Section 8.5 - Unbounded_String Basics

Here are a few important operations:

• Function To_Unbounded_String takes something of type String and converts it into type Unbounded_String. This is useful for a variety of purposes, for example, for setting Unbounded_String values to some constant value, as we'll show later.
• Function To_String is the reverse; it takes something of type Unbounded_String and converts it to type String.
• Function Length takes an Unbounded_String and returns the number of characters currently stored in the Unbounded_String.
• Procedure Append takes two arguments; it appends to the end of its first argument the contents of the second argument. The first argument has to be an Unbounded_String; the second argument can be a String or an Unbounded_String.
• Function Element extracts from a given Unbounded_String (its Source) the character at a given position (its index). The leftmost character has an index of 1, just like Pascal; C and C++ start their indexes at zero. Here's the official definition of Element:

    function Element (Source : in Unbounded_String; Index  : in Positive)
                      return Character;
  

• Procedure Replace_Element is the opposite of Element; it lets you modify a character at a given position. Here's the official definition of Replace_Element:

    procedure Replace_Element (Source : in out Unbounded_String;
                               Index  : in Positive;
                               By     : in Character);
  

• Function Slice takes a given Unbounded_String and returns a ``slice'' of it, i.e., all the characters between a given low and high index. Slice returns a String, so if you want to use its result as an Unbounded_String, use To_Unbounded_String on the result.

    function Slice (Source : in Unbounded_String;
                    Low    : in Positive;
                    High   : in Natural) return String;
  
  

• Procedure Insert takes a New Item (of type String) and inserts it into an Unbounded_String before a given index. If the string isn't empty, this will change the Unbounded_String's length. This procedure's definition is:

    procedure Insert (Source   : in out Unbounded_String;
                      Before   : in Positive;
                      New_Item : in String);
  

• Procedure Delete takes an Unbounded_String and two indexes, and deletes the characters between those two index positions (including the end points). Its definition is:

  Procedure Delete
    procedure Delete (Source  : in out Unbounded_String;
                      From    : in Positive;
                      Through : in Natural);
  

Comparison operations (such as "=" and "<") are defined in this package. There are other routines to modify or search Unbounded_Strings, including "&" (concatenate two Unbounded_Strings together), Translate, Trim, Head, Tail, Index, and Find_Token.

You can use assignment (:=) as well to assign one value to another. Unlike type String, the lengths of the Unbounded_String's need not be equal.

Quiz:

Given a variable ``Input'' of type Unbounded_String, what expression would return the value of the fourth character in Input?

1. Element(Input, 4)
2. Replace_Element(Input, 4, 'L')
3. Element(4, Input)

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