MATLAB: How to generate a single vector of block-consecutive values from 2 vectors of same size without a loop

Question

Hello, I have 2 vectors of same size let's say a = [3 7 19 22] and b = [5 10 20 24]

And from those 2 vectors I want to generate without a loop a single vector:

v = [a(1):b(1) a(2):b(2) … a(end):b(end)].

so here v=[3 4 5 7 8 9 10 19 20 22 23 24].

I found linspaceNDim(a,b,N) on File Exchange Linearly spaced multidimensional matrix without loop but it doesn't work here as I dont want a fixed number of values between a(i) and b(i), but only block-consecutive values : a(i):b(i). So here N is changing at each index i.

I tried a:.b but this syntax doesn't work on Matlab.

Thanks for you help.

Answer 1

Any particular reason you don't want to use a loop? This is one of those cases where you should use one, if performance is an issue. Arrayfun() is just syntactic sugar around a loop. Plus the added overheads of invoking cell arrays:

a = [3 7 19 22];
b = [5 10 20 24];
tic
your_mat = cell2mat(arrayfun(@(x,y) x:y,a,b,'uniformoutput',false));
toc 
tic
   numEl = b - a;
   pos = 1;
   your_data = NaN * ones(sum(numEl),1);
   for ii = 1:numel(numEl)
       your_data(pos:pos+numEl(ii)) = a(ii):b(ii);
       pos = pos + numEl(ii) + 1;
   end
toc
 Elapsed time is 0.000602 seconds.
 Elapsed time is 0.000038 seconds.

EDIT

I had made a mistake, the pre-allocation should read:

 your_data = NaN * ones(sum(numEl) + numel(numEl),1);

EDIT

A bit cleaner:

vals = reshape(sort(randperm(10^6,1000)),2,[]);
a = vals(1,:);
b = vals(2,:);
tic
   offset = b - a;
   numVal = sum(offset + 1);
   pos = 1;
   your_data = ones(numVal,1);
   for ii = 1:numel(offset)
       your_data(pos:pos+offset(ii)) = a(ii):b(ii);
       pos = pos + offset(ii) + 1;
   end
toc