Why is this simple Table calculation so slow

Question

I have the following code

epsilon[t_, alpha_] := If[t > 3, 1, 2]
L[t_, alpha_] := 
 Min[epsilon[t, 
    alpha] - (x[t - 1, alpha] - 
     Min[epsilon[t, alpha - 1], x[t - 1, alpha]]), x[t - 1, alpha - 1]]
d[t_] := 10
f = 5
x[0, alpha_] := 0
x[t_, f] := x[t - 1, f] + L[t, f] - Min[d[t - 1], x[t - 1, f]]
x[t_, 0] := 10

x[t_, alpha_] := x[t - 1, alpha] + L[t, alpha] - L[t, alpha + 1]
s = TimeUsed[];
Table[x[t, a], {t, 0, 8}, {a, 0, 10}] // MatrixForm
Timeused : TimeUsed[] - s

The timeused output increases dramatically and nonlinearly as I increase the range of t, in Table[...]:

1. 0.0 
2: 0.016 
3: 0.016 
4: 0.063 
5: 0.422 
6: 2.90 
7: 19.344 
8: 131

Why does this happen? It seems to me that my equations should be computable in linear of $t$ time: For every $t$, x[t, alpha] can be calculated solely on the basis of x[t-1, alpha]. Therefore each computation should be roughly equally intensive. So it should be able to just sequentially compute all the $x$'s for $t = 0$, $t = 1$, $t = 2$, ...

What am I doing wrong?

If I turn them all to =, it gives me this: "$RecursionLimit::reclim2: "Recursion depth of 1024 exceeded during evaluation of -Min[10,x[-1+(-1022+t),5]]."" — user56834, Apr 15 '17 at 09:06

Pinti · Answer 1 · 2017-04-15T09:29:13.943

I think your code is a good candidate for using memoization.

    Clear[epsilon, L, x, d, f]

    epsilon[t_, alpha_] := If[t > 3, 1, 2]

    L[t_, alpha_] := Min[
      epsilon[t,alpha] - (x[t - 1, alpha] - Min[epsilon[t, alpha - 1], x[t - 1, alpha]]),
      x[t - 1, alpha - 1]
      ]

    d[t_] := 10
    f = 5;
    x[0, alpha_] := 0
    x[t_, 0] := 10

    (* changes in definition of x *)
    x[t_, f] := x[t, f] = x[t - 1, f] + L[t, f] - Min[d[t - 1], x[t - 1, f]]
    x[t_, alpha_] := x[t, alpha] = x[t - 1, alpha] + L[t, alpha] - L[t, alpha + 1]

Then calculation takes a fraction of a second.

Table[x[t, a], {t, 0, 8}, {a, 0, 10}] // AbsoluteTiming // First
(*0.00019*)

Why is this simple Table calculation so slow

1 Answers1