Functions · 14

Functions

Bundle work under a name, take values and return a result — plus recursion.

A function is a named block of code.
It takes values (parameters) and returns a result.
The example below defines a function that squares a number and a factorial function that calls itself.

def square(x):    return x * x  def factorial(n):    if n <= 1:        return 1    return n * factorial(n - 1)  n = int(input())print(square(n))print(factorial(n))

function square(x) {  return x * x;} function factorial(n) {  if (n <= 1) return 1;  return n * factorial(n - 1);} const n = Number(require("fs").readFileSync(0, "utf8"));console.log(square(n));console.log(factorial(n));

import { readFileSync } from "node:fs"; function square(x: number): number {  return x * x;} function factorial(n: number): number {  if (n <= 1) return 1;  return n * factorial(n - 1);} const n: number = Number(readFileSync(0, "utf8"));console.log(square(n));console.log(factorial(n));

import java.util.Scanner; public class Main {    static int square(int x) {        return x * x;    }     static long factorial(int n) {        if (n <= 1) return 1;        return n * factorial(n - 1);    }     public static void main(String[] args) {        int n = new Scanner(System.in).nextInt();        System.out.println(square(n));        System.out.println(factorial(n));    }}

using System; class Program{    static int Square(int x) => x * x;     static long Factorial(int n)    {        if (n <= 1) return 1;        return n * Factorial(n - 1);    }     static void Main()    {        int n = int.Parse(Console.ReadLine());        Console.WriteLine(Square(n));        Console.WriteLine(Factorial(n));    }}

#include <iostream> int square(int x) {    return x * x;} long long factorial(int n) {    if (n <= 1) return 1;    return n * factorial(n - 1);} int main() {    int n;    std::cin >> n;    std::cout << square(n) << '\n';    std::cout << factorial(n) << '\n';}

#include <stdio.h> int square(int x) {    return x * x;} long long factorial(int n) {    if (n <= 1) return 1;    return n * factorial(n - 1);} int main(void) {    int n;    scanf("%d", &n);    printf("%d\n", square(n));    printf("%lld\n", factorial(n));    return 0;}

square(x){    return x * x;} factorial(n){    if (n <= 1)    {        return 1;    }    return n * factorial(n - 1);} n = Console.ReadLine().ToNumber();Console.WriteLine(square(n));Console.WriteLine(factorial(n));

stdin

stdout

16
24

Key points

Wrap code in a function when the same calculation is needed in several places, or to break a long solution into small pieces.
When return runs, the function ends right there.
The lines below it do not run.
Passing a number copies the value.
Changing it with x = 9 inside the function leaves the caller's variable alone.
Passing a list shares the same list.
Adding to it inside the function changes it outside too (a C++ vector is copied when passed).
Recursion is a function calling itself.
It shrinks a big problem into a smaller one of the same shape, and it must have a stopping case (n <= 1 gives 1).
Without a stopping case, or when it goes too deep, the program stops with an error or crashes, depending on the language.

How each language writes it

Python	`def square(x):` plus indentation. Without `return` it returns `None`. You can only call it after the `def` has run (earlier is a `NameError`). Recursion stops at about 1,000 levels with `RecursionError`
JavaScript	`function square(x) { return x * x; }` or `const square = (x) => x * x;`. Without `return` it returns `undefined`. A `function` declaration may sit below the call. Too deep: `RangeError: Maximum call stack size exceeded`
TypeScript	JavaScript plus types: `function square(x: number): number`. The type checker catches a wrong number of arguments
Java	`static int square(int x)` inside a class; `void` when nothing is returned. It can be anywhere in the class. Too deep: `StackOverflowError`
C#	Like Java: `static int Square(int x)` inside a class, `void` when nothing is returned. Too deep a recursion crashes the whole program
C++	Return type first, as in `int square(int x)`; `void` when nothing is returned. Define it above its use, or declare it first with `int square(int x);`. Too deep a recursion crashes the program
C	Same as C++
Craykon	No keyword: `square(x) { return x * x; }`. It may sit below the call. After a few thousand levels it stops and says the function was called too deeply

Common mistakes

Calling with a missing argumentJavaScript

function square(x) { return x * x; }
console.log(square());

→ NaN — the missing x is quietly filled with undefined

Pass the right number of values.
Python (TypeError), Java, C#, C, C++, TypeScript (compile error) and Craykon ("2 values are needed but 1 was given") all tell you.

No stopping case

def factorial(n):
    return n * factorial(n - 1)

→ It calls itself forever and stops with RecursionError. Other languages overflow the stack and stop or crash

Put a stopping case first, such as if n <= 1: return 1.

Changed inside the function, but the original is unchangedC++

void add(std::vector<int> v) { v.push_back(9); }

std::vector<int> a = {1};
add(a);

→ a is still {1} — the value went into a copy

To change the original, add &: std::vector<int>& v.

How languages differ
Factorials grow fast.
A 32-bit int holds only up to 12! (computing 13! with int in Java silently gives 1932053504), so the Java, C#, C and C++ examples return 64-bit long/long long (good up to 20!), while Python has no size limit and JavaScript and Craykon may be inexact beyond 2^53.

Functions

Key points

How each language writes it

Common mistakes

Calling with a missing argumentJavaScript

No stopping case

Changed inside the function, but the original is unchangedC++

Practice