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Unusual speedup of Table


Why is the second call of Table below so much faster than the first? These
results are from Mathematica 4.0.2.1 for Power Mac (400 MHz PPC 750, OS 9). It
isn't some kind of cache effect because the times are consistent over many
executions of both expressions. (And yes, I know I can do a lot better still
for this particular computation with Drop and Take).

- Steve Tether

n = 10000;
a = Table[Random[], {n}];
Timing[Table[a[[j + 1]] - 2 a[[j]] + a[[j - 1]], {j, 2, n - 1}];]

{10.5333 Second, Null}

Timing[Table[Switch[j, _, a[[j + 1]] - 2 a[[j]] + a[[j - 1]]], {j, 2, n - 1}];]

{0.7 Second, Null}


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