Create a sum with the Do function and modules

Question

I should define a function which sums up from k=0 to 100, with x=1; using the Do function and modules; without using the Sum function

I tried it this way:

blub[x_, p_]:=Do[blub=((-1)^k/x^k), {k, 1, p}]
blub[1, 100]

Problems:

1. I don't get any output
2. I don't get why I would need a module

I would be glad if someone could help me.

Thanks for the fast help, Solution number 2 fits perfect for what I want.

Is there a posibility that I use a module inside the iterator? So that I can use my function this way.

v=0;    
f[x_, n_]:=Do[v +=((-1)^n/x^n), {n, 1, n}]
f[1, 100]
v

Is there a posibility to make this work without changing one of the n's into another variable name (I`m aware that I double defined one variable)

Answer 1

The following I hope will assist you:

b = 0;
blub[x_, p_] := Do[b += ((-1)^k/x^k), {k, 1, p}]
blub[1, 100]
b

or achieving the output:

f[x_, p_] := Module[{v = 0},
  Do[v += ((-1)^k/x^k), {k, 1, p}];
  v]

There are many ways to achieve this result, e.g.

s[x_, p_] := Sum[(-1)^k/x^k, {k, 1, p}]
r[x_, p_] := Total[(-1)^#/x^# & /@ Range[p]]