CODEWITHSUNDEEP
javascriptarraysfor-loop
Melisa
Melisa

Reputation: 19

How to stop a For Loop in a middle and continue from there down back in JavaScript

I have a JavaScript code like so:

var myArray = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20];

for (var i = 0, di = 1; i >= 0; i += di) {    

    if (i == myArray.length - 1)  { di = -1; }

    document.writeln(myArray[i]);
}

I need it to stop right in the middle like 10 and from 10 starts counting down to 0 back.

So far, I've managed to make it work from 0 to 20 and from 20 - 0.

How can I stop it in a middle and start it from there back?

Please help anyone!

Upvotes: 1

Views: 166

Answers (6)

BotNet
BotNet

Reputation: 2809

If you capture the midpoint ( half the length of the array ), just start working your step in the opposite direction.

const N = 20;
let myArray = [...Array(N).keys()];
let midpoint = Math.round(myArray.length/2)

for ( let i=1, step=1; i; i+=step) {

  if (i === midpoint)
    step *= -1
    
  document.writeln(myArray[i])
}

To make things clearer, I've:

  • Started the loop iterator variable (i) at 1; this also meant the array has an unused 0 value at 0 index; in other words, myArray[0]==0 that's never shown
  • Set the the loop terminating condition to i, which means when i==0 the loop will stop because it is falsy
  • Renamed the di to step, which is more consistent with other terminology
  • The midpoint uses a Math.round() to ensure it's the highest integer (midpoint) (e.g., 15/2 == 7.5 but you want it to be 8 )
  • The midpoint is a variable for performance reasons; calculating the midpoint in the loop body is redundant and less efficient since it only needs to be calculated once
  • For practical purpose, made sizing the array dynamic using N
  • Updated to ES6/ES7 -- this is now non-Internet Explorer-friendly [it won't work in IE ;)] primarily due to the use of the spread operator (...) ... but that's easily avoidable

Upvotes: 0

Nina Scholz
Nina Scholz

Reputation: 386604

You could move the checking into the condition block of the for loop.

var myArray = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20];

for (
    var i = 0, l = (myArray.length >> 1) - 1, di = 1;
    i === l && (di = -1), i >= 0;
    i += di
) {
    document.writeln(myArray[i]);
}

Upvotes: 0

Tony D
Tony D

Reputation: 194

Couldn’t you just check if you’ve made it halfway and then subtract your current spot from the length?

for(i = 0; i <= myArray.length; i++){

   if( Math.round(i/myArray.length) == 1 ){
       document.writeln( myArray[ myArray.length - i] );
    } else {
       document.writeln( myArray[i] );
   }

}

Unless I’m missing something?

Upvotes: 0

Ryan Wilson
Ryan Wilson

Reputation: 10765

Here is an example using a function which accepts the array and the number of items you want to display forwards and backwards:

var myArray = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20];


if(myArray.length === 1){
   ShowXElementsForwardsAndBackwards(myArray, 1);
}
else if(myArray.length === 0) {
   //Do nothing as there are no elements in array and dividing 0 by 2 would be undefined
}
else {
   ShowXElementsForwardsAndBackwards(myArray, (myArray.length / 2));
}


function ShowXElementsForwardsAndBackwards(mYarray, numberOfItems){
      if (numberOfItems >= mYarray.length) {
             throw "More Numbers requested than length of array!";
      }

      for(let x = 0; x < numberOfItems; x++){
          document.writeln(mYarray[x]);
      }

      for(let y = numberOfItems - 1; y >= 0; y--){
          document.writeln(mYarray[y]);
      }
}

Upvotes: 1

Francis Leigh
Francis Leigh

Reputation: 1960

Could Array.reverse() help you in this matter? 

const array = [0,1,3,4,5,6,7,8,9,10,11,12,13,14,15]

const getArrayOfAmount = (array, amount) => array.filter((item, index) => index < amount)

let arraySection = getArrayOfAmount(array, 10)

let reversed = [...arraySection].reverse()


console.log(arraySection)
console.log(reversed)

And then you can "do stuff" with each array with watever array manipulation you desire.

Upvotes: 0

Karl Taylor
Karl Taylor

Reputation: 5279

Just divide your array length by 2

var myArray = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20];

for (var i = 0, di = 1; i >= 0; i += di) {    

    if (i == ((myArray.length / 2) -1 ))  { di = -1; }

    document.writeln(myArray[i]);
}

Upvotes: 1

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