Here is a fast implementation using binary search. This file is also available on github
function [b,c]=findInSorted(x,range)
%findInSorted fast binary search replacement for ismember(A,B) for the
%special case where the first input argument is sorted.
%
% [a,b] = findInSorted(x,s) returns the range which is equal to s.
% r=a:b and r=find(x == s) produce the same result
%
% [a,b] = findInSorted(x,[from,to]) returns the range which is between from and to
% r=a:b and r=find(x >= from & x <= to) return the same result
%
% For any sorted list x you can replace
% [lia] = ismember(x,from:to)
% with
% [a,b] = findInSorted(x,[from,to])
% lia=a:b
%
% Examples:
%
% x = 1:99
% s = 42
% r1 = find(x == s)
% [a,b] = myFind(x,s)
% r2 = a:b
% %r1 and r2 are equal
%
% See also FIND, ISMEMBER.
%
% Author Daniel Roeske <danielroeske.de>
A=range(1);
B=range(end);
a=1;
b=numel(x);
c=1;
d=numel(x);
if A<=x(1)
b=a;
end
if B>=x(end)
c=d;
end
while (a+1<b)
lw=(floor((a+b)/2));
if (x(lw)<A)
a=lw;
else
b=lw;
end
end
while (c+1<d)
lw=(floor((c+d)/2));
if (x(lw)<=B)
c=lw;
else
d=lw;
end
end
end