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Using "Sum" (i = 1 ... N) in a function definition


I was trying to define a function using the Sum command in the
following way (simplified example):

 MyData = {3,2,4,5,6};
 MyFunc[i_] = Sum[MyData[[j]],{j,1,i}];

As you can see, the idea is to use the length of the list (i)
as a variable in the function definition, such that

 MyFunc[1] = 3
 MyFunc[2] = 3 + 2 = 5
 MyFunc[3] = 3 + 2 + 4 = 9

etc. Mathematica does not like the abobe statements, and prints the
following error message:

"Part specification K$394 is neither an integer nor a list of
integers."

Does anybody know how to define (correctly) such a function. Should be
possible ...?
Thanks
Rainer


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