如何遍历数组并在到达结束后从头开始？

Question

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.

Answer 1

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")

Answer 2

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.

Answer 3

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)

Answer 4

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);