using sum() with a vector and increment

Hi peeps, sorry about this rather simple question; I'm very new to this and cannot wrap my head around it. Dummy-Code:

A= [1 2 3 4 5; 6 7 8 9 10; 11 12 13 14 15]; %create 3x5 matrix
a = [2 3 4]; %row vector
b = A(1,:);  %extract row 1 from Matrix A
c= b(a+1);   %create vector from positions+1 defined with a
d=[1 c];     %create vector with 1 and positions

dsum=sum(A(1,d:d+1),1); %??????

Why does d:d+1 give me the first two positions of row 1 (and 2 and 3 if I change it so A(2,d:d+1))? I understand that : can be used to create vectors with certain increments, but it simply makes no sense to me. I'm sure there is a logical explanation. Anybody? Help is greatly appreciated.

Answer 1

[Jan]

The colon operator ":" needs scalars as input. Then operation "vector:vector" is not defined. Unfortunately Matlab does not stop with an error, if you provide vectors as input, but only the first element is used.

v = [7,8,9]
v:v+1   % This is the same as:  7:7+1
        % The rest of v is ignored

In your case only the first element 1 is used, such that "d:d+1" is the same as "1:2".

This behavior is a typical source of unexpected behavior in:

for k = 1:size(v)

when size(v,2) or numel(v) is meant.

[Guillaume]

Then operation "vector:vector" is not defined

I disagree, it is very well defined in that it is explicitly documented:

If you specify nonscalar arrays, then MATLAB interprets j:i:k as j(1):i(1):k(1).

However, yes, it would probably make more sense if matlab simply errored.

Answer 2

[Isabella Kratzer]

Thank you both Jan and Guillaume!