How to reverse the order in a FOR loop

Question

I\'ve a simple FOR statement like this:

var num = 10,
    reverse = false;

for(i=0;i

when rever

Answer 1

Roy's is similar to mine, but here's what I was thinking. I'll give you what I wrote in C# and how I think it translates to Javascript.

C#

    int num = 10;
    bool reverse = true;

    for (int i = reverse ? num : 0; (reverse ? 0 : i) < (reverse ? i : num); i += reverse ? -1 : 1)
    {
        Console.Write((reverse ? i - 1 : i).ToString());
    }
    Console.ReadKey();

Javascript

        var num = 10,
        reverse = true;

    for (int i = reverse ? num : 0; (reverse ? 0 : i) < (reverse ? i : num); i += reverse ? -1 : 1)
    {
        console.log(reverse ? i - 1 : i);
    }

And here's another way
Javascript

    var num = 10,
        reverse = false;

    for (int i = 0; i < num; i++)
    {
      console.log((reverse ? abs(-num + (i + 1)) : i));

    }

Answer 2

It seems to work:

  var num = 10;
  var z = 1;
  var k = -10;
  if (reverse ){
    k = -1;
    z = -1;
  }
  for(int i = 0; i < 10; i++){
    console.log(num+i*z+k);
  }

Answer 3

Surely in a language like Javascript there must be a way to define a local function and use that in the loop?

function SubtractFrom(val, subtractor) {
    return val - subtractor;
}

function PassThrough(val) {
    return val;
}

var num = 10;
var processor = reverse ? SubtractFrom(num-1) : PassThrough;

for (i = 0; i < num; i++) {
    console.log(processor(i));
}

Not knowing Javascript though, I don't know what actual form the function definitions would take.

Answer 4

I think this meets your requirements:

var num = 10;
var reverse = false;
var diff = 0;

if (reverse) {
    diff = num - 1;
}

for (i = 0; i < num; i++) {
    console.log(Math.abs(diff - i));
}

Answer 5

var start; var end; var inc;
if (reverse) {
    start = num-1; end = 0; inc = -1;
}
else {
    start = 0; end = num-1; inc = 1;
}
for(i=start;i!=end;i+=inc){
    console.log(i)
}

Answer 6

I just came across the need for this the other day. Here's how I did it:

var num = 10,
    i = 0,
    direction = 1, 
    reverse = false;

if(reverse)
    i = num + (direction = num = -1);

for(; i !== num; i += direction) {
    console.log(i);
}

No need for separate loops, and no need to do math to calculate the proper i in the loop.

So if reverse is true...

• i (which represents our first item) becomes num - 1, so we're now starting on what would have been the last item

• num (which represents out of bounds) becomes -1, so we're now stopping on what would have been the first item

• direction is -1, which means it will decrement when we do i += direction

So by swapping our starting point with our ending point and changing the alteration of i from 1 to -1, we'll be going up or down based on those modifications.