Review: Array Manipulation

Module 3: NumPy

Session 3.4: Array Manipulation

import numpy as np

Exercise

How would you join the two NumPy arrays a and b together into a single 1-D array?

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

# Option A
c = np.concatenate([a, b], axis=0)

# Option B
c = np.append(a, b, how="join")

# Option C
c = np.join(a, b)

# Option D
c = a + b

Exercise

What does the following code do?

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

Solution

1. Reshapes the array into 2 rows and 3 columns.

2. Flattens the array.

3. Causes an error.

Exercise

How would you stack two arrays a = [1, 2, 3] and b = [4, 5, 6] vertically?

1. np.vstack([a, b])

2. np.hstack([a, b])

3. np.concatenate([a, b], axis=0)

Exercise

Consider an array \(Z = [1,2,3,4,5,6,7,8,9,10,11,12,13,14]\), how to generate an array \(R = [[1,2,3,4], [2,3,4,5], [3,4,5,6], ..., [11,12,13,14]]\)?

# Solution with errors - Fix them!
# The following code outputs
# [[2,3,4],[3,4,5],[4,5,6],...,[12,13,14]]
z = np.arange(1, 15)  # Output: [1, 2, 3, ..., 14, 15]
# Initialize empty list
ls = []
for idx in range(1, len(z) - 3):
  # Fill the list with sub-sets of the array
  # For example, the first one will be [1, 2, 3, 4]
  ls.append(z[idx:idx+3])
# Turn the list of arrays into a 2D array
r = np.array(ls)
print(r)

# Solution
z = np.arange(1, 15)
# Initialize empty list
ls = []
for idx in range(len(z) - 3):
  # Fill the list with sub-sets of the array
  # For example, the first one will be [1, 2, 3, 4]
  ls.append(z[idx:idx+4])
# Turn the list of arrays into a 2D array
r = np.array(ls)
print(r)

Exercise

I want to create the matrix \[ \begin{bmatrix} 1 & 2 & 3 \\ 4 & 0 & 6 \\ 7 & 8 & 9 \end{bmatrix} \] But the code below raises the error IndexError: too many indices for array: array is 1-dimensional, but 2 were indexed on line 3. Explain the errors of the code and fix them.

x = np.linspace(1, 9, 9)
x.reshape((3, 3))
x[1, 1] = 0

print(x)

# Fixing the code: "x.reshape()" does not modify the array "x" in place
# we need to catch its output into a new variable
x = np.linspace(1, 9, 9)
y = x.reshape((3, 3))
y[1, 1] = 0

print(x)
print(y)

# Again, notice how "x" does NOT change
x = np.array([1, 2, 3, 4])
y = x.reshape((2, 2))  # numpy method: <array>.<method()>

# The original array "x" remains unchanged
print(x)
# The method "array.reshape()" outputs the new array
print(y)

# Compare the code above with a list
# The ".append()" method changes the list in place
x = [1, 2, 3]
y = x.append(4)  # list method: <list>.<method()>

# The original list "x" has changed
print(x)
# The method "list.append()" outputs nothing
print(y)

Exercise

The following function is failing in line 3: TypeError: array() missing required argument 'object' (pos 0). Explain the error(s) and fix the code.

def fibonacci_sequence(n: int):
  # Initialize an empty array
  arr = np.array()

  for i in range(n):
    if i <= 1:
      # The first two elements of Fibonacci are 1
      new_element = 1
    else:
      # Sum the last two elements of the sequence
      new_element = arr[-2] + arr[-1]
    # Add the element to the sequence
    arr = np.append(arr, new_element)
  # Ensure the array contains integer values
  return int(arr)

print(fibonacci_sequence(10))

---