How Can I Accurately Round Floating-Point Numbers to Two Decimal Places in Python?

Limiting Floats to Two Decimal Points

When working with floating-point numbers, it's essential to consider the limitations of this data type. In particular, floating-point numbers may not represent certain values exactly, leading to unexpected results when attempting to round.

Issue: When trying to round a float to two decimal places, you may encounter the following issue:

>>> a
13.949999999999999
>>> round(a, 2)
13.949999999999999

The problem arises because floating-point numbers store values as integers divided by a power of two. This representation may not capture all values precisely. In the case of double-precision numbers (used by Python's floating point type), they have 53 bits (16 digits) of precision, while regular floats have 24 bits (8 digits).

Solution:

To display only two decimal places when rounding a float, several options are available:

• Use integers and store values in cents: Instead of storing dollars, store values in cents and divide by 100 to convert to dollars.
• Use a fixed point number library like decimal: Fixed point numbers provide more precise control over the number of decimal places.

Examples:

>>> a = 13.946
>>> print("%.2f" % a)
13.95
>>> print("{:.2f}".format(a))
13.95

Note that using round(a, 2) directly will still result in floating-point precision issues, but formatting the value as a string with %.2f or {:.2f} will produce the desired two decimal place representation.

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