Review: NumPy Fundamentals

Module 3: NumPy

Review of Session 3.1: Fundamentals

Exercise

What is going to be the final value of x?

np = 4

arr = np.array([2, 3, np, 5])

print(arr)

# a) [2, 3, 4, 5]
# b) numpy.ndarray([2, 3, 4, 5])

Solution

Remember that Python will only remember the last variable declaration with the same name. Doing the code above with np is similar to:

x = 10
x = 2
print(x)

The code fails because np ceases to be NumPy!

import numpy as np

Exercise

How would you replace the for loop in the code below, for a 1D NumPy array?

arr = np.array([1, 2, 3, 4, 5])

result = 0
for i in arr:
  result = result + i

print(result)

arr = np.array([1, 2, 3, 4, 5])

# Option A
result = np.cumsum(arr)

# Option B
result = np.sum(arr)

# Option C
result = np.loop(arr)

Solution

When we do cumsum to a higher dimensional array, NumPy first flattens that array.

arr = np.array([
    [1, 2, 3],
    [4, 5, 6]
])

print(np.cumsum(arr))

Exercise

How do you check the number of dimensions of a NumPy array?

arr = np.array([[1, 2, 3], [4, 5, 6]])

# Option A
print(arr.shape)

# Option B
print(arr.ndim)

# Option C
print(arr.ndim())

# Option D
print(np.arr.ndim)

Exercise

I have a colored image represented by a 3D array img. I want to know its width and height. Which one of the following lines of code will give me that information?

img = np.array([[[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]]])

# Option A
height, width = img.shape

# Option B
height, width, _ = img.shape

# Option C
height, width = img.shape(:2)

Exercise

An empty array is a NumPy array with no elements inside. What of the following lines will NOT initialize an empty array?

# Option A
x = np.array()

# Option B
x = np.array([])

# Option C
x = np.zeros(0).astype(int)

# Option D
# All of the above fail.

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