Procedure to open, write and append in Ada

Answer 1

The thing about plain String in Ada is that a particular string, like your File_Name, has to be fixed-length; but different Strings can be of different length.

You can write

S1 : String := "1234";
S2 : String := "12345";

in which case S1 is of length 4, and assignments to it have to be of length 4. You can write

S1 := "abcd";

but if you try to write

S1 := "pqrst";

or

S1 := S2;

you will get a Constraint_Error.

In the case of String parameters to subprograms, like your Open_Data, the String parameter Name takes on the length -- and of course the value! of the actual parameter in the call. So you can say

Open_Data (X_File, "x.dat");
Open_Data (Y_File, "a_very_long_name.dat");

You were having problems earlier with

procedure Open_Data(File : in out Seq_Float_IO.File_Type; 
                    Name : in String) is
begin
   Seq_Float_IO.Open (File => File,
                      Mode => Seq_Float_IO.Append_File,
                      Name => ????);

I'm reluctant to just tell you the answer, so -- consider the File => File part. The first File is the name of the formal parameter of Seq_Float_IO.Open and the second File is what is to be passed, in this case Open_Data's File parameter.

It might help if I point out that I could have written the calls above as

Open_Data (File => X_File, Name => "x.dat");
Open_Data (File => Y_File, Name => "a_very_long_name.dat");

Answer 2

@Simon Wright's answer is correct, and you may find it helpful to compare his answer to the second one I wrote earlier. Note that if you had

Name_X : constant String := "domainvalues.dat";
Name_Y : constant String := "ordinatevalues.dat";

Either string, Name_X or Name_Y, could be used as the actual Name parameter to Open_Data. The formal parameter, Name, is of type String. String is unconstrained, and it may be any (implementation-defined) maximum length. In contrast, Name_X and Name_Y each have a fixed length determined by their initial assignment.

Addendum: You wrote a subprogram with a formal parameter (Name) of type String, having this signature

procedure Open_Data(
    File : in out Seq_Float_IO.File_Type;
    Name : in String) is ...

In the implementation, you want to forward to Open the String you received as the actual parameter (Name), not the name of a global constant (Name_X).

Seq_Float_IO.Open (
    File => File,
    Mode => Seq_Float_IO.Append_File,
    Name => Name );