OhDaeSu
OhDaeSu

Reputation: 525

How to loop through an array and continue at beginning once it reaches end?

My problem:

I have an array called "weekdays":

const weekdays = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];

Imagine it is Saturday and I want to know how many days it is until Tuesday (obviously 3 days). How can I loop through the array - starting at "Sat" and continue at the beginning until it reaches "Tue"?

My code so far:

const weekdays = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];

const start = weekdays.indexOf("Sat"); 
const end = weekdays.indexOf("Tue"); 
let howManyDays = 0;

for (let i = start; i < end; i = (i + 1) % weekdays.length) {
  howManyDays = howManyDays + 1;
}

However, it seems to be that "howManyDays" is still 0 when I run the code in the console in the browser.

Upvotes: 1

Views: 1577

Answers (5)

Bekir Kaplan
Bekir Kaplan

Reputation: 76

You can simply use this. Without looping.

const weekdays = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];

const weekLength = weekdays.length;
const startDate = weekdays.indexOf("Wed");
const endDate = weekdays.indexOf("Thu"); 

const result = (weekLength - startDate + endDate) % weekLength;

console.log(result); // -> 1

Edit: Above example gives 0 (zero) if you put the same day as start and end date. If you want to get 7 instead of 0. you can add a check.

const weekdays = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];

const weekLength = weekdays.length;
const startDate = weekdays.indexOf("Sat");
const endDate = weekdays.indexOf("Sat"); 

const result = (weekLength - startDate + endDate) % weekLength;

console.log(!result ? weekLength : result)

Upvotes: 0

dippas
dippas

Reputation: 60553

UPDATE

In order to make this work properly we need to take into account array wrapping - which I didn't in my initial answer.

So I'll just leave here a solution similar to the already existing.

const howManyDaysBetween = (start, end) => {
  const weekdays = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
  const week = weekdays.length;
  const startDay = weekdays.indexOf(start);
  const endDay = weekdays.indexOf(end);
  const howManyDays = startDay > endDay ? (week - startDay) + endDay : endDay - startDay;
  
  console.log(`How many days between ${start} and ${end}?: ${howManyDays}`);
}

howManyDaysBetween("Sat", "Tue")
howManyDaysBetween("Tue", "Sat")
howManyDaysBetween("Mon", "Sun")
howManyDaysBetween("Fri", "Wed")

Upvotes: 3

Jack_Hu
Jack_Hu

Reputation: 1379

const weekdays = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];

const length = weekdays.length;        // 7
const start = weekdays.indexOf("Sat"); // 5
const end = weekdays.indexOf("Tue");   // 1

let numOfDays = 0;

// If start comes after end, then we can take the difference between the start and the length,
// and add that to the end.
if (start > end) {
  numOfDays = (length - start) + end;
  //               (7 - 5)     + 1     // 3
}

// If start is before end, then just subtract one from the other.
if (start < end) {
  numOfDays = end - start;
}

// If the start and end are the same, then it's 0, which is what our variable was initialised as.
return numOfDays;                      // 0

The only consideration is, whether you want the same day to be 0, as in the example, or 7, if it's next week.

Upvotes: 3

IT goldman
IT goldman

Reputation: 19485

This loop seems most appropriate to the question asked. Although a bit silly if you run 2 indexOf you already got the distance. just need to substract and module array length. But this approach is good for the loop, because you can just compare the values as you go until you find "Tue"

const weekdays = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];

const start = weekdays.indexOf("Sat");
const end = weekdays.indexOf("Tue");
let howManyDays = 0;

for (let i = start; i != end; i++) {
  i = i % weekdays.length;
  howManyDays = howManyDays + 1;
}

console.log(howManyDays)

Upvotes: 2

anthonyb
anthonyb

Reputation: 372

You're needing to count in a base-7 numbering system. The following code will do it simply for you.

const weekdays = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'];

const start = weekdays.indexOf('Sat');
const end = weekdays.indexOf('Tue') + weekdays.length;

const howManyDays = end - start;

console.log(howManyDays);

Upvotes: -1

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