Re: My vectorized code is slower than a loop

[Francesco Potortì]

No profiling, no runs, just looking at the code, so I may be off.

w = zeros(1, n);
j = 1
for i = 1:n
  if (vectorised)
    if(left(i) <= right(i))
      w(j:j+right(i)-left(i)) = x(i) - x(n+1-[left(i):right(i)]);
      j = j + right(i)-left(i)+1;
    endif
  else                          # looping
    if(left(i) <= right(i))
      for jj = left(i):right(i)
        w(j) = x(i) - x(n+1-jj);
        j += 1;
      endfor
  endif
endfor

In the loop version, the if is useless, because the for range will be
empty, so the for will not run when the if clause is false.  This may
result in a speedup or not.

In the vectorised version, I would write it like this, which minimises
the number of operations at the expense of the number of instructions,
which may result in a speedup or not:

   li = left(i);
   d = right(i) - li;
   if (d >= 0)
     dr = 0:d;
     w(j+dr) = x(i) - x(n+1-li+dr);
     j += d+1;
   endif

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