MATLAB repeat numbers based on a vector of lengths

Question

Is there a vectorised way to do the following? (shown by an example):

input_lengths = [ 1 1 1 4       3     2   1 ]
result =        [ 1 2 3 4 4 4 4 5 5 5 6 6 7 ]         


        

Answer 1

More of a comment than anything, but I did some tests. I tried a for loop, and an arrayfun, and I tested your for loop and arrayfun version. Your for loop was the fastest. I think this is because it is simple, and allows the JIT compilation to do the most optimisation. I am using Matlab, octave might be different.

And the timing:

Solution:     With JIT   Without JIT  
Sam for       0.74       1.22    
Sam arrayfun  2.85       2.85    
My for        0.62       2.57    
My arrayfun   1.27       3.81    
Divakar       0.26       0.28    
Bentoy        0.07       0.06    
Daniel        0.15       0.16
Luis Mendo    0.07       0.06

So Bentoy's code is really fast, and Luis Mendo's is almost exactly the same speed. And I rely on JIT way too much!

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And the code for my attempts

clc,clear
input_lengths = randi(20,[1 10000]);

% My for loop
tic()
C=cumsum(input_lengths);
D=diff(C);
results=zeros(1,C(end));
results(1,1:C(1))=1;
for i=2:length(input_lengths)
    results(1,C(i-1)+1:C(i))=i*ones(1,D(i-1));
end
toc()

tic()
A=arrayfun(@(i) i*ones(1,input_lengths(i)),1:length(input_lengths),'UniformOutput',false);
R=[A{:}];
toc()