how do I find the locations of values in a matrix?

I want to make a matrix with the location of values in an array. This is what i'm trying to do. The first column of TT is in matlab serial date time.

j = 1;
tol = 0.001;
for i = datenum(2018,1,1,00,00,00):0.020833:datenum(2018,12,4,00,00,00)
    [ii,jj]= find(abs(TT(:,1)-i)< tol);
    I(j,:) = size(ii);
    
switch I(j,1)
    case 0
        MAT(j,:) = nan(1,length(TT(1,:)));
    otherwise
        MAT(j,:) = TT(I(j,1),:);
end
    
    j = j+1;
end

As it is, I'm pretty sure I is just giving me the number of data points with that value but i'd like to have I be the location of the value in TT. For example, if the first value of i is in the 45th row of TT, I'd like I(1,1) to be 45. Is there a way to do that? Thanks in advance!

Answer 1

[Stephen23]

Use ismembertol, something like this (untested, but should get you started):

DN = datenum(2018,1,1,00,00,00):0.020833:datenum(2018,12,4,00,00,00);
[ida,idb] ismembertol(DN,TT(:,1),0.001)

You will probably find this of interest:

idb(ida)

[Susan Santiago]

I tried this and just got a 1x16000 array of 1s. I want to make a matrix where the location of each i values is a row of the matrix.

[Stephen23]

"I tried this and just got a 1x16000 array of 1s."

ida is a logical vector, which you can use as an index into DN:

DN(ida)

idb is a vector of linear indices corresponding to TT, you use this to find the rows of TT that correspond to values in DN:

TT(idb(ida),:)

You might also be interested in swapping the first two inputs to ismembertol: you know what output you need, and are perfectly capable of reading the ismembertol help and experimenting with your data. Experiment!

[Susan Santiago]

yeah but did you read what I'm trying to do? Your suggestion gives me pretty much the same thing I already had.

[Stephen23]

"yeah but did you read what I'm trying to do? Your suggestion gives me pretty much the same thing I already had."

This seems to be your main query:

"For example, if the first value of i is in the 45th row of TT, I'd like I(1,1) to be 45. Is there a way to do that?"

When I look at your code you seem to be attempting to construct a matrix MAT whose rows are taken from TT, and which correspond to the datenumbers that you defined. So that is what I tried to help you with, by showing you a simpler way of doing this than using a loop. Here is a complete working example:

>> DN = [737061;737063;737069]; % i.e. your datenum():30/60/24:datenum() vector.
>> TT = [737060,0;737061,111;737062,222;737063,333] % the input matrix.
TT =
                737060                     0
                737061                   111
                737062                   222
                737063                   333
>> [ida,idb] = ismembertol(DN,TT(:,1)); % Find DN values in first colum of TT.
>> mat = nan(numel(DN),size(TT,2)); % preallocate output matrix.
>> mat(ida,:) = TT(idb(ida),:) % put matched rows of TT into output matrix.
mat =
                737061                   111
                737063                   333
                   NaN                   NaN                   

This is also the advice that you were given in an answer to your earlier question:

https://www.mathworks.com/matlabcentral/answers/430473-trying-to-create-a-loop-that-finds-a-value-in-an-array-and-creates-a-matrix-with-it

Of course, if that is not what you are trying to achieve, then please explain what inputs you have and what outputs you need to get, and how the outputs should be derived from the inputs. It is most useful for us when you explain what you are trying to do, i.e. what your task/goal is.