Matlab large rows in Anonymous Functions

Question

In the Matlab Anonymous Functions, I want to have Functions fun_total like this

fun_total = @(x) [ 0;
               1*x(1);
               1*x(1);
               2*x(2);
               2*x(2);
               ...
               100000*x(100000);
               100000*x(100000);]

so here is my code

fun_total = @(x) [0];
for i = 1 : 100000
  fun_temp = @(x) i*x(i);
  fun_total = @(x) [ fun_total(x); fun_temp(x) ];
  fun_total = @(x) [ fun_total(x); fun_temp(x) ];
end

My problem is it's too slow when loop iteration become bigger. Every time fun_total = @(x) [ fun_total(x); fun_temp(x) ];
The fun_total(x) will expand first ,and then merge.

Now I have a solution is output my fun_total as a text file, and then change to function. Is this can work? or someone have other efficient solution? Thanks!!

knedlsepp · Accepted Answer · 2015-02-04 12:57:39Z

3

The problem obviously is, that you are generating 100000 anonymous functions fun_temp (and 100000 nested functions fun_total). This is essentially the same as generating all the m-file-functions function result=xTimes1(x), function result=xTimes2(x), function result=xTimes3(x),... Which when you look at it that way is totally absurd. The best solution is certainly going with a vectorized solution like Dan's, but you can also always consider using a single m-function file.

In terms of efficiency you should expect this hierarchy:

Lots of (anonymous) function calls < lots of for-loop iterations < vectorized code.

So as an intermediate step to the vectorized solution you could use:

function total = fun_total(x)
total = zeros(2*length(x)+1,1);
for i = 1:length(x)
    total([2*i,2*i+1]) = [i*x(i); i*x(i)];
end

and then generate the function handle using @fun_total.

edited Feb 4, 2015 at 12:57

answered Feb 4, 2015 at 11:00

knedlsepp

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1 Comment

Luis Mendo Over a year ago

specially for the hierarchy: +1

Dan · Accepted Answer · 2015-02-04 11:04:48Z

2

Maybe try:

fun_total = @(x)reshape(repmat(1:numel(x).*x, 2, 1),[],1)

or

fun_total = @(x)reshape([1;1]*(1:numel(x).*x),[],1)

edited Feb 4, 2015 at 11:04

answered Feb 4, 2015 at 9:47

Dan

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