what is the correct way to sum loop

Question

var startX = 501;
var finalX = 320;
var multiply = .01;
var res = finalX - startX;
var extract = multiply * res;
var toEval;

if (res > 0) {
  toEval = "<=";
} else {
  toEval = ">=";
}
console.log("extract= " + extract + "");

while (eval(startX + toEval + finalX)) {

  startX += (-1.81);
  console.log(startX + " " + extract);

}

the "problem" is in the while loop, in the thirteenth iteration happen this in console:

482.9 -1.81
proof2.html:39 481.09 -1.81
proof2.html:39 479.28 -1.81
proof2.html:39 477.46999999999997 -1.81
proof2.html:39 475.65999999999997 -1.81

479.28 - 1.81 = 477.47

so what is really happen in the loop?

I don't know; I can't tell what the code is trying to accomplish. — Pointy, Jul 12 '16 at 19:51
You should always explain what you are trying to do before giving the example code. Please edit your question to explain what you want to do, and how your code is going wrong. — lwassink, Jul 12 '16 at 19:55

Nina Scholz · Accepted Answer · 2017-11-26T16:59:25.070

2

You could use a function for the comparing, instead of an use of eval.

var startX = 501,
    finalX = 320,
    multiply = .01,
    res = finalX - startX,
    extract = multiply * res,
    toEval;

if (res > 0) {
    toEval = function (a, b) { return a <= b; };
} else {
    toEval = function (a, b) { return a >= b; };
}
console.log("extract= " + extract + "");

while (toEval(startX, finalX)) {
    startX += (-1.81);
    console.log(startX + " " + extract);
}

edited Nov 26 '17 at 16:59

answered Jul 12 '16 at 19:49

Nina Scholz

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Really helpful, thanks. but my questios is there, why is javascript machine using so mani decimals matematicaly wrong 479.28 - 1.81 = 477.47 not 477.46999999999997 i know that i could round() some way but... what happen? – G. Jaen Jul 12 '16 at 19:58
1

you may have a look here [Is floating point math broken?](http://stackoverflow.com/questions/588004/is-floating-point-math-broken). It's matter of storing a floating-point number and their calculation. – Nina Scholz Jul 12 '16 at 20:01
totally helpful, i am new programming, so cloud you give me some guide or videos to watching. i really want to learn now am reading "speaking javascript". Thank you. – G. Jaen Jul 14 '16 at 07:07
sorry, i do not now any videos. just the documentation of [mozilla](https://developer.mozilla.org/en-US/docs/web/javascript/reference), which is very helpful. but if aou have questions, the just ask at so. – Nina Scholz Jul 14 '16 at 07:10

what is the correct way to sum loop

1 Answers1