Data · 12

Building strings

Build a string from pieces, join with a separator, print values on one line.

When you can't write the output string in one go, you build it by adding one piece at a time.
The example below reads some words and prints a string of their first letters, the words joined with commas, and the word lengths on one line separated by spaces.

words = input().split()initials = ""for w in words:    initials += w[0]         # 한 조각씩 붙여 가요print(initials)print(", ".join(words))      # 사이에만 ", "lens = [len(w) for w in words]print(*lens)                 # 빈칸으로 이어 찍기

const words = require("fs").readFileSync(0, "utf8").trim().split(/\s+/);let initials = "";for (const w of words) {  initials += w[0]; // 한 조각씩 붙여 가요}console.log(initials);console.log(words.join(", ")); // 사이에만 ", "const lens = words.map((w) => w.length);console.log(lens.join(" ")); // 빈칸으로 이어 찍기

import { readFileSync } from "node:fs";const words: string[] = readFileSync(0, "utf8").trim().split(/\s+/);let initials: string = "";for (const w of words) {  initials += w[0]; // 한 조각씩 붙여 가요}console.log(initials);console.log(words.join(", ")); // 사이에만 ", "const lens: number[] = words.map((w) => w.length);console.log(lens.join(" ")); // 빈칸으로 이어 찍기

import java.util.Scanner; public class Main {    public static void main(String[] args) {        String[] words = new Scanner(System.in).nextLine().trim().split(" +");        StringBuilder initials = new StringBuilder();        for (String w : words) {            initials.append(w.charAt(0)); // 한 조각씩 붙여 가요        }        System.out.println(initials);        System.out.println(String.join(", ", words)); // 사이에만 ", "        int[] lens = new int[words.length];        for (int i = 0; i < words.length; i++) lens[i] = words[i].length();        for (int i = 0; i < lens.length; i++) {            if (i > 0) System.out.print(" "); // 첫 값 앞에는 안 찍어요            System.out.print(lens[i]);        }        System.out.println();    }}

using System;using System.Text; class Program{    static void Main()    {        string[] words = Console.ReadLine().Split(' ', StringSplitOptions.RemoveEmptyEntries);        var initials = new StringBuilder();        foreach (string w in words)        {            initials.Append(w[0]); // 한 조각씩 붙여 가요        }        Console.WriteLine(initials);        Console.WriteLine(string.Join(", ", words)); // 사이에만 ", "        int[] lens = new int[words.Length];        for (int i = 0; i < words.Length; i++) lens[i] = words[i].Length;        Console.WriteLine(string.Join(" ", lens)); // 빈칸으로 이어 찍기    }}

#include <iostream>#include <string>#include <vector> int main() {    std::vector<std::string> words;    std::string w;    while (std::cin >> w) words.push_back(w);    std::string initials;    for (const std::string& x : words) {        initials += x[0]; // 한 조각씩 붙여 가요    }    std::cout << initials << '\n';    for (size_t i = 0; i < words.size(); i++) {        if (i > 0) std::cout << ", "; // 사이에만 ", "        std::cout << words[i];    }    std::cout << '\n';    std::vector<int> lens;    for (const std::string& x : words) lens.push_back(x.size());    for (size_t i = 0; i < lens.size(); i++) {        if (i > 0) std::cout << ' '; // 첫 값 앞에는 안 찍어요        std::cout << lens[i];    }    std::cout << '\n';}

#include <stdio.h>#include <string.h> int main(void) {    char words[100][51], initials[101];    int n = 0, lens[100];    while (n < 100 && scanf("%50s", words[n]) == 1) n++;    for (int i = 0; i < n; i++) {        initials[i] = words[i][0]; // 한 조각씩 붙여 가요    }    initials[n] = '\0'; // 문자열의 끝 표시    printf("%s\n", initials);    for (int i = 0; i < n; i++) {        if (i > 0) printf(", "); // 사이에만 ", "        printf("%s", words[i]);    }    printf("\n");    for (int i = 0; i < n; i++) lens[i] = strlen(words[i]);    for (int i = 0; i < n; i++) {        if (i > 0) printf(" "); // 첫 값 앞에는 안 찍어요        printf("%d", lens[i]);    }    printf("\n");    return 0;}

words = Console.ReadLine().Split();initials = "";foreach (w in words){    initials += w[0]; // 한 조각씩 붙여 가요}Console.WriteLine(initials);Console.WriteLine(words.Join(", ")); // 사이에만 ", "lens = [];foreach (w in words){    lens.Add(w.Length);}Console.WriteLine(lens); // 목록은 빈칸으로 이어 찍어요

stdin

red green blue

stdout

rgb
red, green, blue
3 5 4

Key points

Building: start from an empty string and add a piece to the end on each lap (initials += w[0]).
It is the same shape as collecting a sum with total += x.
In Java and C#, += makes a whole new copy of the string so far every time.
So they have a separate StringBuilder that collects the pieces and turns them into a string only once, at the end.
Some languages turn a number into a string for you when you append it; in others you must convert it yourself.
Joining puts a separator only between the items of a list and makes one string.
red, green, blue joined with ", " is red, green, blue.
Nothing is added at the end.
Where there is no join, use a loop.
Print the separator first, only when the item is not the first one, then the item, and no separator is left at the end.
Printing several numbers on one line is the same job.
Some languages have a short way to print them all at once; in others you print them one by one without a newline.

How each language writes it

Python	Append with `s += piece` — convert numbers with `str(n)` first. With many pieces, `append` them to a list and finish with `"".join(parts)`. `", ".join(words)` accepts only a list of strings. One line of output: `print(nums)`; change the separator with `print(nums, sep=", ")`
JavaScript	Append with `s += piece` — numbers become strings by themselves. `words.join(", ")` also works on arrays of numbers, and `join()` with empty parentheses joins with commas. One line of output: `console.log(nums.join(" "))`
TypeScript	Same as JavaScript
Java	`StringBuilder sb = new StringBuilder();` then `sb.append(piece)` — it takes characters, numbers and strings. At the end, `sb.toString()` or simply `println(sb)`. `String.join(", ", words)` accepts only strings. Print numbers one by one with `System.out.print`, or collect them in a `StringBuilder`
C#	`var sb = new StringBuilder();` (`using System.Text;`) then `sb.Append(piece)`, and `sb.ToString()` at the end. `string.Join(", ", words)` also accepts an array of numbers, so one line of output is `Console.WriteLine(string.Join(" ", nums))`
C++	A `std::string` can be changed, so `s += piece` appends in place. Convert numbers with `std::to_string(n)` first. There is no join, so use a loop with `if (i > 0) std::cout << ", ";`. You can also collect with `<<` into a `std::ostringstream` and take the result with `.str()`
C	A string is a `char` array: reserve enough room, put a character in each slot, and put `'\0'` at the end. Append a string with `strcat(buf, "cd")` and a number with `sprintf(buf + len, "%d", n)`. There is no join, so use a loop and `printf`
Craykon	Append with `s += piece` — numbers become strings by themselves. `words.Join(", ")` also works on lists of numbers. `Console.WriteLine(list)` prints the items separated by spaces; to print one by one, use `Console.Write(value)`, which does not end the line

Common mistakes

Putting the separator after each item

words = ["red", "green", "blue"]
line = ""
for w in words:
    line += w + ","
print(line)

→ red,green,blue, — one extra comma at the end

Use join, or add the separator before the item only when it is not the first: if line: line += ",", then line += w.

Joining a list of numbers as it isPython · Java

print(", ".join([1, 2, 3]))

→ Python raises TypeError: sequence item 0: expected str instance, int found. In Java, String.join(", ", nums) with int[] nums is a compile error too

In Python use ", ".join(map(str, nums)) or print(*nums, sep=", "); in Java collect into a StringBuilder.
Join in JavaScript, C# and Craykon accepts numbers.

Adding two quoted strings with `+`C · C++

char *s = "ab" + "cd";

→ Compile error invalid operands to binary + — a C string is an address, and addresses cannot be added. std::string s = "ab" + "cd"; in C++ is a compile error for the same reason

In C: char buf[10] = "ab"; strcat(buf, "cd");.
In C++ make one side a std::string: std::string("ab") + "cd".

How languages differ
With many pieces, how you append makes a big difference in time.
Appending one character 200,000 times, in the same setup as this site's judge, took Java about 1 s with += and 1–2 ms with StringBuilder; C# took 2–3 s with += and 1–2 ms with StringBuilder.
Python, with += outside a function as in the example, took about 0.5 s, and under 10 ms when collecting in a list and calling join.
JavaScript (Node.js) took around 10 ms even with +=.
The times differ from computer to computer.
Also, in C++ always go through std::to_string when appending a number: s += 65 appends not "65" but A, the character with code 65.