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I'm doing some computations that involve summations with many indices over the same range - consider the simplified example Sum[a[i,j],{i,1,2},{j,1,2}]. To prettify my code I'm attempting to define a helper function so I can write this as S2[a[i,j],i,j]. My first thought was pretty simple - just build the sequence of inputs to Sum and fire it off:

S2[expr_, indices__] :=  Sum @@ Join[{expr}, Map[{#, 1, 2} &, {indices}]];

This works for many inputs, but has some issues with premature evaluation that Sum,Table, etc. do not. Consider for example the input function a[i_,j_]:=D[u[i],u[j]], which should produce the same outputs as KroneckerDelta. With our long-hand code we get the expected output:

In[1] := Sum[a[i, j], {i, 1, 2}, {j, 1, 2}]
Out[1] = 2

but with our S2 we do not:

In[2] := S2[a[i, j], i, j]
Out[2] = 0

This is due to a being evaluated before the values of i and j are assigned. I've tried setting HoldFirst on S2, replacing expr with Hold[expr] and releasing the hold at various places in the definition, but nothing I can think of gives me the desired behaviour.

What's the best way to accomplish what I want here (robust evaluation behaviour similar to that of Sum)?

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The following also works

SetAttributes[S3, HoldFirst];

S3[expr_, indices__] := 
 With[{ex = Unevaluated[expr]}, (Sum @@ 
    Join[{ex}, Map[{#, 1, 2} &, {indices}]])]
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One possibility:

a[i_, j_] := D[u[i], u[j]]
ClearAll@S2
SetAttributes[S2, HoldFirst]
S2[expr_, indices__] := Sum @@ Join[Inactivate@{expr}, Map[{#, 1, 2} &, {indices}]] // Activate;
S2[b[i, j], i, j]
S2[a[i, j], i, j]

enter image description here

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  • $\begingroup$ Thanks, but I'm stuck on Mathematica 9 at the moment - any way to do it without the new Activate/Inactivate? $\endgroup$ – Anthony Carapetis Jan 15 '16 at 6:58

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