Exponential Number Conversion

[Tuurbo46]

Hello,

I am trying to pass 8.4e9 to a function to send to an instrument. However when I print this out it comes out in the wrong format (8.4000000e+06). How can I confirm it stays in the correct 8.4e9 format?

I can find how to convert from float to exponential etc, but I cannot find how to shift / format the exponential base number?

Thanks,
Tuurbo46

[Gribouillis]

You could post-process the result with the re module

>>> def replace(match):
...     return match.group(1) + (match.group(2) or '0')
... 
>>> import re
>>> x = 8.4e9
>>> s = f'{x:e}'
>>> re.sub(r'(e[+-])0*(\d*)$', replace, s)
'8.400000e+9'

or perhaps

>>> x = 8.4e9
>>> s = f'{x:e}'
>>> s
'8.400000e+09'
>>> L = re.split(r'(e[+-])0*(\d*)$', s)
>>> L[-2] = L[-2] or '0'
>>> ''.join(L)
'8.400000e+9'

[bowlofred]

I am trying to pass 8.4e9 to a function to send to an instrument. However when I print this out it comes out in the wrong format (8.4000000e+06).

I'm assuming that the +06 is really a +09 and the actual number isn't changing.

Python has some facility for changing the form of the mantissa.

>>> x = 8.4e9
>>> f"{x:e}"
'8.400000e+09'
>>> f"{x:.1e}"
'8.4e+09'

Unfortunately according to https://docs.python.org/3/library/string.html, the exponent always has at least two digits with this style of formatting. Also the sign is always present, even when positive. So if you need a single digit and to remove a positive sign indication, you would have to resort to string manipulation or a different exponential rendering.

>>> f"{x:.1e}".replace("+0","")
'8.4e9'

[Pedroski55]

I don't understand the problem. How is 9 being changed to 6?

Just working in Idle, I get:

x = 8.4e9
print(x) # returns 8400000000.0

Which is correct, if you count the columns up to the 8. Pass x to a function.

x = 8.4e9
def do_it(num):
    print(num)
    
do_it(x) # prints 8400000000.0

def do_it(num, power):
    print(num*10** power)

x = 8.4
power = 9
do_it(x, power)

Maybe you are using an old version of Python?

[DeaD_EyE]

def custom_scientiffic_notation(
    value: float, min_value: int | None = None, max_value: int | None = None
) -> str:
    """
    Convert a float to scientific notation with one decimal place.
    The sign of the exponent is stripped, if the value is positive.
    Leading 0 of the exponent is being stripped.

    Optionally a ValueError is risen, if min_value or max_value is given and value
    is out of range.

    A formatted str is returned.
    """
    if min_value is not None and value < min_value:
        raise ValueError("value is too small")
    if max_value is not None and value > max_value:
        raise ValueError("value is too big")

    v, e = f"{value:.1e}".split("e")
    return f"{v}e{int(e)}"