Functional form of the colon (:) operator?

This is a post about "how to write neat, efficient, readable MATLAB code". I get the job done by doing work-arounds, but I would rather not do those.

Assume that a, b are two vectors of unknown orientation, and we want to compute something similar to the dot-product.

It might look something like this:

function y = myfunc(a,b)
//make sure that vectors are oriented the same way
y = a(:) .* b(:);

But what if I need to calculate y for elements 2:N? We might do something like this, but I think it clutters the code:

function y = myfunc(a,b)
a = a(2:end)
b = b(2:end)
y = a(:) .* b(:);

I am wishing for a functional form of the colon operator that might look something like this:

function y = myfunc(a,b)
y = colon(a(2:end)) .* colon(b(2:end));

Answer 1

[José-Luis]

function y = myfunc(a,b)
//make sure that vectors are oriented the same way
y = reshape(a(2:end),[],1) .* reshape(b(2:end),[],1);

[Jan]

Nice, short, efficient: I have accepted this answer.

Answer 2

[Daniel Shub]

I thought that squeeze would do what you want, but apparently it doesn't "work" on row vectors. What would be nice is if squeeze had a row flag. Barring that, you can easily create your own colon function. From

type squeeze

it should be obvious what you need to change.

Answer 3

[Royi Avital]

I really wish MATLAB would add function to vectorize arrays into column vector as the colon operator does.

[Steven Lord]

We did, a very long time ago. It's called reshape.

A = magic(5);
B = reshape(A, [], 1);

% or to avoid the need to implicitly compute the "missing" size
% inside reshape
C = reshape(A, numel(A), 1);

[Jan]

@Royi: Do you have an example where the mentioned methods are not applicable or "ugly"?

[Royi Avital]

@Jan, I find something like vec(A) to be much more elegant.

Answer 4

[Jan]

What's wrong with:

y = x(:)

or:

reshape(x, [], 1)

There is a functional form of the x(:) operator:

subsref(x, struct('type', '()', 'subs', {{':'}}))

[Walter Roberson]

Sometimes it is easiest to define an auxillary function, such as

column = @(M) M(:);

after which you can

column(a(2:end)) .* column(b(2:end))