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Consider the following function that generates a sparse diagonal matrix:

W[n_] := W[n] = Exp[(2 Pi I)/n]//N;

Clear[X];

X[n_, L_, power_ : 1][i_, chain_] := 
  X[n, L, power][i, chain] =
    DiagonalMatrix@
     SparseArray@
       Table[
         W[n]^(-IntegerDigits[a, n, 2 L][[chain*L - i + 1]] power),
         {a, 0, n^(2 L) - 1}
       ]

Due to the f[x_] := f[x] = SlowFunction definition, I expect this code to be much faster on a second run. Indeed, if I evaluate the following on my laptop several times

X[3, 7][1, 1] // Timing

I get $12$ seconds on the first run then around $2.8$ seconds on any evaluation after that. Clearly the memoization trick seems to have made this faster, but it is still much slower than it should be. For example if I run

a = X[3, 7][1, 1]
a // Timing

I get $10^{-6}$ seconds. Running X[3, 7][1, 1] the second time should also be this fast, since the matrix is already computed and saved. But it seems that instead it is still doing some computation.

Why does this happen, and how could I avoid it so I can take full advantage of memoization to speed up my repeated calculations?

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    $\begingroup$ I think the problem is with the missing optional argument in the memoized version. $\endgroup$ – mikado Feb 11 at 6:57
  • $\begingroup$ I just tried to make the "power' argument not be optional and it worked! Is it possible to keep the argument as optional and still make it work? If you make your comment into an answer (and maybe elaborate on why the optional variable causes this) I will be happy to accept it. $\endgroup$ – Heidar Feb 11 at 7:06
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One way is to use multiple dispatch instead of optional arguments:

X[n_, L_, power_][i_, chain_] := X[n, L, power][i, chain] = 
  DiagonalMatrix@SparseArray@
    Table[W[n]^(-IntegerDigits[a, n, 2 L][[chain*L-i+1]]*power), {a, 0, n^(2L)-1}]
X[n_, L_][i_, chain_] := X[n, L, 1][i, chain]

Here the memoization always happens in the full-argument form. Missing arguments are filled in by the multiple dispatch and forwarded to the full-argument form via delayed assignment.

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