Vectorize Matlab sum

Question

I have the following Matlab function:

function [res] = a3_funct(x)
    res = 0;
    for i = 1:size(x,1)
        res = res + abs(x(i))^(i+1);
    end
end

It's emulating this equation:

\$f(x) = \sum_{i=1}^{n}|x_i|^{i+1}\$

Here is an example use:

>> a3_funct([1,2,3]')

ans =

    90

I know I should be able to use the sum() function to make this faster, but how do I get the exponents in there?

In Revs 1–4, your equation was inconsistent with your code. Your code actually does $$f(x) = \sum_{i=1}^{n}\left|x_i\right|^i$$ if you use one-based arrays, according to MATLAB convention, or $$f(x) = \sum_{i=0}^{n-1}\left|x_i\right|^{i + 1}$$ if you use the zero-based array indexing convention. — 200_success♦, Jun 29 '14 at 22:25

200_success · Accepted Answer · 2014-06-29 23:01:47Z

up vote 6 down vote accepted

Vectorized approach:

res = sum(abs(x(:).'.^(2:numel(x)+1)))

Read more about vectorization techniques here.

Thus, your function would look like this:

function res = a3_funct(x)
    res = sum(abs(x(:).'.^(2:numel(x)+1)));
return

200_success♦
74.3k787279

answered Jun 25 '14 at 16:45

Divakar
26628

Feel free to flag a question for moderator attention if the code in a question doesn't work as advertised. – 200_success♦ Jun 29 '14 at 23:02

@200_success Thank you! Would keep that in mind. – Divakar Jun 30 '14 at 3:48

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