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Comment by f2f | Hacker News Reader

Any time you see a decimal floating-point constant with a nonzero fractional part that doesn't end in '5', you're looking at a bug.

depends on the language. for example here it is in go:

    Pi  = 3.14159265358979323846264338327950288419716939937510582097494459 // http://oeis.org/A000796

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What the person above you is saying, I think, is to remember that computers usually work in base 2. This applies to IEEE floating points, where the mantissa is in base 2; when you represent fractions in base two, they're powers of two: 1/2 (.5), 1/4 (.25), 1/8 (.125), etc. What he's asserting, I think, is that any power-of-two fraction, or any combination of those (in binary), result in a number ending in 5 when represented in decimal. Anything else is going to be rounded to the nearest representable number (that ends in 5).

So, go might have that value in its source, but it's getting rounded to something that would, if represented in decimal, end in 5.

In Go, floating-point constants may have very high precision, so that arithmetic involving them is more accurate. The constants defined in the math package are given with many more digits than are available in a float64.

Having so many digits available means that calculations like Pi/2 or other more intricate evaluations can carry more precision until the result is assigned, making calculations involving constants easier to write without losing precision. It also means that there is no occasion in which the floating-point corner cases like infinities, soft underflows, and NaNs arise in constant expressions.

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