Elena Elena - 2 years ago 71
Python Question

Float difference in string and int format. Why?

I need the current time with nanosec.
As string

"%.9f" % float_time
it is 1502872986.653693676. You can see 76 at the end.
If I try to write the same time as int
int(float_time*10**9)
, it will be 1502872986653693696 and 96 at the end. Why? And which is the right way to get nano format?

from time import time

float_time = time()
print("float_time:", float_time)
int_nano_time = int(float_time*10**9)
print("int_nano_time:", int_nano_time)
str_nano_time = "%.9f" % float_time
print("str_nano_time:", str_nano_time)



float_time: 1502872986.6536937

int_nano_time: 1502872986653693696

str_nano_time: 1502872986.653693676


Solution:

time.monotonic()



float_time: 536596.296

int_nano_time: 536596296000000

str_nano_time: 536596.296000000

Answer Source

Both are correct within the IEEE754 double precision of ~15.7 decimal digits. The reason is that the multiples of ten cannot be produced exactly, so while multiplying by 2 ** 10 is exact, multiplying by 5 ** 10 will have rounding errors again, and that's why the last numbers differ.

'1502872986.653693675994873046875000000000'
>>> "%.30f" % (1502872986.653693676 * 10 ** 9)
'1502872986653693696.000000000000000000000000000000'

The hex representations of these 2 floating point numbers are wildly different:

>>> (1502872986.653693676 * 10 ** 9).hex()
'0x1.4db4704d00617p+60'
>>> (1502872986.653693676).hex()
'0x1.6650166a9d61ep+30'

As for the time returned from the time() call - well it is highly probable that should the computer have a timer that gives a timestamp with high enough precision any time value will be between the exact time time() was called and when it returned:

>>> (time() - time()) * 10 ** 9
-715.2557373046875
>>> (time() - time()) * 10 ** 9
-953.67431640625
>>> (time() - time()) * 10 ** 9
-953.67431640625

your error was 20 nanoseconds, but the time difference between 2 consecutive timestamps returned from time() call is 700-100 nanoseconds on my computer.

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