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    <p>No matter how hard I study the code that's been posted, I can't see why the call to <code>vdist</code> is made inside the loop. </p> <p>When I'm trying to optimise a block of code inside a loop one of the things I look for are statements which are invariant, that is which are the same at each call, and which can therefore be lifted out of the loop.</p> <p>Looking at</p> <pre><code>130 for res=1:6 131 U_neu=p0+NV1; 132 [pos,a12] = vdist(p0(:,2),p0(:,1),U_neu(:,2),U_neu(:,1)); 133 a12=a12+1/6.*res*totalRotation; 134 ddist=1852*safety_distance; 135 [lat2,lon2] = vreckon(p0(:,2),p0(:,1),ddist, a12); 136 extendedPoly(f,:)=[lon2,lat2];f=f+1; 137 end </code></pre> <p>I see</p> <ul> <li>in l131 the variables <code>p0</code>, <code>NV1</code> appear only on the rhs, and they only appear on the rhs elsewhere inside the loop, so this statement is loop-invariant and can be lifted out of the loop; only a small time saving perhaps;</li> <li>in l134 again, I see another loop-invariant statement, which can again be lifted out of the loop for another small time saving;</li> <li>but then I started to look very closely, and I can't see why l132, where the call to <code>vdist</code> is made, is inside the loop either. None of the values on the rhs of that assignment are modified in the loop (other than <code>U_neu</code> but I've already lifted that out of the loop).</li> </ul> <p>Tidying up what was left a bit, this is what I ended up with:</p> <pre><code>U_neu=p0+NV1; [pos,a12] = vdist(p0(:,2),p0(:,1),U_neu(:,2),U_neu(:,1)); ddist=1852*safety_distance; for res=1:6 extendedPoly(f,:) = vreckon(p0(:,2),p0(:,1),ddist, a12+1/6.*res*totalRotation); f=f+1; end </code></pre>
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    1. CO@Mark- Thank you very much for your reply, it has been very helpful. Now, every-time i see a loop i look for the invariant statements inside. While the code was written the performance was not an issue,and that is why it had some invariant statements there. Thanks again!
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