question concerning rounding of numbers

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Jakob Sievers 2015-3-26

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https://ww2.mathworks.cn/matlabcentral/answers/194305-question-concerning-rounding-of-numbers

回答： Jakob Sievers 2015-3-27

Hi

I am having trouble rounding a number to a fixed number of digits. Can anyone explain the problem I am encountering? (As shown in this figure)

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Answer 1

James Tursa 2015-3-26

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https://ww2.mathworks.cn/matlabcentral/answers/194305-question-concerning-rounding-of-numbers#answer_172900

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John D'Errico 2015-3-26

As has been said, no matter how hard you try, you cannot represent 30.7 exactly as a double precision number. That is a binary representation, and your number is not exactly representable.

In fact, this is true of almost all the numbers you will write in decimal form. The exceptions are integers of a reasonable size, and those numbers that can be written as sums of limited negative powers of 2. So 0.5, and 0.375 will be exactly represented.

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