Replace random characters in string

Question

I'm trying to replace random strings characters with a a set amount of a specific character (i.e: "*"), I'll explain more in the code below.

/**
 * Returns a string with replaced amount of strings with a set string
 * @param {string} str The input
 * @param {number} amountToReplace The amount of letters you want to replace
 * @param {string} generalReplace The general letter to replace with
 * @returns {string} Returns a string with the string thats finished
 */

var str = 'thisisastring';
var amountToReplace = 5; 
var generalReplace = '*'

// Function over here, e.g. with the name replacer
var returnMe = replacer(str, amountToReplac

Sorry if the params sound confusing, I'm still new to them!!

So the string, str is the string that's the input the amountToReplace is the amount of letters you want to replace and the generalReplace is general character you want to replace with.

If the amount is 5, the output I'm expecting is something like:

original: thisisastring
output: th*s**as*ri*g

The string that is returned is supposed to replace random parts in the string(but still do the amount that was inputted, instead of doing 5 replacers but returning 3 as common matches were found).

Thanks!

Yes, but also with a set amount of strings I want to replace with — Jeffplays2005
– Jeffplays2005, Commented Sep 21, 2021 at 1:51

Roko C. Buljan · Accepted Answer · 2021-09-26 16:00:27Z

2

The smallest (arguably simplest):

Split the string into Array [...str] // (is now an Array)
Loop inserting at random points an array with the replacer character like i.e: ["t", "h", "i", ["*"], "i", ["*"], ...
continue looping until i (the amount) is not 0 and until the current value at the iterating index is an Array.
Once all done, since we have how a 2D array, use Array.prototype.flat() to make it one dimensional like i.e: ["t", "h", "i", "*", "i", "*", ...
and convert it back to String using Array.prototype.join()

const replacer = (str, i, rep) => {
  if (!str) return;                      // Do nothing if no string passed
  const arr = [...str];                  // Convert String to Array
  const len = arr.length
  i = Math.min(Math.abs(i), len);        // Fix to Positive and not > len 
  while (i) {
    const r = ~~(Math.random() * len);
    if (Array.isArray(arr[r])) continue; // Skip if is array (not a character)
    arr[r] = [rep];                      // Insert an Array with the rep char
    --i;
  }
  return arr.flat().join("");
};


console.log(replacer("thisisastring", 5, "*"));

The above example is battle-tested and works amazingly even if:

There's no string or it's empty
The string contains already all asterisks "*****"
The amount is greater then the string length
The amount is a negative integer

The longest:

Generate an Array of N (amountToReplace) random unique integers
Convert the input string to Array, iterate it replacing every character in that random index to the generalReplace "*" character

/**
 * Create an array of unique integers
 * @param {Integer} len Array length
 * @param {Integer} max Max integer value
 * @returns {Array} Array of random Unique integers
 */
function genArrRandInt(len, max) {
  const nums = new Set();
  while (nums.size !== len) nums.add(Math.floor(Math.random() * max));
  return [...nums];
};

/**
 * Returns a string with replaced amount of strings with a set string
 * @param {string} str The input
 * @param {number} amountToReplace The amount of letters you want to replace
 * @param {string} generalReplace The general letter to replace with
 * @returns {string} Returns a string with the string thats finished
 */
function replacer(str, amountToReplace, generalReplace) {
  const strArr = [...str];
  const uniqueArr = genArrRandInt(amountToReplace, str.length);
  uniqueArr.forEach(i => strArr[i] = generalReplace);
  return strArr.join("");
}


const str = "thisisastring";
const amountToReplace = 5;
const generalReplace = "*";

// Function over here, e.g. with the name replacer
const returnMe = replacer(str, amountToReplace, generalReplace);
console.log(returnMe);

edited Sep 26, 2021 at 16:00

answered Sep 21, 2021 at 2:07

Roko C. Buljan

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6 Comments

Jeffplays2005 Over a year ago

Thank you so much for being so detailed, params were extremely useful and descriptive, amazing in-depth answer!

Roko C. Buljan Over a year ago

@Jeffplays2005 you're welcome

Roko C. Buljan Over a year ago

@Jeffplays2005 there's another fix you could do in genArrRandInt(len, max) and that's to use max = Math.min(len, max); to prevent max being greater than the the string length - in case you used something like : replacer("aaa", 5, "*") since "aaa" is smaller in length than 5. Or you could do the same inside the replacer function.

Jeffplays2005 Over a year ago

I just wanted to mention, there was a really small minor mistake for the first example, instead of return res.flat().join("");, it would be return arr.flat().join(""); as res is undefined!

Roko C. Buljan Over a year ago

@Jeffplays2005 thank you so much for the extra eye. Thought arr would be a better term than res and forgot to rename the latest one :D thank you

|

StackSlave · Accepted Answer · 2021-09-21 02:25:57Z

1

Code might be something like:

function rand(min, max){
  let mn = min, mx = max;
  if(mx === undefined){
    mx = mn; mn = 0;
  }
  return mn+Math.floor(Math.random()*(mx-mn+1));
}
function randCharReplace(string, replaceWith = '*', replaceCount = 1){
  const a = string.split(''), m = a.length-1;
  const f = ()=>{
    return rand(0, m);
  }
  for(let i=0,r; i<replaceCount; i++){
    while(r === undefined || a[r] === replaceWith){
      r = f();
    }
    a[r] = replaceWith;
  }
  return a.join('');
}
console.log(randCharReplace('thisisastring',  '*', 5));

answered Sep 21, 2021 at 2:25

StackSlave

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Comments

DecPK · Accepted Answer · 2021-09-21 02:27:44Z

1

A better efficient approach using Set

function replacer(str, len = 0, generalReplacer) {
  const length = len > str.length ? str.length : len,
           set = new Set(),
           newStr = [];
  
  while (set.size !== length) {
    set.add(Math.floor(Math.random() * str.length));
  }

  for (let i = 0; i < str.length; ++i) {
    set.has(i) ? newStr.push(generalReplacer) : newStr.push(str[i]);
  }

  return newStr.join("");
}

console.log(replacer("thisisastring", 5, "*"));
console.log(replacer("thisisastring", 1, "*"));
console.log(replacer("thisisastring", 7, "*"));

edited Sep 21, 2021 at 2:27

answered Sep 21, 2021 at 2:26

DecPK

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1 Comment

Roko C. Buljan Over a year ago

newStr.push("*") should be newStr.push(generalReplacer)

nart · Accepted Answer · 2021-09-21 03:46:06Z

0

function replacer(string, amount) {
    // If string contains all '*' return string
    if ([...string.matchAll(/\*/g)].length === string.length) {
        return string
    }
    // copy the ori string convert to array
    let res = [...string]
    let i = amount
    let e = ['*', ' ']
    // iterate amount times
    while (i >= 1) {
        // get random index in array
        const randomIdx = Math.floor(Math.random() * string.length)
        // get char at index
        const randomChar = res[randomIdx]
        // check if it not '*' or ' '
        if (!e.includes(randomChar)) {
            // replace char with '*'
            res.splice(randomIdx, 1, '*')
            i--
        }
    }
    return res.join('')
}

You can try this.

edited Sep 21, 2021 at 3:46

answered Sep 21, 2021 at 2:23

nart

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2 Comments

Roko C. Buljan Over a year ago

replacer("asdf", 3) returns wrong results like asd*, a*s* instead of i.e: *s** and if the string is replacer("***", 3) we enter an infinite loop.

nart Over a year ago

@RokoC.Buljan Thanks you for pointing out the err. I edited my answer.

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