Section 8.6 - Unbounded_String Input and Output

Ada 95 does not define input and output packages for Unbounded_String; instead, it defines operations on type String and operations to convert a String into an Unbounded_String.

Personally, I find that it's easier to use a package that defines Input-Output operations directly for an Unbounded_String. It's easy to define such a package, so I've provided one for you.

I call my package ``Ustrings'', which is a nice short name - I'll explain later why it has that name. It has a procedure Get_Line, which reads in a whole text line into an Unbounded_String. Procedure Put prints the Unbounded_String. Procedure Put_Line first Puts the Unbounded_String, and then starts a new line. Here's a shortened version of this package's declaration:

package Ustrings is
  procedure Get_Line(Item : out Unbounded_String);
  procedure Put(Item : in Unbounded_String);
  procedure Put_Line(Item : in Unbounded_String);
end Ustrings;

If you're curious you can see the complete declaration (specification) and body of my package Ustrings.

Also, I also believe that ``Unbounded_String'' is too long a name for such a widely-used type, so I define in package ``Ustrings'' a new name for Unbounded_String called ``Ustring''. You can declare variables of type ``Ustring'' and they'll simply be Unbounded_Strings. You do not need to use the type name "Ustring" instead of "Unbounded_String"; I simply find it convenient.

Let's look at a short Unbounded_String demonstration program named `Unbound'. It reads in text, one line at a time, and then does various things with the line of text. Study the following program and see if you can figure out what it does.

with Ada.Strings.Unbounded, Ustrings,
      Text_IO, Ada.Integer_Text_IO;
use  Ada.Strings.Unbounded, Ustrings,
      Text_IO, Ada.Integer_Text_IO;

procedure Unbound is -- Demonstrate Unbounded_String.
  Input : Unbounded_String;
  Stop  : constant Unbounded_String := To_Unbounded_String("stop");
  Put_Line("Please type 'stop' to end this program.");

    Put_Line("Please type in a line:");

  exit when (Input = Stop);

    Put("You just typed in:"); Put_Line(Input);

    Put("This input line contains ");
    Put_Line(" characters.");

    for I in reverse 1 .. Length(Input) loop
      Put(Element(Input, I));
    end loop;

  end loop;
end Unbound;


What does subprogram Unbound do after it prints the line showing the number of characters in the input string?

  1. It prints the input string in reverse.
  2. It prints the input string as it was entered.
  3. Nothing.

You may also:

PREVIOUS Go back to the previous section

NEXT     Skip to the next section

OUTLINE  Go up to lesson 8 outline

David A. Wheeler (

The master copy of this file is at "".