Build Arrays Fast with Comprehensions
Read and shape input in one line.
Build Arrays Fast with Comprehensions is a free Coding Interview Prep lesson on CoddyKit — lesson 2 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.
Build Lists in One Line
A list comprehension builds a list from a loop in a single expression. It is shorter, faster, and the contest standard for shaping data.
squares = [i * i for i in range(5)]The Basic Shape
The pattern is [expr for item in iterable]. The expression on the left runs once for each item the loop yields.
doubles = [x * 2 for x in nums]Read a Line of Integers
The most common contest line: split the input and convert each token. This maps text to ints in one readable step.
nums = [int(x) for x in input().split()]map() Does the Same Job
map(int, ...) applies int to each token. Wrap it in list(...) to materialize the values for repeated use.
nums = list(map(int, input().split()))Filter While You Build
Add an if condition at the end to keep only items that pass. Here only the even numbers survive the build.
evens = [x for x in nums if x % 2 == 0]Transform and Filter Together
You can transform and filter at once: change the kept items on the left while screening them with the trailing if.
out = [x * x for x in nums if x > 0]Make a Zero-Filled Array
Need an array of a fixed size? [0] * n creates n zeros instantly, perfect for counters and DP tables.
dp = [0] * nBeware Repeated Inner Lists
Use a comprehension for a 2D grid. [[0] * c for _ in range(r)] makes independent rows, while [[0] * c] * r shares one row.
grid = [[0] * c for _ in range(r)]Index with enumerate
When you need positions too, loop with enumerate so each step gives both the index and the value cleanly.
pairs = [(i, v) for i, v in enumerate(nums)]Read a Whole Matrix
Combine reading and looping to pull in r rows of numbers at once, building the full grid in two short lines.
grid = [list(map(int, input().split()))
for _ in range(r)]Set and Dict Versions
The same syntax builds sets and dicts: use curly braces. {x for x in nums} dedupes, and key:value pairs build a map.
seen = {x for x in nums}Quick Check
Pick the idiom that reads a line of space-separated integers.
Recap: Build It Fast
You can now build, filter, and reshape arrays in one line and read full matrices fast. Comprehensions keep input parsing short and bug-free. 💡
Frequently asked questions
Is the “Build Arrays Fast with Comprehensions” lesson free?
Yes — the full text of “Build Arrays Fast with Comprehensions” 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 “Build Arrays Fast with Comprehensions”?
Read and shape input in one line. 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 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Build Arrays Fast with Comprehensions” 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
- Lists, Indexing & Slicing for CP
- Build Arrays Fast with Comprehensions
- Min, Max, Sum & Running Totals
- Find the Index, Not Just the Value