Python 3 Float Decimal Points/Precision

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I am reading a text file with floating point numbers, all with either 1 or 2 decimal points. I am using float() to convert a line into a float, and raising a ValueError if that fails. I am storing all floats in a list. When printing it out, I'd like to print it out as a 2 decimal places floating point.

Assume I have a text file with the numbers -3,65, 9,17, 1. I read each one, and once I convert them to float and append them to a list. Now in Python 2, calling float(-3.65) returns -3.65. In Python 3 however, float(-3.65) returns-3.6499999999999999` which loses its precision.

I want to print the list of floats, [-3.6499999999999999, 9.1699999999999999, 1.0] with 2 decimal points only. Doing something along the lines of '%.1f' % round(n, 1) would return a string. How can I return a list of all two decimal points of floats, and not strings? So far, I rounded it using [round(num, 2) for num in list] but would need to set the decimal points / precision instead of round().

In a word, you can't.

3.65 cannot be represented exactly as a float. The number that you're getting is the nearest number to 3.65 that has an exact float representation.

The difference between (older?) Python 2 and 3 is purely due to the default formatting.

I am seeing the following both in Python 2.7.3 and 3.3.0:

In [1]: 3.65
Out[1]: 3.65

In [2]: '%.20f' % 3.65
Out[2]: '3.64999999999999991118'

For an exact decimal datatype, see decimal.Decimal.

15. Floating Point Arithmetic: Issues and Limitations, A consequence is that, in general, the decimal floating-point numbers you many digits you're willing to write down, the result will never be exactly 1/3, decimal arithmetic suitable for accounting applications and high-precision applications. I read each one, and once I convert them to float and append them to a list. Now in Python 2, calling float(-3.65) returns -3.65. In Python 3 however, float(-3.65) returns-3.6499999999999999` which loses its precision. I want to print the list of floats, [-3.6499999999999999, 9.1699999999999999, 1.0] with 2 decimal points only

The simple way to do this is by using the round buit-in.

round(2.6463636263,2) would be displayed as 2.65.

decimal, Unlike hardware based binary floating point, the decimal module has a user alterable Decimals interact well with much of the rest of Python. For example, Decimal('3.00000') records all five zeros even if the context precision is only three​. Python 3 Float Decimal Points / Precision. Я читаю текстовый файл с номерами с плавающей запятой, все с 1 или 2 десятичными точками.

The comments state the objective is to print to 2 decimal places.

There's a simple answer for Python 3:

>>> num=3.65
>>> "The number is {:.2f}".format(num)
'The number is 3.65'

or equivalently with f-strings (Python 3.6+):

>>> num = 3.65
>>> f"The number is {num:.2f}"
'The number is 3.65'

As always, the float value is an approximation:

>>> "{}".format(num)
'3.65'
>>> "{:.10f}".format(num)
'3.6500000000'
>>> "{:.20f}".format(num)
'3.64999999999999991118'

I think most use cases will want to work with floats and then only print to a specific precision.

Those that want the numbers themselves to be stored to exactly 2 decimal digits of precision, I suggest use the decimal type. More reading on floating point precision for those that are interested.

Precision Handling in Python, Python in its definition allows to handle precision of floating point numbers in used to eliminate all decimal part of the floating point number and return the integer 3. floor() :- This function is used to print the greatest integer smaller than the  Python float decimal precision. The decimal module provides support for decimal floating point arithmetic.It offers several advantages over the float datatype The context for arithmetic is an environment specifying precision, rounding rules, limits on exponents, flags indicating the results of operations, and trap..

Try this:

num = input("Please input your number: ")

num = float("%0.2f" % (num))

print(num)

I believe this is a lot simpler. For 1 decimal place use %0.1f. For 2 decimal places use %0.2f and so on.

Or, if you want to reduce it all to 2 lines:

num = float("%0.2f" % (float(input("Please input your number: "))))
print(num)

How do I round to 2 decimals in python? · GitHub, In python, you have floats and decimals that can be rounded. it to a decimal x = Decimal(16.0/7) # Then we round it to 2 places output = round(x,2) print output decimal import getcontext, Decimal # Set the precision. getcontext().prec = 3  Python 3 Float Decimal Points / Précision. Je suis de la lecture d'un fichier texte avec des nombres à virgule flottante, le tout avec 1 ou 2 points décimaux.

Try to understand through this below function using python3

def floating_decimals(f_val, dec):
    prc = "{:."+str(dec)+"f}" #first cast decimal as str
    print(prc) #str format output is {:.3f}
    return prc.format(f_val)


print(floating_decimals(50.54187236456456564, 3))

Output is : 50.542

Hope this helps you!

1.14. Decimals, Floats, and Floating Point Arithmetic, Floating point numbers like 12.345 are a basic type, but there are some complications due to their More on that in String Formats for Float Precision. Python 3 does what you would expect mathematically with an expression like (1/​2)*6.5. When printing it out, I.d like to print it out as a 2 decimal places In Python 3 however, float(-3.65) returns -3.6499999999999999` which loses. Display a float with two decimal places in Python How can I do this in Python How can I stop printing a float 3 spaces after decimal.

14. Floating Point Arithmetic: Issues and Limitations, How do you set a precision of a float in Python? Precision of floating point numbers in C++ (floor(), ceil(), trunc(), round() and setprecision()) Decimal equivalent of 1/3 is 0.33333333333333…. An infinite length number would require infinite memory to store, and we typically have 4 or 8 bytes.

How to Round Numbers in Python – Real Python, Tips for using floating-point and decimal numbers in Python. Floating-Point Types. Almost all platforms map Python floats to IEEE 754 double precision. f = 0.1  I think that using string is the correct way to go in this case, since you don't know a priori the number of digits in the decimal part. But, it won't work for all cases (e.g. 1.005), as mentioned before by @SingleNegationElimination.

Floating-point cheat sheet for Python, We'll use round() this time to round to three decimal places at each step, and seed() the store floating-point numbers as binary decimals with 53-bit precision​. Python 3.3 и установка PyOpenSSL на Mac asyncio.ensure_future vs. BaseEventLoop.create_task против простой сопрограммы? Hello World в Python Как напечатать определенную строку в pandas DataFrame? Python 3 Float Decimal Points / Precision

Comments
  • In python 2, float(-3.65) is -3.6499999999999999 too.. This is normal. float is by it's very nature not 100 precise.
  • Not in mine: Python 2.7.2 (v2.7.2:8527427914a2, Jun 11 2011, 15:22:34) │ [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin │ Type "help", "copyright", "credits" or "license" for more information. │ >>> float(-3.65) │ -3.65
  • Also, let me point out I know that float is not precise and I know the reasoning behind this. I'm looking an answer that will show how to print it out in 2 decimal points in Python 3. Thanks
  • Basically, you just can't really do this because of the way float point works. I suggest you consider using the Decimal class in the decimal module.
  • That is just the representation of Python rounding it to at most 16 positions behind the decimal.
  • So there is no way to limit the number of decimal points as an number type? To print 3.65, I would require to print it out as a string?
  • @Darksky One can only print strings. Sometimes the conversion isn't as explicit: but it is always done.
  • Yes, so how would I represent 3.64999999999999991118 as 3.65 or anything that's 2 decimal points, without it being a string? Any other type than float that I can use?
  • Use Decimal in the decimal module instead of float point.
  • I thought Python had this change awhile ago. A change to float formatting so that it tried to be the shortest string that would convert back into the exact float you were printing. Basically an attempt to make these kinds of rounding issues disappear most of the time. Did that change not make it into Python3? I thought I saw it in the changelog for 2.6 or 2.7.
  • Though be aware of the note from the documentation on round() that round() can give surprising results when given float values. The example mentioned there: "round(2.675, 2) gives 2.67 instead of the expected 2.68."
  • Hadn't seen the update with f-strings. Honestly not sure if I'm a fan but it's good to know!
  • Using f-strings does not seem to be very readable to another user who may not be aware of the feature. On the other hand, the using "{}".format() is much more intuitive when reading.
  • The OP didn't need a variable to define precision - it was set at 2 places. But for variable precision, you don't need to use the arcane string addition prc = "{:."+str(dec)+"f}". Instead, embed the precision: print("The number is {:.{}f}".format(f_val, dec)). The same but with f-strings: print(f"The number is {f_val:.{dec}f}").
  • This was a perfect solution for me