Mathematica : Easy? Function that returns and increases counter.

[Hepth]

I have a statement that looks like :

vD.vD.vD.vD

Where vD is noncommutative (but that doesn't really matter)

I want to replace it so that each vD becomes v[a].D[a]

But I would like it so the "a" is not repeating. So I need somehow a function that each time it is run returns it with an increased index : v[a[1]].D[a[1]]
So that
vD.vD.vD /.{vD :> FXN}
results in
v[a[1]].D[a[1]].v[a[2]].D[a[2]].v[a[3]].D[a[3]]


Does anyone know how to do this?

 

[Hypersphere]

Would something like Product[v[a[j]].d[a[j]], {j, 1, 3}] work for you?

 

[Hepth]

No, i need a replacement rule.

Lets say my chain is like:

chain = A.vD.B.C.vD.vD.E.F

i want something so I can say:

chain/.{vD:> XXXXX}

where the result would be

A.v[a[1]].D[a[1]].B.C.v[a[2]].D[a[2]].v[a[3]].D[a[3]].E.F

and then if I did it again, it would be

A.v[a[4]].D[a[4]].B.C.v[a[5]].D[a[5]].v[a[6]].D[a[6]].E.FSo there is some hidden counter I can set "acount = 0"

Then when I make a replacement rule, it not only replaces it, but increments the counter:

chain/.{vD:> v[a[acount]].D[a[acount]];acount++;}

but I can't force a command like a++ to run in a replacement rule, and if I make it a Do statement, I don't know how to return what I want...Ah, I just tried this and it worked!
cnt=0;
chain /. {vD :> v[a[cnt]].DD[a[cnt++]]}

So obviously simple I am an idiot :)

 

[Hypersphere]

Aha. Yeah, I wondered if the product solution was too simple to suggest but then I remembered that I once spent an hour perfecting a recursive algorithm in java to format a text string before someone reminded me that I could just have used a string replace function.

Nice solution, by the way.