represent decimal numbers in javascript as "%g" formatting in C

Question

I have a number generated as a finite decimal:

var x = k * Math.pow(10,p)

with k and p integers. Is there a simple way to convert it to an exact string representation?

If I use implict string conversion I get ugly results:

""+ 7*Math.pow(10,-1)

gives

"0.7000000000000001"

I tried using .toFixed and .toPrecision but it is very difficult to find the correct precision to use depending on k and p. Is there a way to get the good old "%g" formatting of C language? Maybe I should resort to an external library?

what do you mean by exact string representation? What was the output you were expecting? — Ashish, Apr 16 '19 at 11:06
Numbers of the form `k*pow(10,p)' have a finite decimal expansion. I would like to get that exact expansion with the minimum number of required digits. In the example: 0.7 — Emanuele Paolini, Apr 16 '19 at 11:42
I'm missing it too: A js function like c/php/... `printf()` functions!! — Wiimm, Apr 16 '19 at 12:40

score 1 · Accepted Answer · answered Apr 16 '19 at 12:24

1

One can use math.format from math.js library:

math.format(7 * Math.pow(10, -1), {precision: 14});

gives

"0.7"

answered Apr 16 '19 at 12:24

Emanuele Paolini

9,912
3
38
64

Vadim Hulevich · Answer 2 · 2019-04-16T13:09:26.223

0

You can create your own floor function like this, with option accuracy

  function fixRound(number, accuracy) {

      return ""+Math.floor(number * (accuracy || 1)) / accuracy || 1;

  }

  let num = 7 * Math.pow(10, -1);

  console.log(fixRound(num, 1000))

edited Apr 16 '19 at 13:09

answered Apr 16 '19 at 11:18

Vadim Hulevich

1,803
8
17

1

The arithmetic in `fixRound` cannot work because .7 is not exactly representable in the JavaScript `Number` format, so no calculating can ever round a number to exactly .7. It can get close enough that the default formatting will print “.7”, so this kludge may work for some purposes. But `fixRound` is also subject to rounding errors in the multiply and the divide. The request OP makes ought to be addressed by using formatting features (which internally handle the conversion to decimal with calculations that avoid additional rounding errors), not by attempting to fudge the arithmetic. – Eric Postpischil Apr 16 '19 at 19:36

represent decimal numbers in javascript as "%g" formatting in C

2 Answers2