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

A fully vectorized version:

selector=bsxfun(@le,[1:max(input_lengths)]',input_lengths);
V=repmat([1:size(selector,2)],size(selector,1),1);
result=V(selector);

Downside is, the memory usage is O(numel(input_lengths)*max(input_lengths))

Answer 2

I search an elegant solution, and I think David's solution is a good start. What I have in mind is that one can generate the indexes where to add one from previous element.

For that, if we compute the cumsum of the input vector, we get:

cumsum(input_lengths)
ans = 1     2     3     7    10    12    13

This is the indexes of the ends of sequences of identical numbers. That is not what we want, so we flip the vector twice to get the beginnings:

fliplr(sum(input_lengths)+1-cumsum(fliplr(input_lengths)))
ans = 1     2     3     4     8    11    13

Here is the trick. You flip the vector, cumsum it to get the ends of the flipped vector, and then flip back; but you must substract the vector from the total length of the output vector (+1 because index starts at 1) because cumsum applies on the flipped vector.

Once you have done this, it's very straightforward, you just have to put 1 at computed indexes and 0 elsewhere, and cumsum it:

idx_begs = fliplr(sum(input_lengths)+1-cumsum(fliplr(input_lengths)));
result = zeros(1,sum(input_lengths));
result(idx_begs) = 1;
result = cumsum(result);

EDIT

First, please have a look at Luis Mendo's solution, it is very close to mine but is more simpler and a bit faster (I won't edit mine even it is very close). I think at this date this is the fastest solution from all.

Second, while looking at others solutions, I've made up another one-liner, a little different from my initial solution and from the other one-liner. Ok, this won't be very readable, so take a breath:

result = cumsum( full(sparse(cumsum([1,input_lengths(1:end-1)]), ...
ones(1,length(input_lengths)), 1, sum(input_lengths),1)) );

I cut it on two lines. Ok now let's explain it.

The similar part is to build the array of the indexes where to increment the value of the current element. I use the solution of Luis Mendo's for that. To build in one line the solution vector, I use here the fact that it is in fact a sparse representation of the binary vector, the one we will cumsum at the very end. This sparse vector is build using our computed index vector as x positions, a vector of 1 as y positions, and 1 as the value to put at these locations. A fourth argument is given to precise the total size of the vector (important if the last element of input_lengths is not 1). Then we get the full representation of this sparse vector (else the result is a sparse vector with no empty element) and we can cumsum.

There is no use of this solution other than to give another solution to this problem. A benchmark can show that it is slower than my original solution, because of a heavier memory load.

Answer 3

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!

---

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()

Answer 4

This is a slight variant of @Daniel's answer. The crux of this solution is based on that solution. Now this one avoids repmat, so in that way it's little-more "vectorized" maybe. Here's the code -

selector=bsxfun(@le,[1:max(input_lengths)]',input_lengths); %//'
V = bsxfun(@times,selector,1:numel(input_lengths)); 
result = V(V~=0)

For all the desperate one-liner searching people -

result = nonzeros(bsxfun(@times,bsxfun(@le,[1:max(input_lengths)]',input_lengths),1:numel(input_lengths)))

Answer 5

Benchmark of all solutions

Following the previous benchmark, I group all solutions given here in a script and run it a few hours for a benchmark. I've done this because I think it's good to see what is the performance of each proposed solution with the input lenght as parameter - my intention is not here to put down the quality of the previous one, which gives additional information about the effect of JIT. Moreover, and every participant seems to agree with that, quite a good work was done in all answers, so this great post deserves a conclusion post.

I won't post the code of the script here, this is quite long and very uninteresting. The procedure of the benchmark is to run each solution for a set of different lengths of input vectors: 10, 20, 50, 100, 200, 500, 1000, 2000, 5000, 10000, 20000, 50000, 100000, 200000, 500000, 1000000. For each input length, I've generated a random input vector based on Poisson law with parameter 0.8 (to avoid big values):

input_lengths = round(-log(1-rand(1,ILen(i)))/poisson_alpha)+1;

Finally, I average the computation times over 100 runs per input length.

I've run the script on my laptop computer (core I7) with Matlab R2013b; JIT is activated.

And here are the plotted results (sorry, color lines), in a log-log scale (x-axis: input length; y-axis: computation time in seconds):

So Luis Mendo is the clear winner, congrats!

For anyone who wants the numerical results and/or wants to replot them, here they are (cut the table into 2 parts and approximated to 3 digits, for a better display):

N                   10          20          50          100         200         500         1e+03       2e+03
-------------------------------------------------------------------------------------------------------------
OP's for-loop       8.02e-05    0.000133    0.00029     0.00036     0.000581    0.00137     0.00248     0.00542 
OP's arrayfun       0.00072     0.00117     0.00255     0.00326     0.00514     0.0124      0.0222      0.047
Daniel              0.000132    0.000132    0.000148    0.000118    0.000126    0.000325    0.000397    0.000651
Divakar             0.00012     0.000114    0.000132    0.000106    0.000115    0.000292    0.000367    0.000641
David's for-loop    9.15e-05    0.000149    0.000322    0.00041     0.000654    0.00157     0.00275     0.00622
David's arrayfun    0.00052     0.000761    0.00152     0.00188     0.0029      0.00689     0.0122      0.0272
Luis Mendo          4.15e-05    4.37e-05    4.66e-05    3.49e-05    3.36e-05    4.37e-05    5.87e-05    0.000108
Bentoy13's cumsum   0.000104    0.000107    0.000111    7.9e-05     7.19e-05    8.69e-05    0.000102    0.000165
Bentoy13's sparse   8.9e-05     8.82e-05    9.23e-05    6.78e-05    6.44e-05    8.61e-05    0.000114    0.0002
Luis Mendo's optim. 3.99e-05    3.96e-05    4.08e-05    4.3e-05     4.61e-05    5.86e-05    7.66e-05    0.000111

N                   5e+03       1e+04       2e+04       5e+04       1e+05       2e+05       5e+05       1e+06
-------------------------------------------------------------------------------------------------------------
OP's for-loop       0.0138      0.0278      0.0588      0.16        0.264       0.525       1.35        2.73
OP's arrayfun       0.118       0.239       0.533       1.46        2.42        4.83        12.2        24.8
Daniel              0.00105     0.0021      0.00461     0.0138      0.0242      0.0504      0.126       0.264
Divakar             0.00127     0.00284     0.00655     0.0203      0.0335      0.0684      0.185       0.396
David's for-loop    0.015       0.0286      0.065       0.175       0.3         0.605       1.56        3.16
David's arrayfun    0.0668      0.129       0.299       0.803       1.33        2.64        6.76        13.6
Luis Mendo          0.000236    0.000446    0.000863    0.00221     0.0049      0.0118      0.0299      0.0637
Bentoy13's cumsum   0.000318    0.000638    0.00107     0.00261     0.00498     0.0114      0.0283      0.0526
Bentoy13's sparse   0.000414    0.000774    0.00148     0.00451     0.00814     0.0191      0.0441      0.0877
Luis Mendo's optim. 0.000224    0.000413    0.000754    0.00207     0.00353     0.00832     0.0216      0.0441

Ok, I've added another solution to the list ... I could not prevent myself to optimize the best-so-far solution of Luis Mendo. No credit for that, it's just a variant from Luis Mendo's, I'll explain it later.

Clearly, the solutions using arrayfun are very time-consuming. The solutions using an explicit for loop are faster, yet still slow compared with others solutions. So yes, vectorizing is still a major option for optimizing a Matlab script.

Since I've seen a big dispersion on the computing times of the fastest solutions, especially with input lengths between 100 and 10000, I decide to benchmark more precisely. So I've put the slowest apart (sorry), and redo the benchmark over the 6 other solutions which run much faster. The second benchmark over this reduced list of solutions is identical except that I've average over 1000 runs.

(No table here, unless you really want to, it's quite the same numbers as before)

As it was remarked, the solution by Daniel is a little faster than the one by Divakar because it seems that the use of bsxfun with @times is slower than using repmat. Still, they are 10 times faster than for-loop solutions: clearly, vectorizing in Matlab is a good thing.

The solutions of Bentoy13 and Luis Mendo are very close; the first one uses more instructions, but the second one uses an extra allocation when concatenating 1 to cumsum(input_lengths(1:end-1)). And that's why we see that Bentoy13's solution tends to be a bit faster with big input lengths (above 5.10^5), because there is no extra allocation. From this consideration, I've made an optimized solution where there is no extra allocation; here is the code (Luis Mendo can put this one in his answer if he wants to :) ):

result = zeros(1,sum(input_lengths));
result(1) = 1;
result(1+cumsum(input_lengths(1:end-1))) = 1;
result = cumsum(result);

Any comment for improvement is welcome.

Answer 6

result = zeros(1,sum(input_lengths));
result(cumsum([1 input_lengths(1:end-1)])) = 1;
result = cumsum(result);

This should be pretty fast. And memory usage is the minimum possible.

An optimized version of the above code, due to Bentoy13 (see his very detailed benchmarking):

result = zeros(1,sum(input_lengths));
result(1) = 1;
result(1+cumsum(input_lengths(1:end-1))) = 1;
result = cumsum(result);