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MathGroup Archive 2004

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

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49692] RE: [mg49666] Using "Sum" (i = 1 ... N) in a function definition
  • From: "Ellis, George" <George.Ellis at Cognos.COM>
  • Date: Thu, 29 Jul 2004 07:42:56 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Rainer,

I do not know why you would want to do this; so my response below may be
off target.  Nonetheless, what about this?  (The function "Total" in
version 5.0, in this case, provides the equivalent functionality -- as
would "Apply[Plus,data]" in earlier versions).

In[10]:=  (8:59:39 on 7/27/04)
data=Range[10];
Sum[data[[x]],{x,Length[data]}]

Out[11]=  (8:59:39 on 7/27/04)
55

In[9]:=  (8:59:21 on 7/27/04)
Total[data]

Out[9]=  (8:59:21 on 7/27/04)
55 

Regards,

George
-----Original Message-----
From: Rainer [mailto:wilhelm.rainer at gmx.net] 
To: mathgroup at smc.vnet.net
Subject: [mg49692] [mg49666] 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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