Compile Problem with Nested Sum

[Julian Francis]

Dear all,

I have a simple function which compiles fine:

fC1 = Compile[{{mx, _Integer}, {my, _Integer}, {mat, _Real, 2}},
Sum[
mat[[mx, my]] , {a, 0, 1}]
]

However, a sum within a sum doesn't compile:

fC2 = Compile[{{mx, _Integer}, {my, _Integer}, {mat, _Real, 2}},
Sum[
Sum[
mat[[mx, my]] , {a, 0, 1}],
{b, 1, 1}]
]

which generates the following error message:

Compile::noinfo: No information is available for compilation of \!\
(\*UnderoverscriptBox[\(\[Sum]\), \(b = 1\), \(1\)]\
((\*UnderoverscriptBox[\(\[Sum]\), \(a = 0\), \(1\)]mat[[mx, my]])\)
\). The compiler will use an external evaluation and make assumptions
about the return type. >>

I am confused as to why the first works, but the second doesn't.

Any help would be appreciated.

Thanks,
Julian Francis.

[Frank K]

What version of Mathematica are you running? It compiles ok on 8.0.4.
However, I don't understand why you're not summing over the indices of
the matrix.
Also, you don't need to nest Sum, you can put both indices inside one
sum

[Szabolcs]

On Monday, 19 December 2011 13:19:56 UTC+1, Frank K wrote:

The reason this doesn't give a message in 8.0.4 is that Compile::noinfo is turned off by default. The expression is still being computed using a standard evaluation (rather than the virtual machine), as you can see with CompilePrint[fC2].

I'm not sure why the nested Sum doesn't compile, but as Frank mentioned, a single Sum with two ranges does work.