Algorithm Notesworking through it, one step at a time

Clear the lowest set bit

EasyPractice ↗

Count Enabled Byte Switches

Original & variantNumber of 1 Bits ↗

Original examples and wording. The question below states the exact input format and output contract.

01. The question

A byte describes eight on/off switches. Return how many are enabled. Variant: the display and input use unsigned 8-bit values rather than a 32-bit word.

Before revealing the idea, predict the result and describe what information you would need to track.

02. Worked example

Input182

Output5

  1. 182 is 10110110, with five enabled switches.
  2. remaining & (remaining - 1) removes the lowest enabled bit.
  3. Five removals leave zero; the counter is 5.

Try your code

Define solve(data, parameter="") and return your result. Your code runs locally, not on a server.

Edit your test cases

Use 1-10 objects with data and an optional string parameter. Expected outputs are computed from the lesson. An optional expected field must be correct.

Input contract & reference code

Example data: 182. Parameter: "". Numeric parameters arrive as strings; convert them with int(parameter) when needed.

The example and generated cases use the question’s exact variant. This checks behavior on a small test batch—not performance on every possible input.

Python runtime notices

Python downloads only when you run code, using compressed runtime assets. Code is not saved or uploaded. Enable JavaScript to run it.

Five-second execution limit per batch. Standard-library Python only; additional packages and network access are unavailable. Stop ends execution without blocking the lesson.

03. The idea

Try sketching your own approach first. What would you track, and why would it work?

Reveal the idea
  1. Initialize the count at zero.
  2. Clear one lowest set bit per iteration.
  3. Stop when the remaining byte is zero.

04. Watch it happen

Use Step to move slowly, or Play to follow the whole trace. Change a preset or load your own input.

Input & settings

Small validated inputs: arrays at most 16 entries, text at most 16 characters, tables at most 20 states. Bit questions use unsigned bytes (0-255). See the task for its exact format.

Red = current item / blue = active window or cells / green = completed work. A value docks at its actual destination; the original input stays unchanged. Positions start at 0.

Working state / what changes

Ready to begin

Enable JavaScript to step through the animation. The worked example and solution below remain readable without it.

Python / why it changes

The line behind the step

def solve(data, parameter=''):
    remaining = data
    count = 0
    while remaining:
        remaining &= remaining - 1
        count += 1
    return count
How to use this walkthrough

Hover or focus a control for its label. Play forms the input and follows values into the working state. Step shows one change; Back and the slider restore exact snapshots. Reduced motion uses instant updates. Saved snapshots and sorted heap displays are teaching overhead, not algorithm complexity.

05. Python solution

def solve(data, parameter=''):
    remaining = data
    count = 0
    while remaining:
        remaining &= remaining - 1
        count += 1
    return count

06. Cost & edge cases

Time
O(k), k enabled bits, at most 8
Space
O(1)
  • 0 has no enabled switches.
  • 255 has eight.
  • Input must be a JSON integer from 0 to 255, not a binary-text string.

Keep going

Browse all questionsBack to Bits & binaryAdd Two Byte Counters Build a Small Switch Lookup Table Find the Unscanned Ticket Mirror an Eight-Switch Panel