How to repeat a string array in JavaScript that allows me to get a sum of it?

Question

I need to create an array with the days of the week, and I need to create a function that takes a day and a number n and return the day after the n days (for example getDayAfter('Monday', 4) will return 'Friday').

I've already created my array named day. It has in the array the days of the week.

I'm creating a function called RepeatArray and it's supposed to create a new array.

let day = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday', 'saturday', 'sunday'];

function repeatArray(day, count) {

  var ln = day.length;
  var b = new Array();
  var a = new Array();
  
  for (i = 0; i < count; i++) {
    day.push(day[i % ln]);
  }

  return b;
}

var a = new Array();
a.push("test1");
a.push("test2");
a.push("test3");

var b = repeatArray(a, 13);

console.log(b.length);

I expect the output of a, 13 to be monday, but the output is 0.

day.push shouldn't be b.push? Since you create b but never uses it and then you return it... also I would suggest to change the let day to another name to not mess names with parameter day — Calvin Nunes
– Calvin Nunes, Commented Jul 17, 2019 at 20:09
You should consider using more meaningful variable names. You've got too many as and bs and it makes it harder to reason about what's in which a for example. When you're just starting out, making everything named uniquely and meaningfully will make it harder to introduce meaningless variables (like the b variable in your function. Also note that day is an array of day names, but also the name of an argument to your function; use console.log(day) at the top of your function and you may be surprised by what it contains. I encourage you to use a debugger to step through your code — Heretic Monkey
– Heretic Monkey, Commented Jul 17, 2019 at 20:17
Why are you creating new arrays? Wasn’t the assignment just to get a day, n days after another day? — Kokodoko
– Kokodoko, Commented Jul 17, 2019 at 20:17

IceMetalPunk · Accepted Answer · 2019-07-17 20:19:14Z

Let's walk through what your code currently does, which is often a good way to find any problems.

let day = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday', 'saturday', 'sunday'];

This creates your days of the week array in an outer scope.

function repeatArray(day, count) {

This declares the repeatArray function -- which shadows the "day" variable. Since it has the same name as the "day" array in the global scope, anything inside this function that references "day" will reference this argument and you will have no access to the outer array.

var ln = day.length;
var b = new Array();
var a = new Array();

ln is now the length of the first argument passed to the function. b and a are empty arrays.

for (i = 0; i < count; i++) {
  day.push(day[i % ln]);
}

This pushes to the first argument the value in the first argument, at the position i mod ln, for all i from 0 to count. So if the first argument is the array ['a', 'b', 'c'], and count is 6, it would be pushing 'a', 'b', 'c', 'a', 'b', 'c' to the first argument, resulting in the array ['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b', 'c']. I don't think this is what you want. But let's continue.

return b;

The function returns b, which is still just a new, empty array with nothing in it, since we haven't touched it.

var a = new Array();
a.push("test1");
a.push("test2");
a.push("test3");

This creates an array, a, which ends up being ['test1', 'test2', 'test3'].

var b = repeatArray(a, 13);
console.log(b.length);

In our function call, 'day' will be the array a (so ['test1', 'test2', 'test3']), and count will be 13. Since the function returns an empty array every time, the length will always be 0. The array a, however, will be modified to repeat 'test1', 'test2', 'test3' 13 extra times, making its length 16 (13 plus the initial 3 values).

For your task, you don't need that kind of repetition or to create a new array. You can just use modular math :)

let day = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday', 'saturday', 'sunday'];
function daysAfter(initial, n) {
    const initialIndex = day.indexOf(initial);
    return day[(initialIndex + n) % day.length];
}

So now daysAfter('wednesday', 13) will equal tuesday as expected :)

Pietro Martinelli · Accepted Answer · 2019-07-17 20:28:07Z

1

I'm not quite sure what you need repeatArray for, but if you are just trying to, as you say, 'create a function that takes a day and a number n and return the day after the n days' this is how I would do it:

const days = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday', 'saturday', 'sunday'];

function getDayAfter(day, count) {
  const i = days.indexOf(day);
  return days[(i + count) % days.length];
}

console.log(getDayAfter('tuesday', 2)); // Should output 'thursday'

edited Jul 17, 2019 at 20:28

Pietro Martinelli

1,92615 silver badges19 bronze badges

answered Jul 17, 2019 at 20:18

Collierre

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3 Comments

IceMetalPunk Over a year ago

With this, getDayAfter('sunday', 10) will return undefined.

KarmaDevelop Over a year ago

Thanks, this is pretty useful to solve part of the problem <3 but with this, if i don't create the repeatarray it will return undefined if I type something like getDayafter('saturday',5)

Collierre Over a year ago

@mariettabell yes, other users pointed that out to me. I've now accepted an edit that fixes that.

Cait · Accepted Answer · 2019-07-17 20:32:19Z

0

I do not really understand the question but this should work.

    let weekday = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday', 'saturday', 'sunday'];

    function getDayAfter(day, count) {
      var id = weekday.indexOf(day);
      var ln = weekday.length;
      var targetday = id + count;

      if((id + count) < ln){
          return weekday[id + count];
      }else{
          return weekday[(id + count) % ln];
      }
    }

    var b = getDayAfter("monday", 13);
    document.writeln(b);

answered Jul 17, 2019 at 20:32

Cait

14 bronze badges

Comments

dmmfll · Accepted Answer · 2019-07-17 21:01:00Z

0

Here is the solution I came up with:

const days = ['monday', 'tuesday', 'wednesday', 'thursday', 'friday', 'saturday', 'sunday'],
      length = days.length;

function getDayFrom(day, count){
    //i % 7 will effectively repeat 0 thru 6 for any value of i
    //creating an array of repeated days. The count is the index of the day
    //expected
    return days[(days.indexOf(day) + count) % length]
    //The above is the efficient version of
    //Array.from({length: count + 1}, (_, i) => days[i % 7])[count]

}
console.log(getDayFrom('monday', 4))

The output is 'friday'.

edited Jul 17, 2019 at 21:01

answered Jul 17, 2019 at 20:38

dmmfll

2,8642 gold badges38 silver badges43 bronze badges

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