NSum generates a warning message when getting the sum of a list

Question

I know what I'm doing can be done with Total:

a = Range@3;
Total@a

And if I simply choose Sum, nothing wrong will happen:

a = Range@3;
Sum[a[[i]], {i, 3}]

N@Sum is still OK:

a = Range@3;
N@Sum[a[[i]], {i, 3}]

But when it's changed to NSum, a warning message comes out though I still get the correct result:

a = Range@3;
NSum[a[[i]], {i, 3}]

Part::pspec: Part specification i is neither an integer nor a list of integers.

I found if I add a Hold, I can avoid the warning:

a = Range@3;
NSum[Hold@a[[i]], {i, 3}]

But I still want to know the exact reason.

Answer 1

The first thing I assumed was that the Hold* attributes of the functions must be different, but they both have the attributes HoldAll.

On the other hand, the algorithms behind Sum and NSum are quite different. I certainly don't know all the details but we can examine what's going on with TracePrint.

a = {1, 2, 3};
TracePrint[NSum[a[[i]], {i, 3}], _Part]

(* Print result:
    a[[i]]
    {1,2,3}[[i]]
*)

Note that Part is called with a symbolic argument i. Now, let's do it with Sum.

a = {1, 2, 3};
TracePrint[Sum[a[[i]], {i, 3}], _Part]

(* Print result:
  a[[i]]
  {1,2,3}[[1]]

  a[[i]]
  {1,2,3}[[2]]

  a[[i]]
  {1,2,3}[[3]]
*)

I suspect that NSum is attempting to do some symbolic evaluations of the summand.

Answer 2

Another possibility is that NSum[ ] wants to have a function as its first argument (and not a list). So if we supply a function

 a[x_]:=x;

to mimic your Range command, and you can NSum without warning using

 NSum[a[i], {i, 3}]

or more concisely

 NSum[i, {i, 1, 3}]