Create a string of variable length, filled with a repeated character

Question

So, my question has been asked by someone else in it\'s Java form here: Java - Create a new String instance with specified length and filled with specific character. Best so

Answer 1

Unfortunately although the Array.join approach mentioned here is terse, it is about 10X slower than a string-concatenation-based implementation. It performs especially badly on large strings. See below for full performance details.

On Firefox, Chrome, Node.js MacOS, Node.js Ubuntu, and Safari, the fastest implementation I tested was:

function repeatChar(count, ch) {
    if (count == 0) {
        return "";
    }
    var count2 = count / 2;
    var result = ch;

    // double the input until it is long enough.
    while (result.length <= count2) {
        result += result;
    }
    // use substring to hit the precise length target without
    // using extra memory
    return result + result.substring(0, count - result.length);
};

This is verbose, so if you want a terse implementation you could go with the naive approach; it still performs betweeb 2X to 10X better than the Array.join approach, and is also faster than the doubling implementation for small inputs. Code:

// naive approach: simply add the letters one by one
function repeatChar(count, ch) {
    var txt = "";
    for (var i = 0; i < count; i++) {
        txt += ch;
    }
    return txt;
}

Further information:

• Run speed test in your own browser
• Full source code of speed test
• Speed test results