how to find correlation between two columns, selecting rows on the basis of third column

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Muhammad
Muhammad 2012-3-28
Hi
I have three column data say A, B and C and the number of rows be 1000. Column A is either empty or it has values from 1 to 3 randomly. B and C are two variables. Let's say values of B and C in each row is a case and value of A in the same row assigns the category to that case. So there are (1+3)four categories of cases in b and C. I need to find correlation between B and C with in each category.
Any idea how to do that?

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David Young
David Young 2012-3-28
I'm not sure what you mean by column A possibly being empty. Anyway, assuming that it contains the categories, you select the values in a particular category like this:
category = 2; % for example - could be any valid value
k = A == category; % find indexes of elements in this category
Bselected = B(k);
Cselected = C(k);
Now you can use your usual correlation function to find the correlation between Bselected and Cselected. You could put the whole lot in a loop to cover each category in turn.
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Muhammad
Muhammad 2012-3-28
I mean to say that column A has cells which are randomly empty, 1,2 and 3.
In other words, it means that among 1000 rows, some of the cells in column A are empty, some have 1, some 2 or 3 as value.
I hope now it is clear.

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Muhammad
Muhammad 2012-3-28
%For example this the data i have
A B C
1 23 4
2 43 7
1 64 8
43 6
2 65 11
3 46 9
2 76 12
3 76 3
1 32 5
%I need to get following matrix from above matrix A B C 1 23 4 1 64 8 1 32 5 %Then i can find correlation with in category 1 (i.e when A=1)
%similarly, i need to extract another matrix from the top matrix as;
A B C
2 43 7
2 65 11
2 76 12
% and then calculate the correlation between B and C.
% i hope this makes it clear
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David Young
David Young 2012-3-28
See above. Is A a cell array?
The code in my answer above gives you exactly what you are asking for, when the category has a number.
If this still doesn't help, perhaps you can show the code that assigns values to the variables A, B and C. (They are variables in MATLAB, aren't they?)

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