How to get a value from a matrix?

Question

Danny Helwegen 2019-1-12

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https://ww2.mathworks.cn/matlabcentral/answers/439475-how-to-get-a-value-from-a-matrix

评论： Danny Helwegen 2019-1-12

Hi guys, I have a question that on the first glance seems really simple. I have a matrix of 2 columns and I have a value of the first column and want to know the corresponding value from the second column, For instance I have the following matrix:

Data = 
1  0.50
15 0.72
2  0.37
25 0.18
3  0.65

I have the first value, 0.25 and now i want to know how to get the 0.18 from the matrix. The matrix of my problem has much more rows, but it's the same principle. I already written a code that works when I have 100 rows, but when I have 200 rows it doesn't work anymore (data of the second and first column also depend on the number of rows). This is the code I already wrote:

Uva = [x; T_plot]'             %This is my matrix
ind1 = Uva(:,1) == 0.591;      %Look for the right x
A1 = Uva(ind1,:);              %Pick the row 
Tsim = A1(2);                  %Assign the outcome to a new name

Can someone please help me?

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Answer 1

Stephen23 2019-1-12

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https://ww2.mathworks.cn/matlabcentral/answers/439475-how-to-get-a-value-from-a-matrix#answer_356124

编辑：Stephen23 2019-1-12

You should never compare for exact equivalence of floating point numbers, read this to know why:

https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

Always compare the absolute difference of values against a tolerance, like this:

>> data = [0.1,0.50;0.15,0.72;0.2,0.37;0.25,0.18;0.3,0.65]
data =
   0.10000   0.50000
   0.15000   0.72000
   0.20000   0.37000
   0.25000   0.18000
   0.30000   0.65000
>> val = 0.15;
>> idx = abs(data(:,1)-val)<1e-5;
>> out = data(idx,2)
out =  0.72000