Question

Consider the following code snippet in Python:

m = int(math.sqrt(n))

For n = 25, it should give m = 5 (and it does in my shell). But from my C experience I know that using such expression is a bad idea, as sqrt function may return a slightly lower value than the real value, and then after rounding i may get m = 4 instead of m = 5. Is this limitation also involved in python? And if this is the case, what is be the best way to write such expressions in python? What will happen if I use Java or C#?
Besides, if there is any inaccuracy, what factors controls the amount of it?

Answer 1

If you are very concerned with the accuracy of sqrt, you could use the decimal.Decimal class from the standard library, which provides its own sqrt function. The Decimal class can be set to greater precision than regular Python floats. That said, it may not matter if you are rounding anyways. The Decimal class results in exact numbers (from the docs):

The exactness [of Decimal] carries over into arithmetic. In decimal floating point,
0.1 + 0.1 + 0.1 - 0.3 is exactly equal to zero. In binary floating point, the result is 5.5511151231257827e-017. While near to zero, the
differences prevent reliable equality testing and differences can
accumulate. For this reason, decimal is preferred in accounting
applications which have strict equality invariants.