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MathGroup Archive 1998

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Re: Help please: Summing a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13509] Re: [mg13480] Help please: Summing a list
  • From: Carl Woll <carlw at fermi.phys.washington.edu>
  • Date: Fri, 31 Jul 1998 04:33:24 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Rob,

There are several problems with the statement

> PP[[i]]:=Sum[P[[n]],{n,0,i}];

The first problem is the use of double brackets instead of single
brackets. Using double brackets like you do above implies that PP is
already an object like a list with elements. This is what causes the
error message which Mathematica gives you.

The second problem is that in a function definition you should use an
argument with a pattern. That is, PP[i_] instead of PP[i]. Otherwise,
your definition only takes action when the argument is the letter i.

The final problem is that you probably don't want to have your sum go
from 0 to i, since the zeroth part of an expression is the head. For
example, if

s = {1,2,3}

then s[[0]] is List and not 1. So, putting the above together, you
probably want the definition

PP[i_] = Sum[ P[[n]], {n,1,i} ]

Carl Woll
Dept of Physics
U of Washington

On Sun, 26 Jul 1998 robpetersonSPAMME at iname.com wrote:

> I make a list of numbers, eg:
> 
> P=Table[6,{x,0,110}]; (I have a more interesting list to use later if I
> get this working)
> 
> Now I want to make another list PP in which each entry PP[[i]] is the
> sum of P's first i entries.  I try
> 
> PP[[i]]:=Sum[P[[n]],{n,0,i}];
> 
> At the definition, I get the following error: Part::"pspec": 
>     "Part specification \!\(n\) is neither an integer nor a list of
> integers."
> 
> I don't know how to make n an integer.  In the definition of Sum[], it
> seems n is an integer unless you add a forth parameter "di" in the
> specification list such as 
> Sum[f, {i, imin, imax, di}]
> 
> Can anyone help me to generate this second list?
> 
> Thanks, Rob
> 



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