为什么2.4 mod 0.8返回0.8 [重复]

Question

This question already has an answer here:

• Why does modulus operator return fractional number in javascript? 7 answers

Why modulus calculation of 2.4 by 0.8 returns 0.8? Shouldn't it return 0?

It happens both in PHP, via fmod, and JavaScript, via %.

Even a google research with this operation fails: http://www.google.com/search?q=2.4+mod+0.8

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Answer 1

If you look closer at what happens in this concrete example, the influence of the binary format becomes more visible.

The input is stored as a binary floating point approximation. This then can be printed as decimal number uniquely identifying the binary,

In [1]: print "%.25f"%2.4
2.3999999999999999111821580

In [2]: print "%.25f"%0.8
0.8000000000000000444089210

The first step is the computation of the quotient and of its integer part. Obviously, as the stored numbers are smaller 2.4 and larger 0.8, the quotient is smaller than 3,

In [3]: print "%.25f"%(2.4/0.8)
2.9999999999999995559107901

The modulo result, the remainder, is then finally the difference after removing the integer multiple,

In [4]: print "%.25f"%(2.4-2*0.8)
0.7999999999999998223643161