float precision / round() precision

[MichaSU99]

Hi everybody,

I'm using a ESP32 DevKit C V2

I have some issues with the precision of floatingpoint calculations.
Here an example:

>>> 9487.26 + 0.01
9487.269

>>> round(9487.26 + 0.01, 2)
9487.269

I would expect that, at least with the round() function, a correct result is achieved.
Or is this normal behavior?

Thanks in advance, Michael.

[robert-hh]

1. It could be a problem of print
2. Single precision float has a little bit less than 7 significant digits. The number may be at the limit.

[robert-hh]

Actually it works fine here:

>>> 9487.26 + 0.01
9487.27

You could try to update the firmware. The print mechanism for floating point numbers has been updated.

[MichaSU99]

Yeah, this works.
Now using:
MicroPython v1.22.1 on 2024-01-05; Generic ESP32 module with ESP32

Thanks!

[MichaSU99]

Hi robert,
I'm sorry for inconvenience, but there must still be a problem(?)
9487.26 + 0.01 now works, but:

i:float = 9486.0

for n in range(0, 100):
    i+=0.01
    # print(i)
    
print(i)

Result is 9486.977 instead of 9487.0

If you print out the interim values, the inaccuracy starts at:

>>> 9486.02 + 0.01
9486.029

Merely in combination with round(i + 0.01, 2) I will get a correct answer...

regards, Michael.

[robert-hh]

I assume it's because of the fact, that there is no precise binary representation of 0.01. If you add that over and over, the error will sum up. Rounding after each step prevents this accumulation. If you write e.g.:

i:float = 9486.0

for n in range(0, 128):
    i += 1/128 
    # print(i)
    
print(i)

You will get a proper result. 1/128 can be exactly represented as binary number.

[MichaSU99]

Ok, many thanks for your explanation. round() is a appropriate workaround...

[kjm1102]

One of the things that has always puzzled me is that while the round function in upython can't seem to do the round up

>>> round(9487.26+.01, 2)
9487.269

whatever it is that does stringifying can

>>> '%.2f'%(9487.26+.01)
'9487.27'

[robert-hh]

Please update your ESP32 firmware. With a recent firmware I get:

MicroPython v1.23.0-preview.47.g16c6bc47c on 2024-01-17; Generic ESP32 module with ESP32
Type "help()" for more information.
>>> round(9487.26+.01, 2)
9487.27