I didn’t even think about the reshape command… :-(

I like your solution. Let me tell you why.

You use a test case that is built right into the code. You could comment it out later so that is available not only for testing, but also for documenting the function.

You do one distinct operation per line. When it comes time to debug, you would be able to watch each step to understand what is going on.

You use the “readable functions” (cat instead of [b; c], you could read your code out loud and understand it better)

You explicitly call out the direction of summing and concatenation so there is no doubt what is going on.

While some of your variable names are not implicitly descriptive, they do show an orderly progression of intermediate results.

Variable names that do deserve descriptive names have them.

This is solid code that anyone, including you in a week, will be able to understand and maintain. I think this is very important.

Thank you,
Doug

 
%% Reshape challenge
a = [1 2 3; 1 2 3; 1 2 3; 3 2 1; 2 1 3; 1 3 2];
check = [1 2 3; 1 2 3; 1 2 3; 3 6 9; 3 2 1; 2 1 3; 1 3 2; 6 6 6];
b = reshape( a', 3, 3, [] ); % Turn into a multidimensional matrix for easy handling
c = sum(b, 2); %Calculate sum of each segment.
d = cat( 2, b, c ); %Stick them together.
e = reshape ( d, 3, [] )'; %Revert to original shape.
% Check that the answer is correct.
verify = ( e == check );
disp ( all(verify(:)) )
 

I would like to take this oppertunity to bring up a large reason for confusion when I work with Matlab. When reshaping matrices, I find it very difficult to remember in what way I need to transpose the data first and then in what order I need to set my size aguments and finally how I have to permute the result to get the data in the format I want. The fact that the ordering of coordinates (row/column or column/row) is not perfectly consistent doesn’t really help. Maybe this is a fundamental difficulty of matrix problems, but I feel that it should be possible to either make a tool that helps or write up a simple non-nonsens cheat-sheet for these kinds of problems.

Just my €0.02.

–DA

 
b = puzzler(a)

[Nrows,Ncols] = size(a);
newrows = floor(Nrows/3);
b = zeros(Nrows+newrows,Ncols);

for n=1:newrows
    b(4*n-3:4*n-1,:) = a(3*n-2:3*n,:);
    b(4*n,:) = sum(a(3*n-2:3*n,:));
end

if (Nrows/3 != newrows)
    b(4*n+1:Nrows+newrows,:) = a(3*newrows+1:Nrows,:);
end

 

 
% reshape the 3xN-by-M array into a 3-by-N*M array
b = reshape(a,3,[]) ;
% concatenate the sum over each 3 elements
b = [b ; sum(b)] ;
% reshape into a (4xN)-by-M array
b = reshape(b,[],size(a,2)) ;
 

or in one line

 
% add proper comments ...
b = reshape([reshape(a,3,[]) ; sum(reshape(a,3,[]))],[],3)
 

Jos