Classic Binary Search Without Bugs
Nail the low, high, and mid loop.
Classic Binary Search Without Bugs is a free Coding Interview Prep lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Coding Interview Prep learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Halve the Search Space
Binary search finds a value in a sorted list by halving the range each step. That turns a slow O(n) scan into a fast O(log n) search.
a = [1, 3, 5, 7, 9] # must be sortedSorted Is the One Rule
Binary search only works on sorted data. If the list is unordered, sort it first or the result is meaningless and wrong.
a.sort() # ascending order requiredTwo Boundaries
Start with two pointers: low at index 0 and high at the last index. The target, if present, always lives between them.
low, high = 0, len(a) - 1Find the Middle Safely
Compute mid as low + (high - low) // 2. In Python overflow is not an issue, but this form is the safe habit everywhere.
mid = low + (high - low) // 2Three Outcomes
Compare a[mid] to the target. Either you found it, it is too small, or it is too big. Each case shrinks the range differently.
if a[mid] == target:
return midToo Small, Go Right
If a[mid] is less than the target, the answer must be to the right. Move low to mid + 1 and discard the left half.
elif a[mid] < target:
low = mid + 1Too Big, Go Left
If a[mid] is greater than the target, search the left half. Move high to mid - 1 so you never re-check mid again.
else:
high = mid - 1The Loop Condition
Keep going while low is less than or equal to high. When they cross, the range is empty and the target is not present.
while low <= high:
mid = low + (high - low) // 2Report Not Found
If the loop ends without a match, the value is absent. Return -1 by convention so callers can tell success from failure.
return -1 # target not in listThe Off-by-One Trap
The classic bug is forgetting the +1 or -1 when moving a pointer. Skip it and mid gets retested forever, causing an infinite loop.
low = mid + 1 # not low = midUse the Library When You Can
For a plain membership test, Python's bisect module already has bug-free search. Hand-write the loop only when you need custom logic.
import bisect
i = bisect.bisect_left(a, target)Quick Check
Think about what keeps the loop honest.
Recap: Search Without Bugs
You can now set low and high, compute mid safely, shrink the right side, and avoid the off-by-one trap. Logarithmic search is yours. 🎯
Frequently asked questions
Is the “Classic Binary Search Without Bugs” lesson free?
Yes — the full text of “Classic Binary Search Without Bugs” is free to read here on the web, and the Coding Interview Prep course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Coding Interview Prep course, upgrade to CoddyKit PRO.
What will I learn in “Classic Binary Search Without Bugs”?
Nail the low, high, and mid loop. You practise Coding Interview Prep with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Coding Interview Prep?
No prior experience is required. Coding Interview Prep on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Classic Binary Search Without Bugs” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Coding Interview Prep lesson?
Yes. Every Coding Interview Prep lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Classic Binary Search Without Bugs
- bisect_left and bisect_right
- First True: Predicate Binary Search
- Binary Search on the Answer