Control flow · 06

Loops

Do the same thing many times: for and while.

A loop runs the same code many times.
The example below prints the numbers from 1 to n one by one, adds them up, and prints the total at the end.

n = int(input())total = 0for i in range(1, n + 1):    print(i)    total += iprint(total)

const n = Number(require("fs").readFileSync(0, "utf8"));let total = 0;for (let i = 1; i <= n; i++) {  console.log(i);  total += i;}console.log(total);

import { readFileSync } from "node:fs";const n: number = Number(readFileSync(0, "utf8"));let total: number = 0;for (let i = 1; i <= n; i++) {  console.log(i);  total += i;}console.log(total);

import java.util.Scanner; public class Main {    public static void main(String[] args) {        int n = new Scanner(System.in).nextInt();        int total = 0;        for (int i = 1; i <= n; i++) {            System.out.println(i);            total += i;        }        System.out.println(total);    }}

using System; class Program{    static void Main()    {        int n = int.Parse(Console.ReadLine());        int total = 0;        for (int i = 1; i <= n; i++)        {            Console.WriteLine(i);            total += i;        }        Console.WriteLine(total);    }}

#include <iostream> int main() {    int n, total = 0;    std::cin >> n;    for (int i = 1; i <= n; i++) {        std::cout << i << '\n';        total += i;    }    std::cout << total << '\n';}

#include <stdio.h> int main(void) {    int n, total = 0;    scanf("%d", &n);    for (int i = 1; i <= n; i++) {        printf("%d\n", i);        total += i;    }    printf("%d\n", total);    return 0;}

n = Console.ReadLine().ToNumber();total = 0;for (i = 1; i <= n; i++){    Console.WriteLine(i);    total += i;}Console.WriteLine(total);

stdin

stdout

Key points

The most common shape is "from 1 to n".
You choose a start value, a condition to keep going, and how much changes each lap.
Accumulating a value as you go (total += i) is the basis of sum, count and maximum problems.
To walk through a list or string item by item, there is a shorter form that hands you each value directly, with no index.
A while loop looks only at a condition and keeps going as long as it is true.
break leaves the loop and continue skips the rest of this lap.
Both affect only the innermost loop, so to leave a nested loop at once, use a flag variable or return from a function.

How each language writes it

Python	`for i in range(1, n + 1):` — the end value of `range` is not included. Step: `range(10, 0, -2)`; each item: `for x in nums:`; condition: `while cond:`. There is no `do-while`
JavaScript	`for (let i = 1; i <= n; i++)`, step with `i -= 2`. Each item: `for (const x of nums)`. `while (cond) { }`, `do { } while (cond);`. To leave an outer loop, use a label: `break outer;`
TypeScript	Same as JavaScript
Java	`for (int i = 1; i <= n; i++)`, each item: `for (int x : nums)`. `while`, `do { } while`. Labels (`break outer;`) work too
C#	`for (int i = 1; i <= n; i++)`, each item: `foreach (var x in nums)`. `while`, `do { } while`
C++	`for (int i = 1; i <= n; i++)`, each item: `for (int x : nums)`. `while`, `do { } while`
C	`for (int i = 1; i <= n; i++)` — there is no for-each form, so loop over indexes. `while`, `do { } while`
Craykon	`for (i = 1; i <= n; i++)`, each item: `foreach (x in nums)`, condition: `while (cond) { }`. Leave with `break`, skip with `continue`. There is no `do-while`. In a scene (animation) `while` pauses one frame per lap, but in console solutions it runs without pausing

Common mistakes

A `while` loop that never changes its condition

i = 1
while i <= 5:
    print(i)

→ It never ends, printing 1 forever

Put the line that changes the condition (i += 1, i++) in the body.

Using `i` after the loopJava · C# · C · C++ · JavaScript · TypeScript · Craykon

for (int i = 0; i < 3; i++) { }
printf("%d\n", i);

→ An error — an i created in the for exists only inside the loop

If you need it afterwards, create it before the loop.
Python's i stays around with its last value.

Looping over values with `for … in`JavaScript · TypeScript

const nums = [10, 20];
for (const x in nums) console.log(x);

→ 0, 1 — the indexes, not the values (and as strings "0", "1")

Loop over values with for (const x of nums).

How languages differ
Python's range(a, b) does not include b, so 1 to n is range(1, n + 1).
In the C family and Craykon, double-check i <= n versus i < n.
Being off by one is the most common way a loop gets the wrong answer.

Loops

Key points

How each language writes it

Common mistakes

A while loop that never changes its condition

Using i after the loopJava · C# · C · C++ · JavaScript · TypeScript · Craykon

Looping over values with for … inJavaScript · TypeScript

Practice

A `while` loop that never changes its condition

Using `i` after the loopJava · C# · C · C++ · JavaScript · TypeScript · Craykon

Looping over values with `for … in`JavaScript · TypeScript