How Can I Reliably Round Floats to Two Decimal Places for Display?

Limiting Floats to Two Decimal Points

Trying to round a float to two decimal points using standard methods like round() may produce unexpected results. This is due to the inherent limitations of floating-point representation, where certain numbers cannot be represented exactly.

In binary computers, floating-point numbers are stored as integers divided by a power of two. For example, 13.95 is approximately represented as 125650429603636838/(2**53). Since floating-point types have limited precision (24 bits for floats and 53 bits for double precision), these values cannot always be represented precisely.

As a result, rounding a floating-point number using round() may produce a result that is slightly off. To achieve the desired two-decimal-place precision for display purposes, alternative approaches can be taken:

• Use integers to store monetary values in cents and divide by 100 to display dollars.
• Use fixed-point number libraries like decimal to ensure exact representation.

For example, the following operations will correctly round a to two decimal places for display:

print("%.2f" % a)  # -> 13.95
print("{:.2f}".format(a))  # -> 13.95

Using "{:.15f}".format(round(a, 2)) will return 13.949999999999999 due to the precision limitations of floating-point numbers. However, for display purposes, this level of precision is not necessary.

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