Comparison operations in NumPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the comparison operation arr1 > arr2 do in NumPy?

It compares each element of arr1 with the corresponding element of arr2 and returns a boolean array where each value is True if the element in arr1 is greater than the element in arr2, otherwise False.

intermediate

How does NumPy handle comparison operations between arrays of different shapes?

NumPy uses broadcasting rules to compare arrays of different shapes. It stretches the smaller array along dimensions of size 1 to match the larger array's shape, then performs element-wise comparison.

beginner

What is the output type of a comparison operation between two NumPy arrays?

The output is a NumPy array of boolean values (True or False) indicating the result of the comparison for each element.

beginner

Which comparison operators can be used element-wise on NumPy arrays?

You can use <, <=, >, >=, ==, and != for element-wise comparisons on NumPy arrays.

intermediate

How can you find if any element in a NumPy boolean array is True after a comparison?

Use the numpy.any() function on the boolean array. It returns True if at least one element is True, otherwise False.

What does arr == 5 return if arr is a NumPy array?

A A boolean array showing where elements equal 5

B A single boolean value

C An array of integers

D An error

The operation compares each element to 5 and returns a boolean array with True where elements equal 5.

Which operator checks if elements in one array are not equal to another array?

A ==

B !=

C >=

D <

The != operator returns True where elements differ between arrays.

If arr1 has shape (3,1) and arr2 has shape (3,4), what happens when you do arr1 < arr2?

A An error occurs due to shape mismatch

B arr2 is reshaped to (3,1)

C Only first column is compared

D Broadcasting stretches arr1 to (3,4) and compares element-wise

NumPy broadcasts arr1 to match arr2's shape and compares each element.

What type of array results from arr1 >= arr2?

A Float array

B Integer array

C Boolean array

D String array

Comparison operations produce boolean arrays indicating True or False per element.

How do you check if any element in a boolean NumPy array is True?

A numpy.any()

B numpy.all()

C sum()

D len()

numpy.any() returns True if at least one element is True.

Explain how element-wise comparison works between two NumPy arrays.

Think about how NumPy compares each element and what it returns.

Describe how to use comparison operators to filter data in a NumPy array.

Consider how True/False arrays help pick data.

Practice

1. What does the numpy expression arr == 5 return when arr is a numpy array?

easy

A. The sum of all elements equal to 5

B. An array of True/False values showing where elements equal 5

C. The count of elements equal to 5

D. A single True or False value

Solution

1. Step 1: Understand element-wise comparison

   The expression arr == 5 compares each element of the array to 5 individually.

2. Step 2: Result type of comparison

   This returns a new array of the same shape with True where the element equals 5, and False otherwise.

3. Final Answer:

   An array of True/False values showing where elements equal 5 -> Option B

4. Quick Check:

   Element-wise comparison = Boolean array [OK]

Hint: Remember == returns True/False array element-wise [OK]

Common Mistakes:

• Thinking it returns a count or sum
• Expecting a single True/False value
• Confusing == with assignment =

2. Which of the following is the correct syntax to check if elements in numpy array arr are greater than 10?

easy

A. arr >> 10

B. arr => 10

C. arr > 10

D. arr >= 10

Solution

1. Step 1: Identify correct comparison operator

   The operator for 'greater than' is >, so arr > 10 checks elements greater than 10.

2. Step 2: Check other options

   arr => 10 is invalid syntax, arr >> 10 is a bitwise shift, and arr >= 10 checks greater or equal, not strictly greater.

3. Final Answer:

   arr > 10 -> Option C

4. Quick Check:

   Greater than operator is > [OK]

Hint: Use > for greater than, >= for greater or equal [OK]

Common Mistakes:

• Using => instead of >
• Confusing bitwise >> with comparison
• Using >= when > is asked

3. What is the output of the following code?

import numpy as np
arr = np.array([3, 7, 2, 9])
result = arr >= 5
print(result)

medium

A. [False False False False]

B. [True False True False]

C. [True True True True]

D. [False True False True]

Solution

1. Step 1: Compare each element with 5

   Elements: 3, 7, 2, 9. Check if each is >= 5: 3>=5 is False, 7>=5 is True, 2>=5 is False, 9>=5 is True.

2. Step 2: Form boolean array

   The result is [False, True, False, True].

3. Final Answer:

   [False True False True] -> Option D

4. Quick Check:

   Element-wise >= 5 = [False True False True] [OK]

Hint: Check each element against 5 carefully [OK]

Common Mistakes:

• Mixing up >= with >
• Misreading element values
• Expecting integer output instead of boolean

4. Identify the error in this numpy comparison code:

import numpy as np
arr = np.array([1, 2, 3])
result = arr = 2
print(result)

medium

A. Using single = instead of == for comparison

B. Array must be converted to list before comparison

C. print() cannot display numpy arrays

D. Missing parentheses in print statement

Solution

1. Step 1: Check comparison syntax

   The code uses arr = 2 which assigns 2 to arr, not compares. Comparison needs ==.

2. Step 2: Understand effect of assignment

   Assignment overwrites arr with 2, so print(result) prints 2, not a boolean array.

3. Final Answer:

   Using single = instead of == for comparison -> Option A

4. Quick Check:

   Comparison needs ==, not = [OK]

Hint: Use == for comparison, = is assignment [OK]

Common Mistakes:

• Confusing = and ==
• Thinking print can't show arrays
• Believing arrays must convert to lists

5. Given two numpy arrays:

import numpy as np
arr1 = np.array([1, 4, 6, 8])
arr2 = np.array([2, 4, 5, 10])

Which expression returns a boolean array showing where elements in arr1 are less than or equal to elements in arr2?

hard

A. arr1 <= arr2

B. arr1 < arr2

C. arr1 == arr2

D. arr1 >= arr2

Solution

1. Step 1: Understand element-wise comparison between arrays

   Comparing two arrays with operators like <= compares each element pair at the same index.

2. Step 2: Identify operator for 'less than or equal'

   The operator <= checks if elements in arr1 are less than or equal to corresponding elements in arr2.

3. Final Answer:

   arr1 <= arr2 -> Option A

4. Quick Check:

   Use <= for less or equal element-wise [OK]

Hint: Use <= to compare arrays element-wise for less or equal [OK]

Common Mistakes:

• Using < instead of <= when equal counts
• Using == which only checks equality
• Using >= which is opposite