Basics · 03

Reading input

Read text and numbers from the keyboard (standard input).

To solve problems you need to read input (standard input).
The example below reads a name on one line and a number on the next, then greets the name and prints twice the number.

name = input()n = int(input())print("Hi, " + name + "!")print(n * 2)

const lines = require("fs").readFileSync(0, "utf8").split("\n");const name = lines[0];const n = Number(lines[1]);console.log("Hi, " + name + "!");console.log(n * 2);

import { readFileSync } from "node:fs";const lines: string[] = readFileSync(0, "utf8").split("\n");const name: string = lines[0];const n: number = Number(lines[1]);console.log("Hi, " + name + "!");console.log(n * 2);

import java.util.Scanner; public class Main {    public static void main(String[] args) {        Scanner sc = new Scanner(System.in);        String name = sc.nextLine();        int n = Integer.parseInt(sc.nextLine().trim());        System.out.println("Hi, " + name + "!");        System.out.println(n * 2);    }}

using System; class Program{    static void Main()    {        string name = Console.ReadLine();        int n = int.Parse(Console.ReadLine());        Console.WriteLine("Hi, " + name + "!");        Console.WriteLine(n * 2);    }}

#include <iostream>#include <string> int main() {    std::string name;    int n;    std::getline(std::cin, name);    std::cin >> n;    std::cout << "Hi, " << name << "!" << '\n';    std::cout << n * 2 << '\n';}

#include <stdio.h> int main(void) {    char name[100];    int n;    scanf("%99s", name);    scanf("%d", &n);    printf("Hi, %s!\n", name);    printf("%d\n", n * 2);    return 0;}

name = Console.ReadLine();n = Console.ReadLine().ToNumber();Console.WriteLine("Hi, " + name + "!");Console.WriteLine(n * 2);

stdin

Lara
3

stdout

Hi, Lara!
6

Key points

Whatever you read is always a string.
To use it as a number, convert it.
You can read input a whole line at a time, or one space-separated token (a word or a number) at a time.
For several numbers on one line, split the line at the spaces and convert each piece.
When you don't know how many lines there are, read until the input ends.
Each language signals "the end" in its own way.

How each language writes it

Python	A line is `input()`, convert with `int(...)`. Several numbers on a line: `map(int, input().split())`. Until the end: `for line in sys.stdin:`
JavaScript	The usual way is to read everything at once with `require("fs").readFileSync(0, "utf8")` and split lines with `split("\n")`. Convert with `Number(...)`. For word-by-word reading, `.trim().split(/\s+/)` is safer
TypeScript	Same as JavaScript
Java	Read with `Scanner`'s `nextLine()` and convert with `Integer.parseInt(...)`. There is also `nextInt()` for a single number. Until the end: `while (sc.hasNextInt())`
C#	Read with `Console.ReadLine()` and convert with `int.Parse(...)`. Several numbers: `Split(' ')`. Until the end: `while ((line = Console.ReadLine()) != null)`
C++	`std::cin >> n` skips spaces and newlines and reads one token straight into a number. A whole line: `std::getline(std::cin, s)`. Until the end: `while (std::cin >> x)`
C	`scanf("%d", &n)` — the `&` before the variable is required. `%s` reads only one word, up to a space; for a whole line use `fgets`. Until the end: `while (scanf("%d", &x) == 1)`
Craykon	A line is `Console.ReadLine()`, convert with `.ToNumber()`. Several numbers on a line: `Console.ReadNumbers()` (gives a list of numbers). Until the end: until `Console.ReadLine()` returns `null`

Common mistakes

Reading a line right after a numberJava · C++

int n = sc.nextInt();
String name = sc.nextLine();

→ name is an empty string "", not the next line: it only read the newline left after the number

As in this example, read whole lines with nextLine() from the start and convert with Integer.parseInt.
C++ getline after cin >> n has the same problem.

Converting an empty line to a numberJavaScript · TypeScript

Number("")

→ Silently 0, which hides the mistake of reading an empty line

A newline at the end of the input leaves one extra empty string at the end.
.trim() before splitting.

How languages differ
What you read is a string, so calculating without converting goes wrong.
In JavaScript, adding 1 to the input 3 gives "3" + 1 → "31", and in Python input() + 1 is a TypeError.

Reading input

Key points

How each language writes it

Common mistakes

Reading a line right after a numberJava · C++

Converting an empty line to a numberJavaScript · TypeScript

Practice