MATLAB: Replacing a for loop

for loopMATLABmatrix

Hello all,

I have a simple piece of code that I would like to get rid of the for loop. I would appreciate your help.

 for n=1:K
   idx=row((((n-1)*dc)+1):(n*dc));
 end

where as an example K=4, dc=2, and row=[1;3;2;3;1;4;2;4];

Best Answer

t = reshape(row, dc, []);
idx = t(:,1:K);

Related Solutions

MATLAB: For loops are too slow

for i = 1:1:N
  for j = 1:1:norigin
    jstart = (j-1)*N + i;
    for k = nmin:1:nmax
      kend = jstart + k*N;
      xmsd(k,1:3) = xmsd(k,1:3) + (md_msd(kend,1:3)-md_msd(jstart,1:3)).^2;
    end
  end
end

Start in the inner loop...first off

md_msd(jstart,1:3)

is invariant w/ k so replace the indexed expression w/ the equivalent

...
  for j = 1:1:norigin
    jstart = (j-1)*N + i;
    mdj=md_msd(jstart,1:3);
    for k = nmin:nmax
      kend = jstart + k*N;
      xmsd(k,1:3) = xmsd(k,1:3) + (md_msd(kend,1:3)-mdj).^2;
    end
  end

Then, kend takes on values of jstart+N, jstart+2N, jstart+3N, ... so define an index vector as

kdx=jstart+N:N:jstart+nmax*N;

and then the loop on k can be written as

xmsd(nmin:nmax,1:3) = xmsd(nmin:nmax,1:3) + (md_msd(kdx,1:3)-mdj).^2;

So, after the first loop reduction you're left with

for i = 1:1:N
  for j = 1:1:norigin
    jstart = (j-1)*N + i;
    kdx=jstart+nmin*N:N:jstart+nmax*N;
    mdj=md_msd(jstart,1:3);
    xmsd(nmin:nmax,1:3) = xmsd(nmin:nmax,1:3) + (md_msd(kdx,1:3)-mdj).^2;
  end
end

Now, see what you can do from here... :)

ADDENDUM: Forgot to add -- xmsd IS preallocated, correct????

ERRATUM: "Then, kend takes on values of jstart+N, jstart+2N, jstart+3N, ..."

Actually, the first value is jstart+nmin*N not jstart+N. The increment is N, however, so only the lower bound needs must be corrected.

MATLAB: Replace column

% 2D example:
A = zeros(5)
A(:,3) = 1
% 3D example:
A = zeros(3,3,3)
A(:,2,:) = 1

Best Answer

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MATLAB: For loops are too slow

MATLAB: Replace column

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