All Indices of all Numbers in Array

Lets say I have an array that looks something like this:

    A = [3 2 1;...
         5 1 2;...
         1 3 5]

I want to know in which row every number can be found in A my result should look like this:

ind = [1 2 3;...
       1 2;...
       1 3;... 
       0;...
       2 3]

so e.g. 1 can be found in row 1, 2 and 3, 4 can't be found in any row and so on

I'm struggling with linear indexing because I dont search for a specicif value. Solution with a loop works, but is way too computationally intensive for my purposes.

Thanks in advance

[Adam]

I assume the 3rd row of ind should be

1 3

?

[Hannes]

yes, my bad

Answer 1

[Adam]

maxElem = max( A(:) );
res = cell( maxElem, 1 );
for i = 1:max( A(:) )
res{i} = ceil( find( A' == i ) / 3 );
end

gives the correct answer, in a cell array because there aren't the same number of results per row and with a [] instead of 0 where the number does not appear at all.

Not sure if it is doable without a loop though...maybe...

[Hannes]

not exactly what i was hoping for, but i guess there is no better answer.

thanks!

[Adam]

The use of cell arrays makes any code ugly, but they are convenient for operations like this.

The for loop really shouldn't be seen as a problem though. Achieving something like this without a for loop could be possible, but it is not trivial and you have to question what would be the purpose. Vectorisation is always better than a for loop, but some other fancy syntaxes just to avoid a for loop are often not.

Two of the biggest misconceptions from people working with Matlab seem to be at opposite ends of the spectrum:

• I need to do something on a multi-dimensional array, I'll use nested for loops.
• For loops are always bad in Matlab, I should do everything possible to avoid them.

If you can't vectorise a for loop is usually perfectly fine.