Question

I have matrix

p=[1 2 3 4; 
   5 6 7 8; 
  10 20 30 50];

I’d like to compute the sum of the elements of each row that are between column iMin and iMax, with iMin and iMax being different for each row

e.g for

iMin = [3 2 1];
iMax = [4 4 3];

the result is

[7 21 60]

Is there an easy Octave / Matlab way to do this without loops ?

Answer 1

You can achieve this using logical indexing, following a similar approach to my answer to your other question.

Assuming that iMin and iMax have the same number of entries as the number of rows in p you can horizonally tile the column indices for p i.e. [1:size(p,2)] and compare this to vertical tilings of iMin and iMax to generate a logical index into p for the entries that satisfy your criterion so:

c_min=repmat(iMin',1,size(p,2))
c_max=repmat(iMax',1,size(p,2))
c_ind=repmat([1:size(p,2)],size(p,1),1)
result=sum(p.*(c_ind>=c_min & c_ind<=c_max),2)

Giving:

result =

   7
  21
  60

No loops 🙂