Question

Say I have an array of values:

values = 1:100;

an array of indices:

interval_indices = [40 45 80];

and an array of quantities that I would like to add to the elements in values:

quantities_to_add = [5 -9 30];

I am looking for a compact expression in MATLAB (maybe using accumarray?) that allows me to add the elmements of quantities_to_add to the elements in values depending on the indices specified by indices.

If I were to do this manually:

values(1:interval_indices(1))                         = values(1:interval_indices(1)) + ...
                                                         quantities_to_add(1);
values(interval_indices(1):interval_indices(2))       = values(interval_indices(1):interval_indices(2)) + ... 
                                                         quantities_to_add(2);
% and so forth

values(interval_indices(end-1):interval_indices(end)) = values(interval_indices(end-1):interval_indices(end)) + ...
                                                         quantities_to_add(end);

Answer 1

EDIT:

Actually, this is a much smarter and vectorized way of doing the same:

lastIndx=interval_indices(end);   
quantitiesVector=zeros(1,lastIndx);
quantitiesVector([1,interval_indices(1:end-1)+1])=[quantities_to_add(1) diff(quantities_to_add)];

newValues=[values(1:lastIndx)+cumsum(quantitiesVector),values(lastIndx+1:end)];

---

Previous answer:

One way of doing it is using arrayfun to generate a vector of quantities and then adding.

intervalLength=diff([0 indices]);
lastIndx=interval_indices(end);
quantities=cell2mat(arrayfun(@(x)ones(1,intervalLength(x))*quantities_to_add(x),1:numel(indices),...
    'UniformOutput',false));

newValues=[values(1:lastIndx)+quantities, values(lastIndx+1:end)];