Matlab给出了错误的答案 [英] Matlab gives wrong answer

问题描述

如果执行以下代码，则MATLAB会出错.有人可以验证吗?

If the following code is executed MATLAB makes a mistake. Can someone verify this?

floor([0.1:0.1:2]/0.01)

那129在这里做什么?

So what is the 129 doing here??

ans = 10 20 30 40 50 60 70 80 90 100 110 120 129 140 150 160 170 180 190 200

推荐答案

由于冒号生成的向量，这是一个浮点数舍入错误.
就像拉斯曼说的那样，如果您这样做:

It is a floating point rounding error because of the colon-generated vector.
Like Rasman said, if you do:

floor((0.1:0.1:2 + eps) / 0.01)

没有舍入错误.

但是，基于冒号运算符如何工作，我建议您进行相同的计算，如下所示:

However, based on how the colon operator works, I suggest that you do the same calculation like this:

floor([(1:20)/10] / 0.01)

[ 编辑 :在Rasman评论之后，我将补充说，后一种方法同样适用于负值，而添加eps有时会失败]

[Edit: following Rasman's comment, I will add that the latter approach works for negative values as well, while adding eps sometimes fails]

最重要的是，最好使用带有 integer 数字的冒号运算符，以最大程度地减少舍入误差.

The bottom line is that it is better using the colon-operator with integer numbers to minimize rounding errors.

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