apply function to a subset of a variable length

Question

I have got a list

list = Row[{#}] & /@ Range[100]

and I want to apply function f to the following elements:

sublist = Row[{5 #}] & /@ Range[20]

It is easy when I specify indexes I want to transform. For instance,

MapAt[f, list, {{1}, {5}}] 

works OK. As soon as I create a new list and use it as "Part":

h = Row[{5 #}] & /@ Range[20]; 
MapAt[f, list, h]

it fails. I suppose that the crux of the problem is using # simultaneously in two arrays - list and h, but I am new to Mathematica and can't figure it out. Is there any way to work with arrays of arbitrary length?

Answer 1

MapAt needs a plain list, not items wrapped in Row. If you omit Row from h it works. Note your function f is applied to Row[{5}], not just {5}. Are you sure you need to use Row at all?