Finding small vector in big vector

Hey everybody,

I am desperately looking for a function that can find a small vector inside a big vector. Ismember and interesect just wouldn't do it right :(

Let's say I have the two vectors x = [ 7 6 9 7 4 3 7 9 0 7 4 3 2 6 7 0 7 5 ]; y = [ 4 3 2 6 ]

As a result I would like to have ans = [ 11 12 13 14 ]

so only the coordinates of where the values are in the correct sequence. Is there anything that can do that?

Thanks in advance!

Answer 1

[Image Analyst]

Try this:

x = [ 7 6 9 7 4 3 7 9 0 7 4 3 2 6 7 0 7 5 ];
y = [ 4 3 2 6 ];
% As a result I would like to have ans = [ 11 12 13 14 ]
out = strfind(x, y) + [0:length(y)-1]

[Lennart]

Wohoo awesome. Thank you guys so much! That just works perfectly. Actually the strfind itself is just what I need, but the idea of "+[0 : length(y)-1] " is quite needed as well.

[Image Analyst]

You're welcome. strfind(x, y) finds only 11 - the start of the sequence - while adding that other vector gives you [11,12,13,14] which is what you said you wanted.

Answer 2

[James Tursa]

I am not on a machine with MATLAB at the moment to test this, but maybe try the strfind(x,y) function to see if it works with doubles (even though the doc doesn't indicate that it does).

[Alexander]

It does. The numbers are internally casted to char. For compatibility with other programs you can use strfind(char(x),char(y)).

[Bruno Luong]

@Alexander Paul "The numbers are internally casted to char."

I don't think so, if it was, the result of the two last commands would be indentiical

d = double('a')+0.1;
a = 'a';
strfind(d,a)

ans =

     []

strfind(char(d),char(a))

ans = 1

[Alexander]

You are right. I checked it only with uint8.

Answer 3

[Matlab Pro]

Hi

A small improvement where the sub-vector is found more than once...

x = x(:)'; % Make sure data is a Row vector

ix = strfind(x, y);

if length(ix)>1 % Fix cases where y is found more than once

idx = [1:length(y)] + ix(1)-1; % Indexes of 1st occurance

else

idx = strfind(x, y) + [0:length(y)-1];

end