MATLAB: Starting with a vector of starting points and a vector of end points, make runs of consecutive numbers between these points

Question

This is probably simple but here is my problem.

I have two vectors, starts and ends. Starts are the starting points of sequences of consecutive numbers and ends are the end points of sequences of consecutive numbers. I would like to create a vector which contains these runs.

So for example, say

starts = [2 7 10 18 24]  
ends = [5 8 15 20 30]

I would like to create the following vector

ans = [2 3 4 5 7 8 10 11 12 13 14 15 18 19 20 24 25 26 27 28 29 30]

Using starts:end only uses the first element of each vector

I would also like to do this without using a (for) loop in order to keep it as fast as possible!

Thanks for reading

Chris

score 2 · Accepted Answer · answered Mar 11 '13 at 23:30

2

Assuming there's always the same number of start and end points, and they always match (e.g. the nth start corresponds to the nth end), then you can do

cell2mat(arrayfun(@(s,e) (s:e), starts, ends, 'UniformOutput', false))

For a bit more detailed explanation, the arrayfun(@(s,e) (s:e), starts, ends, 'UniformOutput', false) part will generate a sequence of n cell arrays, where n is the length of the starts and ends vectors, such that each cell array has the sequence starts(i):ends(i) corresponding to the ith elements of the two vectors. Then the cell2mat function will fuse each of the individual cell arrays into 1 larger matrix.

answered Mar 11 '13 at 23:30

alrikai

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But is it actually faster than a `for` loop? – Ben Voigt Mar 11 '13 at 23:48
1

Yes, it seems like this is lots faster, at least for the problem size shown. – Ben Voigt Mar 11 '13 at 23:56
1

+1: By the way, you can use `@colon` instead of declaring an anonymous function wrapper `@(s, e)(s:e)`. – Eitan T Mar 12 '13 at 00:37
@EitanT very cool, I didn't know about that – alrikai Mar 12 '13 at 00:41

score 0 · Answer 2 · answered Mar 11 '13 at 23:52

When you're worried about making it fast, preallocate:

starts = [2 7 10 18 24]  
ends = [5 8 15 20 30]
a = zeros(1,sum(ends)+numel(ends)-sum(starts));
% or a = zeros(1,sum(ends+1-starts))
j = 1;
for i = 1:numel(ends)
    j2 = j+ends(i)-starts(i);
    a(j:j2) = (starts(i):ends(i));
    j = j2+1;
end

MATLAB: Starting with a vector of starting points and a vector of end points, make runs of consecutive numbers between these points

2 Answers2