MATLAB Central Newsreader - Find function

Find function

Gordon — Sat, 22 Nov 2008 07:24:04 -0500

I’m attempting to run a find command on two vectors A and B, I want to find the number of occurrences in vector a that are <= every occurrence in vector b. I can do this easily with a for loop, but it takes forever to run on large vectors. Here is my code…

y=zeros(length(a),1);
for i = 1:length(a)
a1 = a(i);
y(i) = length(find(b <= a1));
end

a= [1;1;2;3;4;5;7;9;3]
b=[ 1;2;3;4;1;2;3;3;3;4;1;1;2;3;4]
y= [4;4;7;12;15;15;15;15;12]

This works fine with small vectors, but I want to do this with very large vectors. Is there any way to do this in Matlab without using the For Loop?

Any help would be appreciated.

Re: Find function

Johan Carlson — Sat, 22 Nov 2008 08:33:04 -0500

Re: Find function

Roger Stafford — Sat, 22 Nov 2008 09:24:02 -0500

"Gordon " <mloomis@gmail.com> wrote in message <gg8c2k$s8d$1@fred.mathworks.com>...
> I’m attempting to run a find command on two vectors A and B, I want to find the number of occurrences in vector a that are <= every occurrence in vector b. I can do this easily with a for loop, but it takes forever to run on large vectors. Here is my code…
>
> y=zeros(length(a),1);
> for i = 1:length(a)
> a1 = a(i);
> y(i) = length(find(b <= a1));
> end
>
> a= [1;1;2;3;4;5;7;9;3]
> b=[ 1;2;3;4;1;2;3;3;3;4;1;1;2;3;4]
> y= [4;4;7;12;15;15;15;15;12]
>
> This works fine with small vectors, but I want to do this with very large vectors. Is there any way to do this in Matlab without using the For Loop?
>
> Any help would be appreciated.

  Gordon, you have described your problem one way but have computed quite a different quantity. For each element of a you have counted the number of elements in b that it is less than or equal to. This is very different from "the number of occurrences in vector a that are <= every occurrence in vector b". The one is a vector as long as a is and the other is a single scalar number.

  To do what your for-loop does, try this:

[p,p] = sort([a,b]);
t = length(b)-cumsum(p>length(a));
q = 1:length(p); q(p) = q;
y = t(q(1:length(a)));

  To do what you described in words would only require taking the 'min' of b and counting how many a's are less than equal to that minimum. I doubt if that is what you meant.

Roger Stafford

Re: Find function

Roger Stafford — Sat, 22 Nov 2008 09:32:01 -0500

"Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in message <gg8j3i$3ih$1@fred.mathworks.com>...
> ......
> [p,p] = sort([a,b]);
> t = length(b)-cumsum(p>length(a));
> q = 1:length(p); q(p) = q;
> y = t(q(1:length(a)));
> ......

Note: The way I wrote that assumed that a and b are row vectors.

Roger Stafford

Re: Find function

Bruno Luong — Sat, 22 Nov 2008 09:35:02 -0500

Re: Find function

Roger Stafford — Sun, 23 Nov 2008 00:50:17 -0500

"Gordon " <mloomis@gmail.com> wrote in message <gg8c2k$s8d$1@fred.mathworks.com>...
> I’m attempting to run a find command on two vectors A and B, I want to find the number of occurrences in vector a that are <= every occurrence in vector b. I can do this easily with a for loop, but it takes forever to run on large vectors. Here is my code…
>
> y=zeros(length(a),1);
> for i = 1:length(a)
> a1 = a(i);
> y(i) = length(find(b <= a1));
> end
>
> a= [1;1;2;3;4;5;7;9;3]
> b=[ 1;2;3;4;1;2;3;3;3;4;1;1;2;3;4]
> y= [4;4;7;12;15;15;15;15;12]
>
> This works fine with small vectors, but I want to do this with very large vectors. Is there any way to do this in Matlab without using the For Loop?
>
> Any help would be appreciated.

I realized belatedly that the code I sent you has its inequality testing backwards with respect to the for-loop code you gave. Here is the corrected version. It now assumes that a and b are column vectors. For large arrays it should be considerably faster than your for-loop method.

[p,p] = sort([b;a]);
t = cumsum(p<=length(b));
q = (1:length(p)); q(p) = q;
y = t(q(length(b)+1:end));

Roger Stafford

Re: Find function

Gordon — Wed, 26 Nov 2008 00:23:01 -0500

Roger,
Thanks for the help, this solution worked great! Thanks to everyone else who replyed.
ML

"Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in message <gga9c9$l1c$1@fred.mathworks.com>...
> "Gordon " <mloomis@gmail.com> wrote in message <gg8c2k$s8d$1@fred.mathworks.com>...
> > I’m attempting to run a find command on two vectors A and B, I want to find the number of occurrences in vector a that are <= every occurrence in vector b. I can do this easily with a for loop, but it takes forever to run on large vectors. Here is my code…
> >
> > y=zeros(length(a),1);
> > for i = 1:length(a)
> > a1 = a(i);
> > y(i) = length(find(b <= a1));
> > end
> >
> > a= [1;1;2;3;4;5;7;9;3]
> > b=[ 1;2;3;4;1;2;3;3;3;4;1;1;2;3;4]
> > y= [4;4;7;12;15;15;15;15;12]
> >
> > This works fine with small vectors, but I want to do this with very large vectors. Is there any way to do this in Matlab without using the For Loop?
> >
> > Any help would be appreciated.
>
> I realized belatedly that the code I sent you has its inequality testing backwards with respect to the for-loop code you gave. Here is the corrected version. It now assumes that a and b are column vectors. For large arrays it should be considerably faster than your for-loop method.
>
> [p,p] = sort([b;a]);
> t = cumsum(p<=length(b));
> q = (1:length(p)); q(p) = q;
> y = t(q(length(b)+1:end));
>
> Roger Stafford
>