Simple way to repeat a string

Question

I\'m looking for a simple commons method or operator that allows me to repeat some string n times. I know I could write this using a for loop, but I wish to avoid f

Answer 1

a straightforward one-line solution:
requires Java 8

Collections.nCopies( 3, "abc" ).stream().collect( Collectors.joining() );

Answer 2

If you only know the length of the output string (and it may be not divisible by the length of the input string), then use this method:

static String repeat(String s, int length) {
    return s.length() >= length ? s.substring(0, length) : repeat(s + s, length);
}

Usage demo:

for (int i = 0; i < 50; i++)
    System.out.println(repeat("_/‾\\", i));

Don't use with empty s and length > 0, since it's impossible to get the desired result in this case.

Answer 3

public static String rep(int a,String k)

       {
           if(a<=0)
                return "";
           else 
           {a--;
               return k+rep(a,k);
       }

You can use this recursive method for you desired goal.

Answer 4

public static String repeat(String str, int times) {
    int length = str.length();
    int size = length * times;
    char[] c = new char[size];
    for (int i = 0; i < size; i++) {
        c[i] = str.charAt(i % length);
    }
    return new String(c);
}

Answer 5

Simple loop

public static String repeat(String string, int times) {
    StringBuilder out = new StringBuilder();
    while (times-- > 0) {
        out.append(string);
    }
    return out.toString();
}

Answer 6

So you want to avoid loops?

Here you have it:

public static String repeat(String s, int times) {
    if (times <= 0) return "";
    else return s + repeat(s, times-1);
}

(of course I know this is ugly and inefficient, but it doesn't have loops :-p)

You want it simpler and prettier? use jython:

s * 3

Edit: let's optimize it a little bit :-D

public static String repeat(String s, int times) {
   if (times <= 0) return "";
   else if (times % 2 == 0) return repeat(s+s, times/2);
   else return s + repeat(s+s, times/2);
}

Edit2: I've done a quick and dirty benchmark for the 4 main alternatives, but I don't have time to run it several times to get the means and plot the times for several inputs... So here's the code if anybody wants to try it:

public class Repeat {
    public static void main(String[] args)  {
        int n = Integer.parseInt(args[0]);
        String s = args[1];
        int l = s.length();
        long start, end;

        start = System.currentTimeMillis();
        for (int i = 0; i < n; i++) {
            if(repeatLog2(s,i).length()!=i*l) throw new RuntimeException();
        }
        end = System.currentTimeMillis();
        System.out.println("RecLog2Concat: " + (end-start) + "ms");

        start = System.currentTimeMillis();
        for (int i = 0; i < n; i++) {
            if(repeatR(s,i).length()!=i*l) throw new RuntimeException();
        }               
        end = System.currentTimeMillis();
        System.out.println("RecLinConcat: " + (end-start) + "ms");

        start = System.currentTimeMillis();
        for (int i = 0; i < n; i++) {
            if(repeatIc(s,i).length()!=i*l) throw new RuntimeException();
        }
        end = System.currentTimeMillis();
        System.out.println("IterConcat: " + (end-start) + "ms");

        start = System.currentTimeMillis();
        for (int i = 0; i < n; i++) {
            if(repeatSb(s,i).length()!=i*l) throw new RuntimeException();
        }
        end = System.currentTimeMillis();
        System.out.println("IterStrB: " + (end-start) + "ms");
    }

    public static String repeatLog2(String s, int times) {
        if (times <= 0) {
            return "";
        }
        else if (times % 2 == 0) {
            return repeatLog2(s+s, times/2);
        }
        else {
           return s + repeatLog2(s+s, times/2);
        }
    }

    public static String repeatR(String s, int times) {
        if (times <= 0) {
            return "";
        }
        else {
            return s + repeatR(s, times-1);
        }
    }

    public static String repeatIc(String s, int times) {
        String tmp = "";
        for (int i = 0; i < times; i++) {
            tmp += s;
        }
        return tmp;
    }

    public static String repeatSb(String s, int n) {
        final StringBuilder sb = new StringBuilder();
        for(int i = 0; i < n; i++) {
            sb.append(s);
        }
        return sb.toString();
    }
}

It takes 2 arguments, the first is the number of iterations (each function run with repeat times arg from 1..n) and the second is the string to repeat.

So far, a quick inspection of the times running with different inputs leaves the ranking something like this (better to worse):

1. Iterative StringBuilder append (1x).
2. Recursive concatenation log2 invocations (~3x).
3. Recursive concatenation linear invocations (~30x).
4. Iterative concatenation linear (~45x).

I wouldn't ever guessed that the recursive function was faster than the for loop :-o

Have fun(ctional xD).