Access subset of elements in a cell

I have created the following cell C from reading a txt file.

{["0719-3"    ]}    {["0729-2"    ]}    {["0742-5"    ]}    {["0742-6"    ]}    {["0744-5"     ]}    {["0744-6"     ]}
{84444×4 double}    {84444×4 double}    {84444×4 double}    {84444×4 double}    {84444×4 double}     {84444×4 double}    

I want to switch the sign of the 2nd to 4th colums of each matrix (second row). Is there any way I can access all the submatrix (:,2:4) in the cell? I habe tried C{2,:}(:,2:4) (I was hoping to do something like C{2,:}(:,2:4) = -C{2,:}(:,2:4)but it does not work.

Answer 1

[Andres]

When typing

C{2,:}(:,2:4) 

you are probably getting the error message "Expected one output from a curly brace or dot indexing expression, but there were 6 results" which pretty much explains what is happening. Note that

C{2,1}(:,2:4)

does not error.

You could use cellfun on C(2,:) as follows:

C =  [{"A1"},{"A2"},{"A3"}; repmat({magic(4)},1,3) ];  % sample cell array

C(2,:) = cellfun(@(x) x.*[1,-1,-1,-1], C(2,:), 'UniformOutput', false);

>> C
C =
  2×3 cell array
    {["A1"    ]}    {["A2"    ]}    {["A3"    ]}
    {4×4 double}    {4×4 double}    {4×4 double}

>> C{2,1}
ans =
    16    -2    -3   -13
     5   -11   -10    -8
     9    -7    -6   -12
     4   -14   -15    -1

[Luigi]

what is x for? and UniformOutput?

[Andres]

@(x) x.*[1,-1,-1,-1]

is the anonymous function that is applied to each element of the cell array via cellfun

https://www.mathworks.com/help/matlab/matlab_prog/anonymous-functions.html

x is just the input argument of the anonymous function, so it represents any element of the cell array passed to cellfun, i.e. each of the 84444×4 double arrays in C(2,:) here.

'UniformOutput' is one of the name-value pair arguments for cellfun that has to be set to false if the function applied does not return a scalar. This is the case here as the function returns an N×4 array.

https://www.mathworks.com/help/matlab/ref/cellfun.html

[Luigi]

Thx a lot!