Issues with The Colon (:) Operator

Question

Royi Avital 2015-5-20

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/217673-issues-with-the-colon-operator

评论： Stephen23 2020-4-22

采纳的回答： Guillaume

在 MATLAB Online 中打开

Hello,

I have an issue with the colon operator which operates differently than expected.

Let's define the following vectors:

vA = 30:0.0025:60;
vB = 30:0.0025:70;

Now, since their starting point and step size are identical I'd expect their values to match in the overlap zones.

Yet, try this:

a = vA(642);
b = vB(642);
isequal(a, b)
a - b

As one could see, the result is different (46.0249999999 vs. 46.0250000006). This is very different from the documentation of the colon operator .

I was expecting its values no to be dependent on the end value (Only the number of steps depend on the end value).

Is it a bug or am I missing something?

Thank You.

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Answer 1

Guillaume 2015-5-20

3
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/217673-issues-with-the-colon-operator#answer_179743

The behaviour of colon used to be fully documented in older versions and in a technical solution on Mathworks website. Unfortunately, that documentation has been removed in newer versions and the link on Mathworks is dead.

This answer gives you the details.

Basically, with non-integer steps, colon works from both ends to avoid accumulating errors. I can't fathom why Mathworks thinks that information is no longer relevant.

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Aravind C G 2020-4-22

编辑：Aravind C G 2020-4-22

Wow, thank you for this answer that clarifies things. I stumbled upon a similar issue while using the colon operator to pass a range of decimal values into a function. Sometimes the size of the functions output would be [1 19] and at others [1 20]

Now I see where the problem is coming from.

Edit: Ah, now I see it is included in the latest documentation as:

"However, if i is not an integer, then floating point arithmetic plays a role in determining whether colon includes the endpoint k in the vector, since k might not be exactly equal to j+m*i"