Javascript sort array using day of month

Question

I want to change the order of a string based on what day of the month it is.

For example, say the word is "PIZZAS", on the 1st of May it will be "ZZSIPA", and on the 2nd of May it will be "APIZZS" etc...

The code I run works for Math.random(), however when I assign it the value of the day of the month, it does not change.

    const dayOfMonth = new Date().getDate();

function shuffle(s) {
    
  var arr = s.split(''); 

for (let i = arr.length -1; i > 0; i--) {
  let j = Math.floor(dayOfMonth * i)
  let k = arr[i]
  arr[i] = arr[j]
  arr[j] = k
}
    
  s = arr.join(''); 
  return s; // Return shuffled string
    
}
var s = shuffle("PIZZAS");

Samuel Olekšák · Accepted Answer · 2022-04-10T12:13:57.420

1

Math.random() returns a float between 0 and 1, new Date().getDate() returns an integer between 1 and 31, so if you want to get the same behavior you should divide the day of month by 31.

But note that this pseudo-random number generator is not very good and you'd probably get less predictable results with something from question about seeding JS random number generator.

edited Apr 10 '22 at 12:13

answered Apr 10 '22 at 12:08

Samuel Olekšák

389
4
12

2

What's wrong with `Math.random()`? Apart from not being able to seed it, I've never heard anything *bad* about it – J-Cake Apr 10 '22 at 12:18
@J-Cake well the question is about seeding the RNG with current day of the month, isn't it? – Samuel Olekšák Apr 10 '22 at 12:22
Thought you were referring to the RNG itself :/ – J-Cake Apr 10 '22 at 12:23
Yes so I essentially need 31 different orders of the array – Sam Apr 10 '22 at 12:23

score 0 · Answer 2 · answered Apr 10 '22 at 12:49

By using 'Math.sin(dayOfMonth++) * 10000', it's solved the issue.

var dayOfMonth = new Date().getDate();

function random() {
    var x = Math.sin(dayOfMonth++) * 10000;
    return x - Math.floor(x);
}

function shuffle(s) {
    
  var arr = s.split(''); 

for (let i = arr.length -1; i > 0; i--) {
  let j = Math.floor(random() * i)
  let k = arr[i]
  arr[i] = arr[j]
  arr[j] = k
}
    
  s = arr.join(''); 
  return s; // Return shuffled string
    
}
var s = shuffle("PIZZAS");

score 0 · Answer 3 · answered Apr 10 '22 at 12:56

Your shuffle is failed because you are swapping undefined.

I have debugged your code, the implementation of shuffle(s) function remains unchanged.

And the shuffle_new(s) will fix the problem of swapping undefined by using modulo operator: let j = Math.floor(dayOfMonth * i) % arr.length, so that arr[j] never returns undefined.

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Shuffle Demo</title>
</head>

<body>
    <script>
        const writeln = (text = '') => document.writeln(text + '<br/>');
    </script>
    <script>
        const dayOfMonth = new Date().getDate();

        function shuffle(s) {
            var arr = s.split('');
            for (let i = arr.length - 1; i >= 0; i--) {
                writeln();
                let j = Math.floor(dayOfMonth * i)
                writeln('j:' + j);
                let k = arr[i]
                writeln('k:' + k);
                arr[i] = arr[j]
                writeln('arr[j]:' + arr[j]);
                arr[j] = k
                writeln('arr:' + arr);
            }

            s = arr.join('');
            return s; // Return shuffled string

        }

        function shuffle_new(s) {
            var arr = s.split('');
            for (let i = arr.length - 1; i >= 0; i--) {
                writeln();
                let j = Math.floor(dayOfMonth * i) % arr.length
                writeln('j:' + j);
                let k = arr[i]
                writeln('k:' + k);
                arr[i] = arr[j]
                writeln('arr[j]:' + arr[j]);
                arr[j] = k
                writeln('arr:' + arr);
            }

            s = arr.join('');
            return s; // Return shuffled string
        }

        writeln('dayOfMonth:' + dayOfMonth);
        writeln('process: shuffle("PIZZAS")');
        var s = shuffle("PIZZAS");
        writeln('output:' + s);

        writeln();
        writeln();
        writeln();
        writeln();
        writeln('process: shuffle_new("PIZZAS")');
        var s = shuffle_new("PIZZAS");
        writeln('output:' + s);
    </script>
</body>

</html>

Javascript sort array using day of month

3 Answers3