Sum of row and column #?

Say I ask the user to input the size of a square matrices, n.

And say I want each element of this matrix to be the sum of the row # it's in & the column # it's in. So, for n =2:

2 3
3 4

Is there some Matlab function I could use to calculate each of those elements and display the above as a matrix?

Answer 1

[Matt J]

result=(1:n).'+(1:n);

or for older MATLAB versions,

result = bsxfun(@plus,(1:n).',(1:n));

Answer 2

[KSSV]

n = 2 ;
x = 1:n ;

row = repmat(x',1,n) ;
col = repmat(x,n,1) ;
iwant = row+col

Or

x = 1:2*n ;
ind = reshape(x,n,n) ;
[I,J] = ind2sub([n n],ind) ;
iwant = I+J

[Rachel Dawn]

hi! Thank you very much. Could you elaborate on what each line of your code is actually doing/calculating? Sorry, I'm new to Matlab and unfamiliar with all those terms. I tried looking them up but Matlab instructions are quite complicated.

Answer 3

[John D'Errico]

Matt's answer of

result=(1:n).'+(1:n);

is the best way, at least in current versions of MATLAB, because it is most clear what it is doing. But this only applies in 2017 releases and beyond.

Other solutions are less clear, but pretty simple. For example, this should be almost as obvious:

[i,j] = meshgrid(1:n);
result = i + j;

Or

result = cumsum(ones(n),1) + cumsum(ones(n),2);

Or

result = repmat(1:n,n,1) + repmat(1:n,n,1).';

Any of these should be quite clear, and all are equally valid. If unclear still, then it really is time to start reading the getting started tutorials in MATLAB. If you do not follow what they did, then look at each piece of the code. See what it does for some value of n. Read the documentation, perhaps for meshgrid, repmat, bsxfun, cumsum, etc., as appropriate. You won't learn MATLAB until you start reading the documentation.

Answer 4

[Matt J]

Too bad hankel() isn't MEX-implemented. A MEX-optimized implementation of the following would be O(n),

 result = hankel( 2:n+1,n+1:2*n)