Changing a certain index in a string to a preferred character - ada

Question

When I try creating a subprogram that receives a string and a character and I for instance want to change the first index in the string to the character then return the result I'm not getting it to work...
This is how I tried doing it:
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;
procedure Test is
function Change_Index_In_String(
Str : in String;
Char : in Character
) return String is
begin
Str(Str'First) := C
return Str;
end Change_Index_In_String
Str : String(1..5);
Char : Character;
begin
Put("Type a string (5 characters): ");
Get(String);
Put("Type a character: ");
Get(Character);
Put("Your new string is: ");
Put(Change_Index_In_String(Str, Char))
end Test;
How am I supposed to do this? I'm getting errors in my complier like this:
assignment to "in" mode parameter not allowed
expected type "Standard.Character"
found type "Standard.String"
I am aware on how to return the a certain index in a string as a character, like doing return Str(Str'Last) etc but what about changing a certain index and then returning it?
In the terminal if I type the string Stack and the character W it should return and print out Wtack

Answer 1

One of the key design decisions to make is whether your subprogram is modifying state (a procedure), or returning a new value (a function).
If you want to modify an argument to a subprogram, then A) it needs to be marked out or in out, and B) it should probably be a procedure.
If you want to write a function, you should work on a copy of the input data, rather than trying to modify the argument passed into the function.
Consider the below example of both approaches in one program.
with Ada.Text_IO; use Ada.Text_IO;
procedure Test is
function Change_First(s : in String; c : in Character) return String is
s2 : String(s'range) := s;
begin
s2(s2'First) := c;
return s2;
end Change_First;
procedure Sub_First(s : in out String; c : in Character) is
begin
s(s'First) := c;
end Sub_First;
s : String(1..5) := "Hello";
begin
Put_line(Change_First(s, 'W'));
Sub_First(s, 'Y');
Put_Line(s);
end Test;
This gives us as output:
Wello
Yello

Answer 2

Much is undefined for your function. What should happen if the String is null ('Length = 0)? What should the bounds of the returned String be? One simple solution for one set of answers is
function F (S : in String; C : in Character) return String is
(C & S (S'First + 1 .. S'Last) )
with Pre => S'Length > 0 and S'First < Integer'Last,
Post => (F'Result'First = 1 and
F'Result'Length = S'Length) and then
(F'Result (F'Result'First) = C and
F'Result (F'Result'First + 1 .. F'Result'Last) = S (S'First + 1 .. S'Last) );