Thread Subject:

return indices

Hi,

I have a matrix :

F =
                    0.6892 134.5343 25.5343
                    23.108 130 11
                     3.31 108 25
                    55.45 77 32
                  136.7508 6 2

I am given label of the F matrix for each of the 5 values in each of the 3 columns as follows:

I =
                                     2 1 1
                                     3 3 2
                                     4 4 4
                                     5 5 5
                                     6 6 6

For each column and its corresponding row value how can I get the row number and column number index for values <4 ?

The answer I am working towards should be:

[1 4;
 3 6]

where 1 , 3 are representative of the indices of the identified columns with the value<4 and 4 , 6 are the corresponding indices.

Please help me out guys.

thanks

  You seem to have omitted the value 0.6892 which is also less than 4. Assuming you want that included, do this:

 [r,c] = find(F < 4);
 R = [c,I(sub2ind(size(I),r,c))];

Roger Stafford

thank you roger

Well I got this far:
F = [0.6892 134.5343 25.5343;...
  23.108 130 11;...
 3.31 108 25;...
  55.45 77 32;...
136.7508 6 2]
I =[2 1 1;...
    3 3 2;...
    4 4 4;...
    5 5 5;...
    6 6 6]
% Find values in F less than 4
[rows cols] = find(F < 4)
% Convert from row, col coordinates into linear indices.
linearIndices = sub2ind(size(F), rows, cols)
% Just to check, let's see what we found...
extractedFValues = F(linearIndices) % Just to verify...
% Get the indices from the I matrix
extractedIValues = I(linearIndices)

rows =
     1
     3
     5
cols =
     1
     1
     3
linearIndices =
     1
     3
    15
extractedFValues =
    0.6892
    3.3100
    2.0000
extractedIValues =
     2
     4
     6

but your description fails in general because some columns may have different number of numbers that are less than 4, so your "result" array might have 2 numbers in one row, but 3 in another, and 6 in another, for the general case of larger matrices and values that can be anything. Also, I'm not sure why you found 2 values less than 4 and I found 3 but I'm not sure how to put them into your results output array unless I switch to a cell array. I'd put 2 into one row and 1 into the other row.

Looks like Roger found the same inconsistency as me, and solved it by putting the additional stuff on an additional row. I suggest you go with his code - it is essentially what I was going after (until I hit the inconsistency), mine is just a bit more verbose while his is more compact.

Roger,

The find function works well and outputs 'r' and 'c' in [r,c] as a single column when the F matrix is large as the original F matrix.

So in the example I posted,

>> [r,c] = find(F < 4)

r =

     1
     3
     5


c =

     1
     1
     3

However , when the F matrix is small, specifically in the form of the below example, 'r' and 'c' outputs into


>>F = [1 0.2]

     I= [4 5]

>>[r,c] = find(F < 4)

r =

     1 1


c =

     1 2


This directly messes up my output when I use the code you helped me out with,i.e.
R = [c,I(sub2ind(size(I),r,c))];

It wrongly outputs as:

R =

    1 2 4 5

Instead it should be

R = [ 1 4;
        2 5]

I've been trying to sort it out for a while and cant seem how to fix this.My code is an iterative one , where the F matrix gets smaller , this is how I recognised this problem.

Need some help please.

Thanks again
ravi

Roger,

The find function works well and outputs 'r' and 'c' in [r,c] as a single column when the F matrix is large as the original F matrix.

So in the example I posted,

>> [r,c] = find(F < 4)

r =

     1
     3
     5


c =

     1
     1
     3

However , when the F matrix is small, specifically in the form of the below example, 'r' and 'c' outputs into


>>F = [1 0.2]

     I= [4 5]

>>[r,c] = find(F < 4)

r =

     1 1


c =

     1 2


This directly messes up my output when I use the code you helped me out with,i.e.
R = [c,I(sub2ind(size(I),r,c))];

It wrongly outputs as:

R =

    1 2 4 5

Instead it should be

R = [ 1 4;
        2 5]

I've been trying to sort it out for a while and cant seem how to fix this.My code is an iterative one , where the F matrix gets smaller , this is how I recognised this problem.

Need some help please.

Thanks again
ravi

"ravi " <ravi_071@hotmail.com> wrote in message <i8tkmn$9m4$1@fred.mathworks.com>...
> .......
> It wrongly outputs as:
>
> R =
> 1 2 4 5
>
> Instead it should be
>
> R = [ 1 4;
> 2 5]
> .......
- - - - - - - - - -
  Yes, if you are going to use vectors rather than two-dimensional arrays for F and I, you will have to force those two quantities to be column vectors:

 [r,c] = find(F < 4);
 t = I(sub2ind(size(I),r,c));
 R = [c(:),t(:)];

Roger Stafford