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# find coordinates within a matrix

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AA on 3 May 2020
Commented: Ameer Hamza on 3 May 2020
Suppose I have the following matrices:
b= 0.85981
0.87169
and
a=
0.855750000000000 0.858420000000000 0.858110000000000 0.858210000000000 0.858880000000000
0.857680000000000 0.858700000000000 0.858920000000000 0.858480000000000 0.858630000000000
The first value of "b" (0.8591) is contained in the first row of variable "a" whilst the second value of "b" (0.87169) is found in the second row of variable "a".
My question is how can I find which column number or position "b" has in "a"?

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### Answers (1)

Ameer Hamza on 3 May 2020
Edited: Ameer Hamza on 3 May 2020
the elements of 'b' given in the question are not actually present in the matrix 'a', so I used values in b from 'a'
a = [
0.855750000000000 0.858420000000000 0.858110000000000 0.858210000000000 0.858880000000000
0.857680000000000 0.858700000000000 0.858920000000000 0.858480000000000 0.858630000000000];
b = [0.85811
0.8587];
[~,c] = find(a==b);
Result:
c =
2
3
Note that, if the values are not exactly the same, then you will need to do the comparison with a tolerance value like this
[~,c] = find(abs(a-b)<1e-6); % 1e-6 is tolerance value

#### 2 Comments

Image Analyst on 3 May 2020
Or simply use ismembertol() in recent versions of MATLAB.
Ameer Hamza on 3 May 2020
Tolerance value in ismembertol is always a bit non-intuitive. I always find it easier to write it using abs().

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