Quick question on understanding ./ notation

Greetings all,

I have the following "snippet" of code:

Zeta_EM31_West = DrillHoleData2(:,3)/3.67;
%Line above outputs a 16x1 double array, I can provide "DrillHoleData2" if needed
Response_values_H= 0:0.01:9;
R_h=sqrt(4*Response_values_H.^2+1)-(2*Response_values_H);
R_h(Response_values_H>=9.1) = 0;
R_h_Case1A=interp1(Response_values_H,R_h,Zeta_EM31_West);

Now I'm a bit confused on something. Say the next line is this: sigma_2_Case1A=EM31CondMidSwathWest/R_h_Case1A;

where "EM31CondMidSwathWest" is another 16x1 array (and I can provide the numbers if needed). Now just using / will provide me with a 16x16 double, which I understand, but in one column I'll receive values, and the rest are zeros (I also understand this).

What I don't understand is if I say: sigma_2_Case1A=EM31CondMidSwathWest./R_h_Case1A;

which will be a 16x1 double, but the numbers will be different than those with using "/". Shouldn't they be the same?

Thanks! -J

[Stephen23]

This explains everything:

http://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

Answer 1

[John D'Errico]

If they should be the same for / and ./ I imagine they would not give us TWO different operators?

./ does an element-wise division. It divides each element by the corresponding element, as long as the two vectors/arrays are the same size and shape. It will fail if they are not, unless one of them was a scalar.

/ does the element-wise division IF the denominator is a scalar. If the two arguments are arrays, or vectors, then linear algebra comes into play.

B/A is equivalent to B*inv(A), WHEN A is a square matrix. If A is not a square matrix, then a solve is done that employs a factorization of the matrix A.

READ THE HELP. Of course, I admit that it might not be obvious to look at these:

help slash
help rdivide
help ldivide
help mrdivide

After all, I might have naively looked for help under divide. I honestly think it would be a good idea if this:

help divide

produced links to the appropriate functions, and explained the basic differences between all of those operators.

[Jesse]

That's what I did too at first (help divide) - thanks for pointing the others out. I just thought it was ./ in the help.

[Jesse]

John, thanks for your help. Apologies for asking such a basic question, but stepping away from my code for a couple of hours gave me an "a-ha" moment. Sometimes we all need that!

[Stephen23]

Although the cyclist has already given this link, it deserves a bit more prominence:

http://www.mathworks.com/help/matlab/matlab_prog/array-vs-matrix-operations.html

Answer 2

[the cyclist]

I'm not as confident as you are that you understand the difference between the rdivide (./) and mrdivide (/). In particular, the latter command is for solving systems of linear equations, not for simple division.

I suggest you read those documentation pages carefully.

You can read more generally about array vs. matrix operations here.