Merging different arrays depending on condition

Hi

I have a following vector called cases as follows:

cases = [1,4,4,3,1,2,2,4,3,4,4,...];

Additionally, I have four arrays Array1 to Array4.

Now I want to construct a new Array, where the elements of this new array are picked from the Array1,...,Array4 depending on what the "cases" array indicates. So for instance, the first element of cases is "1" which means to pick from Array 1 (and I would like to do this in the right order, so I pick the first element of Array1). Then the second element of cases is a "4", so now I want to sample from Array 4, again the first element. The next element of cases is again "4", now I pick the second element of Array4 and so on. How can I do that effectively? The only thing which comes to my mind is to loop with ifelse loops, but that seems rather not so straightforward. Thanks

Answer 1

[Stephen23]

"How can I do that effectively?"

Like always in MATLAB, using indexing is going to be simple and efficient, and storing data in lots of separate matrices just makes code complicated, so the first thing we will do is put all of the data into one cell array, then the solution is simple:

>> cases = [1,4,4,3,1,2,2,4,3,4];
>> C = {101:110,201:210,301:310,401:410};
>> arrayfun(@(c,x)C{c}(x),cases,1:numel(cases))
ans =
   101   402   403   304   105   206   207   408   309   410
>>

If the matrices are the same size then you could also put them into one numeric array, which would make the code a bit more efficient:

>> M = [101:110;201:210;301:310;401:410]
M =
   101   102   103   104   105   106   107   108   109   110
   201   202   203   204   205   206   207   208   209   210
   301   302   303   304   305   306   307   308   309   310
   401   402   403   404   405   406   407   408   409   410
>> M(sub2ind(size(M),cases,1:numel(cases)))
ans =
   101   402   403   304   105   206   207   408   309   410
>>

[MiauMiau]

I am not so sure this is doing the right thing actually. For instance it seems that the first instance of the case "3" is not picking the first instance of 300:310 which would be 300, but the the third instance..also true for the first instance of case 4, which should be 400..hence this code is throwing me an error for my real data..

So the output for your (edited) example:

>> cases = [1,4,4,3,1,2,2,4,3,4]; >> C = {101:110,201:210,301:310,401:410};

should be:

101, 401,402, 301, 102, 201, 202,...

[Stephen23]

"first instance of 300:310 which would be 300"

Have a look at my code. The first instance of the third "array" is 301, not 300. The first instance of the fourth "array" is 401, not 400. I did that to make the counting simpler: e.g. 301, 302, 303, etc, correspond to the first, second, third, etc, elements of the third "array".

It seems that you want to sequentially pick the elements of the vectors, regardless of the position in cases: is this correct? Your original question was ambiguous on this, and your explanation seemed to me that you wanted to pick each based on the same location as in cases. Give me a minute :)

[MiauMiau]

Edit: Yes, sequentially:) Sorry for the ambiguity!

I see that you changed that, but the code is still not doing what I asked for...see above: If the first instance of the fourth array should be now 401, the output still should not be 101 402 403, ... but 101, 401, 402,...

[Stephen23]

Using a for-loop and some simple logical indexing makes for one easy solution:

cases = [1,4,4,3,1,2,2,4,3,4]
mat = [101:110;201:210;301:310;401:410]
vec = unique(cases);
out = NaN(size(cases));
for k = 1:numel(vec)
    idx = cases==vec(k);
    out(idx) = mat(k,1:nnz(idx));
end

Giving:

out =
   101   401   402   301   102   201   202   403   302   404

[MiauMiau]

many thanks. What can I do however if my arrays are not of equal length?

[Stephen23]

Use a cell array:

cases = [1,4,4,3,1,2,2,4,3,4];
C = {101:110;201:210;301:310;401:410};
vec = unique(cases);
out = NaN(size(cases));
for k = 1:numel(vec)
    idx = cases==vec(k);
    out(idx) = C{k}(1:nnz(idx));
end

[MiauMiau]

ah ok, my own written cell-array version just had a bug then. Many thanks!:)

[KL]

Neat! +1