       Re: Help please: Summing a list

• To: mathgroup at smc.vnet.net
• Subject: [mg13507] Re: [mg13480] Help please: Summing a list
• From: "Jens-Peer Kuska" <kuska at linmpi.mpg.de>
• Date: Fri, 31 Jul 1998 04:33:23 -0400
• Sender: owner-wri-mathgroup at wolfram.com

```Hi Rob,

>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}];

a) Mathematica List[]s index starts with 1, Part[any,0] gives the head.
must be
Sum[P[[n]],{n,1,i}]

b) What do You want ? A function PP[i_] ? that calculates the sums ?
Here is a possible solution

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

or the recursive one

PP:=P[]
PP[i_ /; i>1]:=PP[i]=P[[i]]+PP[i-1]

Notice that this definition does not calculate the sums. But if You say
PP You will
get the value.

Or do You want that PP is a list containing the accumulated sums ?

PPList = Table[Plus @@ Drop[#1,n] &[P],{n,-10,0}]

or using the defined PP[]-Function

PPList=Table[PP[i],{i,1,11}]

Hope that helps
Jens

```

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