Re: Help please: Summing a list

*To*: mathgroup at smc.vnet.net*Subject*: [mg13491] Re: Help please: Summing a list*From*: scottb (Scott Brown)*Date*: Fri, 31 Jul 1998 04:33:12 -0400*Organization*: Wolfram Research, Inc.*References*: <6pebm8$i6j@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

robpetersonSPAMME at iname.com writes: >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. If Plus, instead of Sum, will do, you could try this. If you really want Sum, you can adapt this easily. In[1]:= foo = Range[10] Out[1]= {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} In[2]:= partsum[l_] := MapIndexed[ Apply[ Plus, Take[ l, First[#2] ]]&, l] In[3]:= partsum[foo] Out[3]= {1, 3, 6, 10, 15, 21, 28, 36, 45, 55} Some specific comments about the problems you encountered follow. Your approach was very C-like. Mathematica includes lots of functions (e.g., MapIndexed) for manipulating Lists. Using these functions is faster and more robust than writing your own index-referencing code (you can avoid things like off-by-one errors, for instance). > I try >PP[[i]]:=Sum[P[[n]],{n,0,i}]; Note: Mathematica Lists start their indices at 1. Also, you would use i_, not i, on the left side of := to do this. However, you shouldn't do this :), see above. >At the definition, I get the following error: Part::"pspec": > "Part specification \!\(n\) is neither an integer nor a list of >integers." The problem isn't due to Sum, it's because you're telling Mathematica to assign to PP[[i]], and i is not defined to have a numeric value yet. In[1]:= somesum := Sum[ P[[n]], {n, 0, i} ] In[2]:= PP[[i]] := somesum SetDelayed::pspec: Part specification i is neither an integer nor a list of integers. Out[2]= $Failed In[3]:= PP[[j]] := 2 SetDelayed::pspec: Part specification j is neither an integer nor a list of integers. Out[3]= $Failed In[4]:= PP[[i_]] := 2 SetDelayed::pspec: Part specification i_ is neither an integer nor a list of integers. Out[4]= $Failed >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 ------------------------------------------------------------------------ | Scott Brown | "I may be speaking from Wolfram Research, Inc., | | scottb at wolfram.com | but that doesn't mean I'm speaking for them." | ------------------------------------------------------------------------