← all languages

C

697 problems50 sections📅 2026 curriculum

Learn C by typing real, low-level code.

Type your way through C's fundamentals: pointers and pointer arithmetic, manual malloc/free memory management, structs, arrays, and string handling with char buffers. Every snippet is real C, and an AI tutor explains each line — where a pointer actually points, why a buffer needs its null terminator — so the low-level model becomes concrete as you build speed.

Progress0 / 697 solved · 0/50 sections

best wpm

0

avg wpm

0

accuracy

0%

runs

0

level

1

rating

1000

▶ Start / Continue See details & error analysis →

Run C on your machine

1. Install the toolchain

brew install gcc # or: sudo apt install build-essential

2. Save your code as hello.c, then compile & run

gcc hello.c -o hello && ./hello

Official install docs ↗

progress by section

1Fundamentals0/16start →

Hello, C Variables and printf Booleans The char type const bindings for loop Functions if / else if / else Integer and real division sizeof Ternary operator while loop Arrays switch statement auto type inference constexpr constants

2Strings In Depth0/18start →

strlen Array of strings Walk char by char Fill with memset char* vs char[]strcat strchr strcmp strcpy strncmp prefix test Uppercase in place Length by pointer walk Parse a number Reverse in place snprintf strncpy safely strstr strtok

3Values & References0/18start →

Address and dereference Pointer arithmetic Arrays decay to pointers Pass by pointer mutates Pass by value copies Pointer to const Null pointer as absence Struct through a pointer Swap via pointers Borrow a big struct const pointer vs pointer to const Avoid dangling pointers Heap ownership Return a pointer into data Two-pointer reverse Pointer to pointer Function pointers void pointer

4Slices & Arrays0/18Unlock →

Membership test Fill a slice First and last Pointer plus length Reverse in place A subslice Begin and end pointers Copy with memcpy Min and max Find an index Split at an index A slice struct Binary search Non-overlapping chunks Shift with memmove Sort with qsort Rotate left Overlapping windows

5Data structures0/11Unlock →

A struct Fixed array of structs typedef struct with behavior Dynamic array with realloc A stack A growable vector type Singly linked list A ring-buffer queue A binary tree A hash table A tagged union

6Iterators0/20Unlock →

7Traits0/20Unlock →

8Generics0/20Unlock →

9Lifetimes0/20Unlock →

10Closures0/20Unlock →

11Pattern Matching0/20Unlock →

12Error Handling0/20Unlock →

13Option & Result0/20Unlock →

14Conversions0/15Unlock →

atoi bool as a number Char and int Float to int Format specifiers Numeric cast String as char array Parse hex and auto-base Integer vs real division Number to string strtod strtol with endptr Integer promotion Type punning via memcpy Type punning via union

15Collection Conversions0/19Unlock →

Clone an array Length of a linked list Concatenate two arrays Set membership test Character frequency table Dedup into a set Flatten a 2D array Invert key/value pairs Linked list to array Parallel arrays as a map Reshape 1D into 2D Transpose a matrix Zip two arrays to pairs Array to linked list Unique, keeping order Build a frequency table Build a list by prepending Sort then dedup Array to hash table

16Math0/14Unlock →

Integer arithmetic Factorial Roots and powers Mean of an array min, max, clamp Rounding Float modulo GCD and LCM Logarithms and exp Trigonometry Bit math hypot and atan2 Primality test Median and standard deviation

17Encoding0/23Unlock →

18Random0/20Unlock →

19Data I/O0/18Unlock →

Parse CSV into rows Tab-separated values Build a CSV string Select one column CSV into a number matrix Split off the header Whitespace-separated matrix Read a data file Serialize records to CSV Skip blanks and comments Print an aligned table Write CSV to a file Write a matrix as text Append a row Parse key=value config Round-trip write then read Stream lines with fgets Drop unparseable rows

20Linear Algebra0/21Unlock →

21Text Processing0/16Unlock →

Capitalize the first letter Count lines Word count Count occurrences Reverse a string Count vowels Zero-pad a number Pad and align Character frequency Longest word Reverse word order Title case Trim whitespace Truncate with an ellipsis Replace a substring Levenshtein distance

22Date & Time0/13Unlock →

UNIX timestamp now UTC broken-down time Seconds to HH:MM:SS Leap year test Local broken-down time Format a time Break seconds into parts Time a block of work Day of the week Days in a month Difference in seconds Build a timestamp Parse a clock string

23Memory0/12Unlock →

Zero-initialized memory Allocate and free Copy with memcpy Fill with memset Swap through pointers Avoid a leak Grow a buffer sizeof in bytes Stack vs heap Pointer arithmetic Arena / bump allocation Flexible array member

24Modules0/20Unlock →

25Macros0/20Unlock →

26Testing0/20Unlock →

27Algorithms0/8Unlock →

Greatest common divisor Binary search Palindrome check Fibonacci with memoization Merge sorted arrays Fisher-Yates shuffle Two sum with hashing Quicksort

28Sorting Algorithms0/12Unlock →

Check if sorted Standard library qsort Bubble sort Sort by key and descending Insertion sort Selection sort Counting sort Merge sort Shell sort Heapsort Quicksort Radix sort

29Expression Parsing0/6Unlock →

Evaluate postfix (RPN)Tokenize an expression Evaluate infix Recursive-descent calculator Right-associative power Shunting-yard

30Idioms0/9Unlock →

Designated initializers Early return Array length idiom Const-correctness static for internal linkage Bit flags Compound literals do/while(0) macro X-macros

31OOP & Traits0/8Unlock →

Interfaces via function pointers Composition over inheritance Shared behavior Encapsulation (opaque type)Making a type printable Dynamic polymorphism Static polymorphism Building on an interface

32Trait Objects0/14Unlock →

Default method over an object Take a trait object Return one of several types An array of trait objects Store behaviors together One type vs many Shared ownership A slice of trait objects Static vs dynamic dispatch Extending an object An object that borrows data Recover the concrete type A handler registry Self-taking methods

33Design Patterns0/14Unlock →

Lazy Initialization Singleton Strategy Adapter Builder Command Dependency Injection Factory Method Observer (Pub/Sub)Prototype (Clone)Decorator Object Pool State Actor (Message Passing)

34Async0/3Unlock →

Callbacks and a poll loop A non-blocking state machine poll() over file descriptors

35File I/O0/6Unlock →

Read a whole file Write a file Write then read back Append with mode "a"Count lines helper Read lines with fgets

36Filesystem0/18Unlock →

Does a path exist?Copy a file Files vs directories Build a path Create a directory List a directory Rename or move basename and dirname File extension Is it a directory?File metadata Remove files and dirs Strip the extension Write to the temp dir Last modified time Sorted listing Total size of a tree Walk the tree

37Networking0/16Unlock →

Classify an address Parse the request line Route by path Fill a sockaddr_in Bind an ephemeral port Build an HTTP response Read a line from a socket Shut down a connection TCP client A receive timeout UDP receive UDP send Resolve a hostname Serve a web page A process per connection TCP echo server

38Concurrency0/7Unlock →

Spawn a thread Passing an argument Returning a value Atomic counting A mutex-protected counter Run something once A condition variable

39Unsafe & FFI0/14Unlock →

Dereference a pointer const vs mutable pointers Minding buffer bounds A function pointer Manual memory Pointer arithmetic void* and casts Casting between pointer types Type punning with memcpy offsetof uintptr_t Type punning with a union volatile Reinterpret raw bytes

40Commands0/14Unlock →

Command-line arguments Skip the program name Read an environment variable Env var with a default Iterate all env vars Path to this program Exit with a code Current directory Set and remove a variable Run a command Command exit status Capture command output Resolve an absolute path Read all of stdin

41Array0/10Unlock →

Contains Duplicate Two Sum Best Time to Buy and Sell Stock Maximum Subarray Container With Most Water Find Minimum in Rotated Sorted Array Product of Array Except Self Maximum Product Subarray Search in Rotated Sorted Array 3Sum

42Binary0/5Unlock →

Missing Number Number of 1 Bits Counting Bits Reverse Bits Sum of Two Integers

43String0/10Unlock →

Valid Anagram Valid Palindrome Valid Parentheses Encode and Decode Strings Group Anagrams Longest Substring Without Repeating Characters Longest Repeating Character Replacement Longest Palindromic Substring Palindromic Substrings Minimum Window Substring

44Matrix0/4Unlock →

Rotate Image Set Matrix Zeroes Spiral Matrix Word Search

45Interval0/5Unlock →

Meeting Rooms Merge Intervals Insert Interval Non-overlapping Intervals Meeting Rooms II

46Linked List0/6Unlock →

Reverse Linked List Linked List Cycle Merge Two Sorted Lists Remove Nth Node From End Merge K Sorted Lists Reorder List

47Tree0/14Unlock →

Maximum Depth of Binary Tree Invert Binary Tree Same Tree Lowest Common Ancestor of a BST Binary Tree Level Order Traversal Subtree of Another Tree Implement Trie (Prefix Tree)Kth Smallest Element in a BST Validate Binary Search Tree Add and Search Word Construct Binary Tree from Preorder and Inorder Binary Tree Maximum Path Sum Serialize and Deserialize Binary Tree Word Search II

48Graph0/8Unlock →

Number of Connected Components Number of Islands Clone Graph Course Schedule Longest Consecutive Sequence Graph Valid Tree Alien Dictionary Pacific Atlantic Water Flow

49Heap0/3Unlock →

Kth Largest Element in an Array Top K Frequent Elements Find Median from Data Stream

50Dynamic Programming0/11Unlock →

Climbing Stairs House Robber Unique Paths Coin Change House Robber II Jump Game Combination Sum IV Decode Ways Longest Common Subsequence Longest Increasing Subsequence Word Break