A function that (also) changes a replacement rule

Question

I'd like to write a function operating on a list, returning a list, but also changing a replacement rule. Let's say I have a list of replacement rules:

data={
...
a->3,
...
};

Now I write a function:

f1[{b_,c_}]:={b+1,c+1};

What I would also like this function to do is to change the replacement rule of data list, so that after the function is evaluated (with any list as an input), the variable a would take the value of the evaluated c+1 expression, or f1[[2]] if You will. Seems like an easy task, but somehow confuses me.

Edit:

As an input I would like a list l={2,3}, and {3,4} as an output, and a replacement rule a->3 changed into a->f1[[2]].

In general, my problem is that I want another function later on to access a value produced by f1, but I would like that other function to call this value from external list and not explicitly from f1.

Answer 1

Let me expand the comment. If the list of replacement rules data is to be considered a global variable, you can access it from within your function and alter its value in this way

data = {a->3, ab->2, ac->1, ad->0};

f1[{b_,c_}]:=Block[{tmp},
    tmp = {b+1, c+1};
    data = data /. Rule[a,_]:>Rule[a,tmp[[2]]]
]

This will produce a new value for the list of rules data where the rule associated with a is changed.

f1[{2,3}]

{a -> 4, ab -> 2, ac -> 1, ad -> 0}

If you do not wish to see the list of rules, just put a semicolon at the end of the last line of code. You can even have f1 to return the list {b+1,c+1} in this way:

f1[{b_,c_}]:=Block[{tmp},
    tmp = {b+1, c+1};
    data = data /. Rule[a,_]:>Rule[a,tmp[[2]]];
    tmp
]

Once you have evaluated f1,

f1[{2,3}]

{3, 4}

to get the value of a, you will use

a /. data

4

Answer 2

Simply this?

f1[{b_, c_}] := Module[{a, d},
  data = {a -> 8, d -> 7};
  {a, d} = {b + 1, c + 1};
  {a, d} /. data]
 f1[{2, 3}]

(8, 7}

Further to OP's comment ...

data = {a -> 8, d -> 7};
f1[{b_, c_}] := Module[{},
  {a, d} = {b + 1, c + 1};
  {a, d} /. data]
f1[{2, 3}]

(8, 7}