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ULP (Unit of Least Precision): what is it exactly?  RSS feed

Dario Stepanos
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Here's is what I've found about it:

Unit of least precision. It is used to discuss the accuracy of floating-point data. It represents the minimum step between representable values near a desired number: the true result to infinite precision, assuming the argument is exact. For instance, the ulp of 1.977436582E+22 is 1.0E+13, since the least significant (leftmost) digit of the mantissa is in the 10 13 place. Within 0.5 ulp is the best approximation representable.

My questions are, what is a minimum step and how do we get to 0.5 ulp (what is the calculation)?

Many thanks for your time,

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