Control flow · 05

Conditions

Do different things depending on a condition: if · else if · else, plus switch.

A conditional is "if this, then that" in code.
Conditions are checked from the top, and only the first branch that is true runs.
The example below turns a score into a grade and also says whether it is even.

score = int(input())if score >= 90:    print("A")elif score >= 80:    print("B")elif score >= 70:    print("C")else:    print("F")print("even" if score % 2 == 0 else "odd")

const score = Number(require("fs").readFileSync(0, "utf8"));if (score >= 90) {  console.log("A");} else if (score >= 80) {  console.log("B");} else if (score >= 70) {  console.log("C");} else {  console.log("F");}console.log(score % 2 === 0 ? "even" : "odd");

import { readFileSync } from "node:fs";const score: number = Number(readFileSync(0, "utf8"));if (score >= 90) {  console.log("A");} else if (score >= 80) {  console.log("B");} else if (score >= 70) {  console.log("C");} else {  console.log("F");}console.log(score % 2 === 0 ? "even" : "odd");

import java.util.Scanner; public class Main {    public static void main(String[] args) {        int score = new Scanner(System.in).nextInt();        if (score >= 90) {            System.out.println("A");        } else if (score >= 80) {            System.out.println("B");        } else if (score >= 70) {            System.out.println("C");        } else {            System.out.println("F");        }        System.out.println(score % 2 == 0 ? "even" : "odd");    }}

using System; class Program{    static void Main()    {        int score = int.Parse(Console.ReadLine());        if (score >= 90)            Console.WriteLine("A");        else if (score >= 80)            Console.WriteLine("B");        else if (score >= 70)            Console.WriteLine("C");        else            Console.WriteLine("F");        Console.WriteLine(score % 2 == 0 ? "even" : "odd");    }}

#include <iostream> int main() {    int score;    std::cin >> score;    if (score >= 90) {        std::cout << "A\n";    } else if (score >= 80) {        std::cout << "B\n";    } else if (score >= 70) {        std::cout << "C\n";    } else {        std::cout << "F\n";    }    std::cout << (score % 2 == 0 ? "even" : "odd") << '\n';}

#include <stdio.h> int main(void) {    int score;    scanf("%d", &score);    if (score >= 90) {        puts("A");    } else if (score >= 80) {        puts("B");    } else if (score >= 70) {        puts("C");    } else {        puts("F");    }    puts(score % 2 == 0 ? "even" : "odd");    return 0;}

score = Console.ReadLine().ToNumber();if (score >= 90){    Console.WriteLine("A");}else if (score >= 80){    Console.WriteLine("B");}else if (score >= 70){    Console.WriteLine("C");}else{    Console.WriteLine("F");}Console.WriteLine(score % 2 == 0 ? "even" : "odd");

stdin

stdout

C
odd

Key points

Conditions are checked from the top, so put the strictest test first.
If score >= 70 comes first, even 95 becomes C.
If every condition is false, the else branch runs.
else is optional.
Logical operators join conditions: and &&, or ||, not !.
Python uses and, or, not.
Once the answer is known, the rest is skipped (x != 0 && 10 / x > 1 is safe).
To compare one value against many cases, switch is easier to read.
Languages differ on whether a case "falls through" into the next one.
A short either/or can use the ternary operator: cond ? if true : if false (Python: a if cond else b).

How each language writes it

Python	`if cond:` / `elif cond:` / `else:` — `:` and indentation instead of braces. switch since 3.10: `match x:` + `case 1 \| 2:` + `case _:`
JavaScript	`if (cond) { } else if (cond) { } else { }` — in switch, a `case` without `break` falls through to the next `case`. Values are compared with `===`
TypeScript	Same as JavaScript. Comparing values of different types is caught at compile time
Java	`if (cond) { } else if (cond) { } else { }` — switch needs `break`. Since Java 14, `case 1, 2 -> …` does not fall through
C#	`if (cond) { } else if (cond) { } else { }` — in switch, every non-empty `case` must end with `break` (or `return`); leaving it out is a compile error
C++	`if (cond) { } else if (cond) { } else { }` — switch works only on integers (characters included); without `break` it falls through to the next `case`
C	Same as C++. True and false are any non-zero number and 0
Craykon	`if (cond) { } else if (cond) { } else { }` — `switch (x) { case 1, 2: … default: … }` runs only the matching case and never falls through

Common mistakes

Chaining `1 < x < 10`JavaScript · C · C++ · Craykon

let x = 50;
console.log(1 < x < 10);

→ true — (1 < x) becomes true first, and true < 10 is true

Split it: 1 < x && x < 10.
Only Python chains comparisons like math (TypeScript, Java and C# give a compile error).

`=` instead of `==`C · C++ · JavaScript · TypeScript

if (x = 5) {
    printf("five");
}

→ 5 is stored in x, and the condition is always true

Compare with ==.
Java, C#, Python and Craykon report this mistake as an error.

Forgetting `break` in a switchC · C++ · Java · JavaScript

switch (n) {
    case 1: puts("one");
    case 2: puts("two");
}

→ When n is 1, both "one" and "two" are printed

End each case with break;.
C# makes this a compile error, and Craykon's switch never falls through in the first place.

How languages differ
In JavaScript and TypeScript, use === for equality: == converts types first, so "1" == 1 and "" == 0 are both true.
Craykon's == is strict like ===, so "1" == 1 is false.

Conditions

Key points

How each language writes it

Common mistakes

Chaining 1 < x < 10JavaScript · C · C++ · Craykon

= instead of ==C · C++ · JavaScript · TypeScript

Forgetting break in a switchC · C++ · Java · JavaScript

Practice

Chaining `1 < x < 10`JavaScript · C · C++ · Craykon

`=` instead of `==`C · C++ · JavaScript · TypeScript

Forgetting `break` in a switchC · C++ · Java · JavaScript