Counting with boolean arrays in NumPy - Step-by-Step Execution

Concept Flow - Counting with boolean arrays

Create boolean array

↓

Apply condition to get boolean array

↓

Count True values using sum()

↓

Output count

We create a boolean array by applying a condition, then count how many True values it has using sum.

Execution Sample

NumPy

import numpy as np
arr = np.array([1, 3, 5, 2, 4])
bool_arr = arr > 3
count = bool_arr.sum()
print(count)

This code counts how many numbers in the array are greater than 3.

Execution Table

Step	Action	Variable	Value	Explanation
1	Create array	arr	[1 3 5 2 4]	Original numeric array
2	Apply condition arr > 3	bool_arr	[False False True False True]	Boolean array where True means element > 3
3	Count True values	count	2	Sum counts True as 1, so total is 2
4	Print count	output	2	Output shows how many elements are > 3

💡 All steps done, count of True values calculated and printed

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
arr	None	[1 3 5 2 4]	[1 3 5 2 4]	[1 3 5 2 4]	[1 3 5 2 4]
bool_arr	None	None	[False False True False True]	[False False True False True]	[False False True False True]
count	None	None	None	2	2

Key Moments - 3 Insights

Why does summing a boolean array count True values?

In numpy, True is treated as 1 and False as 0 when summed, so sum counts how many True values exist (see execution_table step 3).

What if the condition returns no True values?

Then the boolean array has all False, sum returns 0, meaning no elements met the condition (not shown here but same logic applies).

Can we count False values the same way?

Yes, by summing the negation (~bool_arr) or using (bool_arr == False), sum counts False values.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of bool_arr after step 2?

A [False True False True False]

B [False False True False True]

C [True True False True False]

D [True False True False True]

💡 Hint

Check the 'Value' column for bool_arr at step 2 in execution_table.

At which step is the count of True values calculated?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for the step where 'count' variable gets a numeric value in execution_table.

If the array was arr = np.array([0, 1, 2]), what would count be after applying arr > 3?

A 1

B 3

C 0

D 2

💡 Hint

Refer to how sum counts True values; none of these numbers are > 3, so sum is zero.

Concept Snapshot

Counting with boolean arrays:
- Apply a condition to get a boolean array (True/False)
- True counts as 1, False as 0
- Use sum() on boolean array to count True values
- Useful for quick counts of elements meeting a condition

Full Transcript

We start by creating a numpy array of numbers. Then we apply a condition (like greater than 3) to get a boolean array where each element is True if it meets the condition, False otherwise. Next, we sum the boolean array. Since True counts as 1 and False as 0, the sum gives the count of elements meeting the condition. Finally, we print this count. This method is simple and fast for counting with conditions in numpy.