and why.

Consider what average means and what happens if there are no numbers.