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Functional form of the colon (:) operator?

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Knut
Knut on 18 Sep 2012
Commented: Royi Avital on 15 Jan 2019
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));

Accepted Answer

José-Luis
José-Luis on 18 Sep 2012
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);
  1 Comment
Jan
Jan on 10 Oct 2017
Nice, short, efficient: I have accepted this answer.

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More Answers (3)

Daniel Shub
Daniel Shub on 18 Sep 2012
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.

Royi Avital
Royi Avital on 10 Oct 2017
I really wish MATLAB would add function to vectorize arrays into column vector as the colon operator does.
  3 Comments
Royi Avital
Royi Avital on 15 Jan 2019
@Jan, I find something like vec(A) to be much more elegant.

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Jan
Jan on 10 Oct 2017
What's wrong with:
y = x(:)
or:
reshape(x, [], 1)
There is a functional form of the x(:) operator:
subsref(x, struct('type', '()', 'subs', {{':'}}))
  1 Comment
Walter Roberson
Walter Roberson on 17 Oct 2018
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))

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