MATLAB: Replacing the for loops

replace for loops

L=(length(time)-1);
 for q=1:numPT
    for  h=1:L
    x(q,h)=SortDate(q,5);
    y(q,h)=SortDate(q,6);
    c(q,h)=SortDate(q,4);
    end
end

Best Answer

cxy = permute(repmat(SortDate(q,4:6),1,1,L),[1 3 2]);

here cxy your arrays c,x and y as cxy = cat(3,c,x,y)

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Use repmat to avoid manually typing all 160 of those ‘%q’ repeats!

Try this:

formatSpec = ['%f',repmat('%q',1,160), '%[^\n\r]'];

MATLAB: Replacing the for loop please

Before you start to vectorize the code, did you pre-allocate the output before the loops? This is essential:

L = (length(time)-1);
x = zeros(numPT, L);
y = zeros(numPT, L);
c = zeros(numPT, L);
for q = 1:numPT
  for h=1:L
    x(q,h) = SortDate{q+numPT*(h-1),6};
    y(q,h) = SortDate{q+numPT*(h-1),7};
    c(q,h) = SortDate{q+numPT*(h-1),5};
  end
end

Now compare the timings. Then in the next step remove the inner loop:

L = (length(time)-1);
x = zeros(numPT, L);
y = zeros(numPT, L);
c = zeros(numPT, L);
for q = 1:numPT
  x(q, :) = [SortDate{q+numPT*(0:L-1), 6}];  % Faster than: x(q, 1:L) = ...
  y(q, :) = [SortDate{q+numPT*(0:L-1), 7}];
  c(q, :) = [SortDate{q+numPT*(0:L-1), 5}];
end

And now the outer loop - and then a pre-allocation is not required:

L = (length(time)-1);
index = (1:numPT).' + (0:numPT:numPT*(L-1)));  % >= R2016b

x = reshape(cell2mat(SortDate(index, 6)), size(index));
y = reshape(cell2mat(SortDate(index, 7)), size(index));
c = reshape(cell2mat(SortDate(index, 5)), size(index));

Do you see how the indices are moved from the for loop into the vector indices?

If you have an older Matlab version, you need bsxfun instead of the modern auto-expanding:

index = bsxfun(@plus, (1:numPT).', (0:numPT:numPT*(L-1)));

If this is still the bottleneck of you code, try to use the faster FEX: Cell2Vec, which seems to have a better memory management. Perhaps it is faster to convert the complete SortDate array at once:

L     = (length(time)-1);
index = (1:numPT).' + (0:numPT:numPT*(L-1)));  % >= R2016b
siz   = size(index);
M     = Cell2Vec(SortDate(:, 5:7));
x     =  reshape(M(index, 2), siz);
y     =  reshape(M(index, 3), siz);
z     =  reshape(M(index, 1), siz);

Best Answer

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