Glossary

Every concept in this sheet, plus the smaller terms it uses. Select a concept to open its card.

.gitignore: A file listing patterns of files and folders Git should not track, such as dependencies, build output, logs and local secrets.
async and await: An async method returns a Task and can await other tasks.

Arrays and amortised growth: An array stores items side by side in memory, so reading by index is O(1).

Big-O notation: Big-O describes how the running time or memory of an algorithm grows as the input size n grows, ignoring constants.
Binary search: Searching a sorted array by checking the middle element and discarding the half that cannot contain the target, until it is found or the range is empty.
Bits, bytes and binary: A bit is a 0 or 1.
Branches: A branch is a movable name that points to a commit.

Calling a public API: Sending an HTTP request (usually GET or POST) to a web service and reading its response, typically a status code plus a JSON body.
Classes and objects: A class is a blueprint that groups data (fields) with the behaviour that works on it (methods).
Code smells and refactoring: A code smell is a sign of a deeper design problem: long functions, duplicated blocks, many parameters, deep nesting.
Comments, docstrings and READMEs: Comments explain why code does something non-obvious.
Commit messages: A short summary line in the imperative mood (about 50 characters), a blank line, then a body that explains why the change was made.
Compiled and interpreted code: A compiler translates source to machine code before running (C++).
Conditionals: Code that runs only when a condition is true, with else if branches checked in order and an optional else for everything left.
Configuration and secrets: Settings that change between machines (API keys, URLs, file paths) are read from environment variables or a local config file instead of being written into the code.

Dictionaries and maps: A collection of key and value pairs where you look a value up by its key, such as a product name to its stock count.
DRY, KISS and YAGNI: DRY: keep each piece of knowledge in one place.

Edge cases: Inputs at or beyond the limits of normal use: empty collections, zero, negative numbers, one item, very large values, missing fields and unexpected text.
Encapsulation: Keeping an object's data private and exposing only the operations that keep it valid.
Exceptions: An exception is an error object thrown when something goes wrong.

Floating point numbers: Floats store numbers in binary with limited precision, so most decimal fractions such as 0.1 are approximations.
Functions: A named block of code that takes inputs (parameters), does one job and can return a result.

Garbage collection: The runtime frees heap objects automatically once nothing can reach them.
Graphs: A graph is a set of nodes (vertices) connected by edges, which may have directions or weights.

How hash tables work: A hash table turns a key into a number with a hash function, uses it to pick a bucket in an array, and stores the entry there.
How the CPU runs a program: The CPU repeatedly fetches an instruction from memory, decodes it and executes it: simple steps such as add, compare, jump and copy.

Inheritance and composition: Inheritance builds a class from a parent class and reuses its code ("is a").
Integer overflow: Fixed-size integers have a maximum.
Interfaces and polymorphism: An interface is a contract listing methods a type must provide.

Linked lists: A chain of nodes where each node holds a value and a pointer to the next (and, in a doubly linked list, the previous) node.
LINQ: LINQ adds query methods such as Where, Select, GroupBy and OrderBy to any collection, with lambdas for the conditions.
Lists and dynamic arrays: An ordered collection that grows as you add items and gives fast access by index: list in Python, arrays in JavaScript, ArrayList in Java, std::vector in C++ and List<T> in C#.
Loops: A loop repeats a block of code: a for-each loop visits every item in a collection, a counting for loop runs over a range of indexes, and a while loop repeats as long as a condition holds.

Merge conflicts: A conflict happens when two branches changed the same lines.
Merging and rebasing: Merging joins another branch into yours, adding a merge commit if both changed.
Modules and imports: A module is a file (or group of files) of related code that other files import by name.
Mutability and copying: A mutable value can change in place; an immutable one cannot.

Naming and small functions: Names that say what a value is or what a function does (unpaid_invoices, send_reminder), and functions short enough to do one thing at one level of detail.
NuGet packages: NuGet is the .NET package manager.
Null and missing values: A special value meaning "nothing here": None in Python, null and undefined in JavaScript, null in Java and C#.
Nullable reference types: With nullable reference types on (the default in new projects), string means never null and string? means may be null.

Passing arguments to functions: Python, JavaScript and Java always pass a copy of the value, but for objects that value is a reference, so the function can change the object but not rebind your variable.
Processes and threads: The operating system runs each program as a process with its own memory.
Properties: Properties look like fields to callers but run get and set accessors.
Publishing on GitHub: Pushing a project to a public GitHub repository with a README, a licence, a .gitignore and a clean history of meaningful commits.
Pull requests: A pull request asks to merge one branch into another on GitHub.

Queues and deques: A queue is first-in, first-out: add at the back, remove from the front, both O(1).

Reading and validating input: Reading text typed by a user or passed to the program, then converting it to the type you need, such as an integer, and rejecting anything that does not fit.
Reading and writing JSON: JSON is a text format for objects, arrays, strings, numbers, booleans and null.
Reading errors and debugging: A stack trace lists the chain of function calls that led to an error, with file names and line numbers.
Reading files and cleaning up: Files, network connections and locks must be closed when you are done, even if an error happens halfway.
Records: A record is a type for data with value-based equality, a readable ToString and non-destructive changes through with.
Recursion: A function that calls itself on a smaller version of the problem, with a base case that stops the calls.
Remotes: clone, fetch, pull and push: A remote is a copy of the repository on a server such as GitHub, usually named origin.
Repositories and commits: A Git repository stores the full history of a project.

Scope: Scope is the part of the code where a name is visible.
Scoping a small project: Cutting an idea down to the smallest version that solves one real problem end to end, then splitting it into a handful of milestones.
Separation of concerns: Splitting a program so each part handles one concern: reading input, business logic, storage and output live in separate functions or modules.
Sets: An unordered collection of unique values with fast membership checks.
Shell navigation: The shell is a text interface for running programs and working with files.
SOLID: Five design principles: a class has one reason to change; extend behaviour without editing working code; subtypes must work wherever the parent does; keep interfaces small; depend on abstractions rather than concrete details.
Stack and heap: The stack holds each running function's local variables in frames that vanish when it returns.
Stacks: A last-in, first-out collection: you push onto the top and pop from the top, both O(1).
Staging area: The staging area holds the changes that will go into the next commit.
Static and dynamic typing: Statically typed languages (Java, C++, C#) check types when the code is compiled.
Strings and formatting: A string is a sequence of text characters.

Test doubles: Stand-ins for real dependencies in tests: a stub returns fixed answers, a fake is a simple working version (an in-memory store), and a mock checks how it was called.
Test-driven development: Writing a failing test first (red), then the least code that makes it pass (green), then cleaning up (refactor), in short cycles.
Text encoding and Unicode: Unicode gives every character a number (a code point).
The dotnet CLI: The dotnet command from the .NET SDK creates, builds, runs and tests projects.
The test pyramid: A guideline for balancing test types: many fast unit tests, fewer integration tests that check parts working together, and a few slow end-to-end tests through the whole system.
Trees and binary search trees: A tree is a hierarchy of nodes with one root, where each node has children and no cycles.

Undoing mistakes: restore discards uncommitted changes to a file, commit --amend fixes the last commit, revert adds a commit that undoes an old one, reset moves your branch back, and reflog lists where HEAD has been.
Unit tests: Small automated tests that call one function or class with known input and assert on the result.

Value types and reference types: Value types (int, bool, struct) are copied on assignment.
Variables and types: A variable is a name that refers to a value.

Writing a README: The front page of a repository: what the project does and for whom, a demo, how to install, run and test it, how it works and its known limits.