Warning message when summing from a elements in a list

Question

Why does adding two elements from a list as follows:

test = {1, 2};
NSum[test[[n]], {n, 1, 2}]

lead to the warning "The expression n cannot be used as a part specification."? It does produce the correct answer but I don't know how to get rid of the warning.

Answer 1

I suspect the reason for the warning is because:

NSum first localizes the values of all variables, then evaluates f with the variables being symbolic, and then repeatedly evaluates the result numerically.

NSum has attribute HoldAll, and effectively uses Block to localize variables.

So under the hood it is doing something like

Clear[n]
{1, 2}[[n]] (* triggers your error *)

n = 1;
{1, 2}[[n]] (* produces your answer *)

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To get rid of the warning does this work for you?

test = {1, 2};
NSum[Indexed[test, n], {n, 1, 2}] (*3.*)