[solved] look's like pre increment in while condition is skipping the first increment

[espruino-discuss]

Posted at 2020-07-24 by @MaBecker

Why do this two do while loops produce different output?

var dataLen4 = 4;
var idx = 0;
do {
      console.log("1. idx:", idx, idx << 2);
      ++idx;
} while(idx <  dataLen4 );
/* output
1. idx: 0 0
1. idx: 1 4
1. idx: 2 8
1. idx: 3 12
*/

idx=0;
do {
      console.log("2. idx:", idx, idx << 2);
} while(++idx <  dataLen4 );
/* output
2. idx: 0 0
2. idx: 2 8
2. idx: 3 12
*/

Code was running on a PICO_R1_3 with 2v06.49.

[gfwilliams]

Posted at 2020-07-24 by @gfwilliams

Wow, thanks. No idea how that happened - and it's been a problem since at least 2v00.

I'll get on to a fix for this now

[gfwilliams]

Posted at 2020-07-24 by @gfwilliams

Ok, now fixed (in cutting edge or 2v07)

[espruino-discuss]

Posted at 2020-07-24 by @MaBecker

Thanks - As quickly as usual!

[espruino-discuss]

Posted at 2020-07-24 by @allObjects

Neat... I use(d) pre-decrement quite a lot (when walking an array (backwards when direction did not matter)... luckily I used just the while () ...; and not the do ... while() construct and never noticed an issue... ...and there is no issue with it... (the 'real' pre-pre-decrement...) - haha (I usually try NOT to enter a loop unconditionally... it is increased risk for an infinite loop (and it has messed with me). Of course, even with an upfront conditioned loop, an infiniteloop is possible, but much less because 'the break/exit thinking' is done upfront rather than an after thought. If I wanted to run the loop at least once, that would go also into the setup of the upfront condition... and thought. I'm though very aware of the fact that the do ... while() construct can be very useful (and is actually very Espruino-economic): it conveys already by its very architecture that the loop body is executed at least once where as with the upfront condition that fact is masked and only understood by wading the start values and the condition. @MaBecker, saw your post the day before. Even though it is fixed now, i wanted to know if I was affected by it.

// predecrement.js

function predecTest() {

  var i=5; // for example: ...=array.legth()
  while(--i >= 0) console.log(i); // walk array
  console.log("----(1)");

  var j=4; // for example: ...=array.legth()-1
  while (j >=0) {
    console.log(j);
    j--; }
  console.log("----(2)");

  var k=5; // for example: ...=array.legth()
  while (k > 0) { k--; // walk array
    console.log(k); }
  console.log("----(2)");

  var l=4; // for example: ...=array.legth()-1
  do { console.log(l);
  } while(--l >= 0); // walk array
  console.log("----(4)");

}

function onInit() {
  predecTest();
}

setTimeout(onInit,999);

Output:

>
 ____                 _
|  __|___ ___ ___ _ _|_|___ ___
|  __|_ -| . |  _| | | |   | . |
|____|___|  _|_| |___|_|_|_|___|
         |_| espruino.com
 2v06 (c) 2019 G.Williams
>
4
3
2
1
0
----(1)
4
3
2
1
0
----(2)
4
3
2
1
0
----(2)
4
2
1
0
----(4)
> 

[espruino-discuss]

Posted at 2020-07-28 by DrAzzy

it's been a problem since at least 2v00.

Says something about how often people use do...while() loops doesn't it?