while loop increment in Javascript

Question

function diamond(n) {
  if (n % 2 == 0 || n < 1) return null
  var x = 0,
    add, diam = line(x, n);
  while ((x += 2) < n) {
    add = line(x / 2, n - x);
    diam = add + diam + add;
  }
  return diam;
}
function repeat(str, x) {
  return Array(x + 1).join(str);
}
function line(spaces, stars) {
  return repeat(" ", spaces) + repeat("*", stars) + "\n";
}
console.log(diamond(5));

The code prints the diamond shape using asterisks.
I was trying to understand this code (other person's code).
I thought the result would look like

  *
 ***
*****
*****
*****
 ***
  *

But it worked perfectly well, and I realized the part while ((x +=2) < n) did make difference.
So my question is: what is the difference between while ((x += 2) < n) { ... } and while (x < n) { ... x += 2 }?

kooskoos · Accepted Answer · 2019-12-11 19:08:34Z

2

 ((x+=2) < n)

x+=2 is a shorthand for x=x+2;

if x=0 initially the condition that would be checked would be 2 < n

(x < n) {...x += 2}

if x=0 initially the condition that would be checked would be 0 < n

Main difference is you are incrementing first and then checking the condition in first one while in second one you check and then increment x.

edited Dec 11, 2019 at 19:08

answered Dec 11, 2019 at 19:06

kooskoos

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3 Comments

SH Kim Over a year ago

Oh, so the statements in while loop have been looped through only two times?

SH Kim Over a year ago

I totally get it. Thanks so much !

kooskoos Over a year ago

Glad I was able to help:)

A Friend · Accepted Answer · 2019-12-11 19:09:51Z

2

So your question is what's the difference between...

while ((x+=2) < n) {}

AND

while(x < n) { x+=2 }

The main difference is that in the first example, the x gets the 2 added on and is then compared to the value of n, whereas in the second example, x is compared to n before the 2 has been added and will therefore most likely run 1 more loop than you would expect

answered Dec 11, 2019 at 19:09

A Friend

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