Convert Python floating-point into C# decimal

Question

I'm trying to convert a floating-point number in Python into decimal in C# using pythonnet.

Let's say there is one number 0.0234337688540165776476565071.

I have tried this in two ways:

using float()

from System import Decimal
Decimal(0.0234337688540165776476565071)
# Same with Decimal(float(0.0234337688540165776476565071))

0.02343377

using native Python Decimal

from System import Decimal
from decimal import Decimal as PyDecimal
Decimal(PyDecimal("0.0234337688540165776476565071"))

# This loses every number under the floating-point
0

What should I do?

The value returned from PyDecimal("0.0234337688540165776476565071") is a decimal do why are you putting Decimal() arond the results? If you want more accuracy use Double. A Double is 64 bits while a Float is 32 bits. Decimal gives better accuracy than float with 32 bits for most numbers because it is using a logrithmic algorithm. Double and Float uses exponential algorithm — jdweng
– jdweng, Commented Mar 28, 2021 at 13:36
PyDecimal above is native Python type, and I'm trying to convert Python's Decimal into System.Decimal of C#. That's why I did Decimal(PyDecimal(0.212....)). — 6991httam
– 6991httam, Commented Mar 28, 2021 at 17:58
Why would Python Decimal be any dffient from Ne3 Decimal? They both are IEEE 754. See : en.wikipedia.org/wiki/Decimal_floating_point — jdweng
– jdweng, Commented Mar 29, 2021 at 0:28

LOST · Accepted Answer · 2021-03-28 20:04:43Z

1

From the examples in your question, it looks like you are trying to convert a string to System.Decimal. For that, System.Decimal has a Parse method:

from System import Decimal

Decimal.Parse("0.0234337688540165776476565071")

Note: you probably also need to pass CultureInfo.InvariantCulture depending on your scenario.

answered Mar 28, 2021 at 20:04

LOST

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1 Comment

6991httam Over a year ago

This is really simple and works for me!! Thank you! Why I didn't know this...Parse()

VersBersch · Accepted Answer · 2021-03-28 18:35:27Z

0

If you cast a string to a python float then it will be truncated to 64-bit and you will lose precision. You will have to use a different constructor for System.Decimal. For instance:

public Decimal (int lo, int mid, int hi, bool isNegative, byte scale);

Skipping validation, it might look something like

from System import Decimal
from System import Int32, Boolean, Byte

def str_to_csp_decimal(number: str) -> Decimal:
    """ convert a string to a C# Decimal """
    is_negative = number[0] == "-"
    abs_value = number[1:] if is_negative else number

    has_fraction = "." in abs_value 
    if has_fraction:
        integer, fraction = abs_value.split(".")
        scale = len(fraction)                # denominator = 10 ** scale
        numerator = int(integer + fraction)  # no precision loss for integers
    else:
        scale = 0                            # denominator == 1
        numerator = int(abs_value)
    
    assert numerator < (2 << 96), "Decimal only has 96 bits of precision"

    # split the numerator into the lower, mid, and high 32 bits
    mask = 0xFFFFFFFF
    low = Int32(numerator & mask)
    mid = Int32((numerator >> 32) & mask)
    high = Int32((numerator >> 64) & mask)

    return Decimal(low, mid, high, Boolean(is_negative), Byte(scale))

str_to_csp_decimal("0.0234337688540165776476565071").ToString()

answered Mar 28, 2021 at 18:35

VersBersch

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2 Comments

LOST Over a year ago

Why not just use Decimal.Parse method?

6991httam Over a year ago

Your answer is also amazing. Thank you so much for this.

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