lua decimals break around -0.1 to 0.1 (exclusive)

Question

I have a Lua for loop that iterates through numbers from -1 to 1 by increments of 0.01, and is producing numbers as bad as 6.6613381477509e-016 in the range of -0.1 to 0.1 (exclusive)

I am using Lua in the LOVE engine (v 0.9.2), written in C++

I would much rather solve the problem than just receive an info dump of an explanation, but I'd rather that than nothing at all.

(also I do understand the general reason floats are inaccurate, so that may be left out of explanations)

I don't see how 6.6613381477509e-016 is a bad number. It appears between -0.01 and 0.01, right? — Andros Rex, Jul 07 '16 at 02:07
Show us the code. If you got that value by starting from `-1.0` and incrementing by `0.01` 100 times, I wouldn't be at all surprised. — Keith Thompson, Jul 07 '16 at 02:08
On my system, this: `lua -e 'x = -1 for i = 1,100 do x = x + 0.01 end print(x)'` prints `7.5286998857393e-16` — Keith Thompson, Jul 07 '16 at 02:10
Don't they teach this in elementary computer courses anymore? — tonypdmtr, Jul 08 '16 at 06:23
@tonypdmtr I am learning computer science almost entirely on my own, I've never had a course go over this. — Pixel, Jul 11 '16 at 22:29

score 6 · Accepted Answer · answered Jul 07 '16 at 02:08

6

You've already known the reason, here's a possible solution: don't loop with floating point numbers, use integers.

Instead of

for i = -1, 1, 0.01 do

use:

for i = -100, 100 do
  print(i / 100)
end

answered Jul 07 '16 at 02:08

Yu Hao

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lua decimals break around -0.1 to 0.1 (exclusive)

1 Answers1