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Re: Using "Sum" (i = 1 ... N) in a function definition
*To*: mathgroup at smc.vnet.net
*Subject*: [mg49687] Re: Using "Sum" (i = 1 ... N) in a function definition
*From*: "David W. Cantrell" <DWCantrell at sigmaxi.org>
*Date*: Thu, 29 Jul 2004 07:42:49 -0400 (EDT)
*References*: <ce5d8p$b4k$1@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
wilhelm.rainer at gmx.net (Rainer) wrote:
> 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 ...?
Yes. Instead of
MyFunc[i_] = Sum[MyData[[j]],{j,1,i}];
use
MyFunc[i_] := Sum[MyData[[j]],{j,1,i}];
that is, use a delayed, rather than an immediate, assignment.
David Cantrell
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