0x1 matrix getting while using find function

2018 9 10
1 个回答
回答已采纳
更新时间：2018 9 10
5 次查看（30 天）

0 个投票

Am getting 0x1 matrix while using find function to retreiv the indices,i have tried to use the max function in the array and then used find function still getting 0x1 empty double matrix error.Could you please help as when am using the same function to retriev the other indices am getting proper results

6 个评论
显示 4更早的评论 隐藏 4更早的评论

Guillaume
Guillaume 2018-9-10
编辑：Guillaume 2018-9-10
Well, if you're getting an empty matrix it's because find didn't find anything. If you were expecting find to find something, then you've got a bug (or the wrong expectations) but without seeing your code we certainly can't guess what the bug could be.
Sandeep Nair
Sandeep Nair 2018-9-10
ok,please find the example code a.values(100) + 1; ans = 2.11 find(a.values(100)+1 ==2.11) ans = 212
Now if i store the first command in some variable lets say b, b = a.values(100) + 1; find(a.values(100) +1 == b) am getting ans =0×1 empty double column vector I have used max function also before using find function but still getting the same error
Stephen23
Stephen23 2018-9-10
编辑：Stephen23 2018-9-10
You are doing an exact comparison of floating point numbers, which is not recommend because of the exact problem that you are having. Always compare the difference of floating point numbers against a tolerance:
abs(A-B)<=tol
The values that you are trying to compare cannot be exactly represented using binary floating point numbers, and are not the same. What you see printed in the command window is the closest representation to 5 or 16 significant digits, depending on your current format setting. To see the "real" value download James Tursa's FEX submission:
https://www.mathworks.com/matlabcentral/fileexchange/22239-num2strexact-exact-version-of-num2str
Use James Tursa's num2strexact and you will see that none of those values really have the exact value 2.11. All you are looking at is a representation of those floating point numbers displayed in the command window, to the precision defined by your format setting. Just because you see 2.11 displayed tells you nothing about the "real" floating point number's value.
Note that you can change how the value is displayed by changing the format.
You need to learn about the limits of floating point numbers. Start by reading these:
https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero
https://www.mathworks.com/matlabcentral/answers/316889-why-two-equal-numbers-are-not-equal
http://www.mathworks.com/matlabcentral/answers/102419-how-do-i-determine-if-the-error-in-my-answer-is-the-result-of-round-off-error-or-a-bug
http://www.mathworks.com/matlabcentral/answers/140656-about-numerical-difficulty-of-equality-comparison-operation-between-two-double-real-numbers-in-matlab
This is worth reading as well:
https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html
http://faculty.tarleton.edu/agapie/documents/cs_343_arch/papers/1991_Goldberg_FloatingPoint.pdf
Sandeep Nair
Sandeep Nair 2018-9-10
thanks for the answer,i applied it in my code and its running fine

请先登录，再进行评论。

请先登录，再回答此问题。

 采纳的回答

the cyclist
the cyclist 2018-9-10

1 个投票

My best guess here, with the limited information you have given, is that you are trying to compare a number to one that is not exactly equal, due to floating point representation.
Take a look at this question/answer for more details.

3 个评论
显示 1更早的评论 隐藏 1更早的评论

John D'Errico
John D'Errico 2018-9-10
And given the comment made by the OP at the same time you posted this answer, this is exactly the case.
Sandeep Nair
Sandeep Nair 2018-9-10
Hi, I looked at the answer and have already applied these solutions to my code but still its not working. Please note i have set of datasets lets say 0 to 1000 each is getting incremented by 0.01 and i want to look at the indices of value 2.11 ,my code is working perfectly for other values like eg value of 2.12 but at some points,the code is not working properly and am getting 0x1 empty double column vector
Guillaume
Guillaume 2018-9-10
Clearly, you've not read properly the answers (also look at Stephen's comment) or not doing the comparison properly. You need to understand that you cannot use == to compare floating point numbers. It's easy to demonstrate that it does not work:
a = 2.09 + 0.01 + 0.01 %display 2.11 but is not actually 2.11 due to floating point precision
a == 2.11 %return false
a - 2.11 %display about -4.4e-16
tolerance = 1e-6 %appropriate tolerance for the magnitude of numbers being compared
abs(a - 2.11) < tolerance %proper way to compare floating point number

请先登录，再进行评论。

更多回答（0 个）

类别

在 帮助中心 和 File Exchange 中查找有关 Linear Algebra 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by