A function is a named, reusable block of code that accepts inputs (called parameters or arguments), performs a computation, and returns an output. Functions are the primary mechanism for organizing code into manageable, understandable pieces.
The concept borrows from mathematics, where a function maps each element of one set to an element of another. In programming, the analogy holds loosely: a function takes input values and produces a result. A function that computes the area of a circle, for example, accepts a radius as input and returns the product of pi and the radius squared.
Functions serve several purposes. They enable abstraction: once a function is written and named, the programmer can use it without thinking about the details of how it works internally. They enable reuse: the same function can be called from many places in a program, avoiding the need to duplicate code. And they enable decomposition: a large problem can be broken into smaller sub-problems, each handled by its own function.
Most programming languages distinguish between a function’s definition (where the code specifies what the function does) and its invocation or call (where the function is actually executed with specific input values). A function may call other functions, and a function may call itself — the latter case is called recursion.
In functional programming, functions are treated as first-class values: they can be stored in variables, passed as arguments to other functions, and returned as results. A function that takes or returns another function is called a higher-order function. A function that produces the same output for the same input and has no side effects is called a pure function.
Related terms
- algorithm — the procedure a function implements
- variable — a named reference to data, often passed to or returned from functions
- pure function — a function with no side effects
- higher-order function — a function that operates on other functions
- recursion — a function calling itself