How can I vectorize the loops of this function in Octave? - matlab

Question

I want to be able to vectorize the for-loops of this function to then be able to parallelize it in octave. Can these for-loops be vectorized? Thank you very much in advance!
I attach the code of the function commenting on the start and end of each for-loop and if-else.
function [par]=pem_v(tsm,pr)
% tsm and pr are arrays of N by n. % par is an array of N by 8
tss=[27:0.5:32];
tc=[20:0.01:29];
N=size(tsm,1);
% main-loop
for ii=1:N
% I extract the rows in each loop because each one represents a sample
sst=tsm(ii,:); sst=sst'; %then I convert each sample to column vectors
pre=pr(ii,:); pre=pre';
% main-condition
if isnan(nanmean(sst))==1;
par(ii,1:8)=NaN;
else
% first sub-loop
for k=1:length(tss);
idxx=find(sst>=tss(k)-0.25 & sst<=tss(k)+0.25);
out(k)=prctile(pre(idxx),90);
end
% end first sub-loop
tp90=tss(find(max(out)==out));
% second sub-loop
for j=1:length(tc)
cond1=find(sst>=tc(j) & sst<=tp90);
cond2=find(sst>=tp90);
pem=zeros(length(sst),1);
A=[sst(cond1),ones(length(cond1),1)];
B=regress(pre(cond1),A);
pt90=B(1)*(tp90-tc(j));
AA=[(sst(cond2)-tp90)];
BB=regress(pre(cond2)-pt90,AA);
pem(cond1)=max(0,B(1)*(sst(cond1)-tc(j)));
pem(cond2)=max(0,(BB(1)*(sst(cond2)-tp90))+pt90);
clear A B AA BB;
E(j)=sqrt(nansum((pem-pre).^2)/length(pre));
clear pem;
end
% end second sub-loop
tcc=tc(find(E==min(E)));
% sub-condition
if(isempty(tcc)==1);
par(ii,1:9)=NaN;
else
cond1=find(sst>=tcc & sst<=tp90);
cond2=find(sst>=tp90);
pem=zeros(length(sst),1);
A=[sst(cond1),ones(length(cond1),1)];
B=regress(pre(cond1),A);
pt90=B(1)*(tp90-tcc);
AA=[sst(cond2)-tp90];
BB=regress(pre(cond2)-pt90,AA);
pem(cond1)=max(0,B(1)*(sst(cond1)-tcc));
pem(cond2)=max(0,(BB(1)*(sst(cond2)-tp90))+pt90);
RMSE=sqrt(nansum((pem-pre).^2)/length(pre));
% outputs
par(ii,1)=tcc;
par(ii,2)=tp90;
par(ii,3)=B(1);
par(ii,4)=BB(1);
par(ii,5)=RMSE;
par(ii,6)=nanmean(sst);
par(ii,7)=nanmean(pre);
par(ii,8)=nanmean(pem);
end
% end sub-condition
clear pem pre sst RMSE BB B tp90 tcc
end
% end main-condition
end
% end main-loop

Answer 1

You haven't given any example inputs, so I've created some like so:
N = 5; n = 800;
tsm = rand(N,n)*5+27; pr = rand(N,n);
Then, before you even consider vectorising your code, you should keep 4 things in mind...
Avoid calulating the same thing (like the size of a vector) every loop, instead do it before looping
Pre-allocate arrays where possible (declare them as zeros/NaNs etc)
Don't use find to convert logical indices into linear indices, there is no need and it will slow down your code
Don't repeatedly use clear, especially many times within loops. It is slow! Instead, use pre-allocation to ensure the variables are as you expect each loop.
Using the above random inputs, and taking account of these 4 things, the below code is ~65% quicker than your code. Note: this is without even doing any vectorising!
function [par]=pem_v(tsm,pr)
% tsm and pr are arrays of N by n.
% par is an array of N by 8
tss=[27:0.5:32];
tc=[20:0.01:29];
N=size(tsm,1);
% Transpose once here instead of every loop
tsm = tsm';
pr = pr';
% Pre-allocate memory for output 'par'
par = NaN(N, 8);
% Don't compute these every loop, do it before the loop.
% numel simpler than length for vectors, and size is clearer still
ntss = numel(tss);
nsst = size(tsm,1);
ntc = numel(tc);
npr = size(pr, 1);
for ii=1:N
% Extract the columns in each loop because each one represents a sample
sst=tsm(:,ii);
pre=pr(:,ii);
% main-condition. Previously isnan(nanmean(sst))==1, but that's only true if all(isnan(sst))
% We don't need to assign par(ii,1:8)=NaN since we initialised par to a matrix of NaNs
if ~all(isnan(sst));
% first sub-loop, initialise 'out' first
out = zeros(1, ntss);
for k=1:ntss;
% Don't use FIND on an indexing vector. Use the logical index raw, it's quicker
idxx = (sst>=tss(k)-0.25 & sst<=tss(k)+0.25);
% We need a check that some values of idxx are true, otherwise prctile will error.
if nnz(idxx) > 0
out(k) = prctile(pre(idxx), 90);
end
end
% Again, no need for FIND, just reduces speed. This is a theme...
tp90=tss(max(out)==out);
for jj=1:ntc
cond1 = (sst>=tc(jj) & sst<=tp90);
cond2 = (sst>=tp90);
% Use nnz (numer of non-zero) instead of length, since cond1 is now a logical vector of all elements
A = [sst(cond1),ones(nnz(cond1),1)];
B = regress(pre(cond1), A);
pt90 = B(1)*(tp90-tc(jj));
AA = [(sst(cond2)-tp90)];
BB = regress(pre(cond2)-pt90,AA);
pem=zeros(nsst,1);
pem(cond1) = max(0, B(1)*(sst(cond1)-tc(jj)));
pem(cond2) = max(0, (BB(1)*(sst(cond2)-tp90))+pt90);
E(jj) = sqrt(nansum((pem-pre).^2)/npr);
end
tcc = tc(E==min(E));
if ~isempty(tcc);
cond1 = (sst>=tcc & sst<=tp90);
cond2 = (sst>=tp90);
A = [sst(cond1),ones(nnz(cond1),1)];
B = regress(pre(cond1),A);
pt90 = B(1)*(tp90-tcc);
AA = [sst(cond2)-tp90];
BB = regress(pre(cond2)-pt90,AA);
pem = zeros(length(sst),1);
pem(cond1) = max(0, B(1)*(sst(cond1)-tcc));
pem(cond2) = max(0, (BB(1)*(sst(cond2)-tp90))+pt90);
RMSE = sqrt(nansum((pem-pre).^2)/npr);
% Outputs, which we might as well assign all at once!
par(ii,:)=[tcc, tp90, B(1), BB(1), RMSE, ...
nanmean(sst), nanmean(pre), nanmean(pem)];
end
end
end