问题
This question already has an answer here:
- Is floating point math broken? 31 answers
I convert degrees to radians (degrees * Math.PI/180) but why does the following:
Math.cos(90 * Math.PI/180)
yield 6.123031769111... and not zero?
I'm trying to perform 2D rotations uses matrixes and the results are completely out of whack.
回答1:
The output of
Math.cos(90 * Math.PI/180)
is
6.123031769111886e-17
Notice the e-17 at the end, which means that this number is 6.123 x 10-17. This is a number so vanishingly close to 0 that it's effectively 0. The reason that it's not identically 0 is due to rounding errors in the IEEE-754 double format which prevents you from getting an exact representation of π / 2 and causes minute rounding errors in the calculation of the cosine.
By the way - I was pretty surprised as well when the result came back starting with a 6! It's only after I looked at the very end that things started to make sense.
Hope this helps!
回答2:
6.123233995736766e-17
is scientific notation for a very small number, close to zero. It is not exactly zero because of rounding errors and so forth.
回答3:
6.123233995736766e-17 is very very close to zero. Just round the number.
来源：https://stackoverflow.com/questions/9652695/why-does-math-cos90-math-pi-180-yield-6-123031769111-and-not-zero