StringBuilder Class

(Inherits from MfObject)

Represents a mutable string of characters. This class cannot be inherited.

Sealed Class

Links

Properties, Methods, Related, Remarks, Example

Constructors

Properties

Methods

Related

MfString

Remarks

This class repesents a string-like object whose value is a mutable sequence of characters.

In this section:

• The String and StringBuilder types
• How StringBuilder works
• Memory allocation
• Instantiating a StringBuilder object
• Calling StringBuilder methods
• Performing StringBuilder operations
• Iterating StringBuilder characters
• Adding text to a StringBuilder object
• Deleting text from a StringBuilder object
• Modifying the text in a StringBuilder object
• Searching the text in a StringBuilder object

The String and StringBuilder types

Although StringBuilder and MfString both represent sequences of characters, they are implemented differently. MfString is an immutable type. That is, each operation that appears to modify a MfString object actually creates a new string.

For example, concat the following example appears to change the value of a string variable named value. In fact, the concat method returns a value object that has a different value and address from the value object that was passed to the method.

value := "This is the first sentence" . "."
Address1 := &Value
value := value . "This is the second sentence. "
value := value . "This is the third sentence. "
Address2 := &value
MsgBox % Address1 = Address2 ; displays 0 for false

For routines that perform extensive string manipulation (such as apps that modify a string numerous times in a loop), modifying a string repeatedly can exact a significant performance penalty. The alternative is to use StringBuilder, which is a mutable string class. Mutability means that once an instance of the class has been created, it can be modified by appending, removing, replacing, or inserting characters. A StringBuilder object maintains a buffer to accommodate expansions to the string. New data is appended to the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, and the new data is then appended to the new buffer.

Consider using the MfString class under these conditions:

• When the number of changes that your app will make to a string is small. In these cases, StringBuilder might offer negligible or no performance improvement over MfString.
• When you have to perform extensive search operations while you are building your string. The StringBuilder class lacks search methods such as IndexOf or StartsWith. You'll have to convert the StringBuilder object to a String for these operations, and this can negate the performance benefit from using StringBuilder. For more information, see the Searching the text in a StringBuilder object section.

Consider using the StringBuilder class under these conditions:

• When you expect your app to make an unknown number of changes to a string at design time (for example, when you are using a loop to concatenate a random number of strings that contain user input).
• When you expect your app to make a significant number of changes to a string.

How StringBuilder works

The StringBuilder.Length property indicates the number of characters the StringBuilder object currently contains. If you add characters to the StringBuilder object, its length increases until it equals the size of the StringBuilder.Capacity property, which defines the number of characters that the object can contain. If the number of added characters causes the length of the StringBuilder object to exceed its current capacity, new memory is allocated, the value of the Capacity property is doubled, new characters are added to the StringBuilder object, and its Length property is adjusted. Additional memory for the StringBuilder object is allocated dynamically until it reaches the value defined by the StringBuilder.MaxCapacity property. When the maximum capacity is reached, no further memory can be allocated for the StringBuilder object, and trying to add characters or expand it beyond its maximum capacity throws either an MfArgumentOutOfRangeException or an MfOutOfMemoryException exception.

The following example illustrates how a StringBuilder object allocates new memory and increases its capacity dynamically as the string assigned to the object expands. The code creates a StringBuilder object by calling its default (parameterless) constructor. The default capacity of this object is 16 characters, and its maximum capacity is more than 2 billion characters. Appending the string "This is a sentence." results in a new memory allocation because the string length (19 characters) exceeds the default capacity of the StringBuilder object. The capacity of the object doubles to 32 characters, the new string is added, and the length of the object now equals 19 characters. The code then appends the string "This is an additional sentence." to the value of the StringBuilder object 11 times. Whenever the append operation causes the length of the StringBuilder object to exceed its capacity, its existing capacity is doubled and the Append operation succeeds.

sb  := new MfText.StringBuilder()
sb1 :=  new MfText.StringBuilder()
SbAppendInfo(sb1, sb)
sb1.Append("This is a sentence.")
SbAppendInfo(sb1, sb)
ctr = 0
While (ctr <= 10)
{
   sb1.Append("This is an additional sentence.")
   SbAppendInfo(sb1, sb)
   ctr++
}

MsgBox % sb.ToString()
/*
Capacity: 16    MaxCapacity: 2147483647    Length: 0
Capacity: 32    MaxCapacity: 2147483647    Length: 19
Capacity: 64    MaxCapacity: 2147483647    Length: 50
Capacity: 128    MaxCapacity: 2147483647    Length: 81
Capacity: 128    MaxCapacity: 2147483647    Length: 112
Capacity: 256    MaxCapacity: 2147483647    Length: 143
Capacity: 256    MaxCapacity: 2147483647    Length: 174
Capacity: 256    MaxCapacity: 2147483647    Length: 205
Capacity: 256    MaxCapacity: 2147483647    Length: 236
Capacity: 512    MaxCapacity: 2147483647    Length: 267
Capacity: 512    MaxCapacity: 2147483647    Length: 298
Capacity: 512    MaxCapacity: 2147483647    Length: 329
Capacity: 512    MaxCapacity: 2147483647    Length: 360

*/
SbAppendInfo(sbIn, sbOut)
{
   sbOut.AppendFormat("Capacity: {0:i}    MaxCapacity: {1:i}    Length: {2:i}", sbIn.Capacity, sbIn.MaxCapacity, sbIn.Length)
   sbOut.AppendLine()
}

Memory allocation

The default capacity of a StringBuilder object is 16 characters, and its default maximum capacity is MfIntInteger.MaxValue. These default values are used if you call the Constructor() and Constructor(value) constructors.

You can explicitly define the initial capacity of a StringBuilder object in the following ways:

• By calling any of the StringBuilder constructors that includes a capacity parameter when you create the object.
• By explicitly assigning a new value to the StringBuilder.Capacity property to expand an existing StringBuilder object. Note that the property throws an exception if the new capacity is less than the existing capacity or greater than the StringBuilder object's maximum capacity.
• By calling the StringBuilder.EnsureCapacity method with the new capacity. The new capacity must not be greater than the StringBuilder object's maximum capacity. However, unlike an assignment to the Capacity property, EnsureCapacity does not throw an exception if the desired new capacity is less than the existing capacity; in this case, the method call has no effect.

If the length of the string assigned to the StringBuilder object in the constructor call exceeds either the default capacity or the specified capacity, the Capacity property is set to the length of the string specified with the value parameter.

You can explicitly define the maximum capacity of a StringBuilder object by calling the Constructor(capacity, maxCapacity) constructor. You can't change the maximum capacity by assigning a new value to the MaxCapacity property, because it is read-only.

As the previous section shows, whenever the existing capacity is inadequate, additional memory is allocated and the capacity of a StringBuilder object doubles up to the value defined by the MaxCapacity property.

In general, the default capacity and maximum capacity are adequate for most apps. You might consider setting these values under the following conditions:

• If the eventual size of the StringBuilder object is likely to grow exceedingly large, typically in excess of several megabytes. In this case, there may be some performance benefit from setting the initial Capacity property to a significantly high value to eliminate the need for too many memory reallocations.
• If your app is running on a system with limited memory. In this case, you may want to consider setting the MaxCapacity property to less than MfIntInteger.MaxValue if your app is handling large strings that may cause it to execute in a memory-constrained environment.

Instantiating a StringBuilder object

You instantiate a StringBuilder object by calling one of its six overloaded class constructors, which are listed in the following table. Three of the constructors instantiate a StringBuilder object whose value is an empty string, but set its Capacity and MaxCapacity values differently. The remaining three constructors define a StringBuilder object that has a specific string value and capacity. Two of the three constructors use the default maximum capacity of Int32.MaxValue, whereas the third allows you to set the maximum capacity.

The following example uses three of these constructor overloads to instantiate StringBuilder objects.

sb := new MfText.StringBuilder()
value := new MfString("An ordinary string")
index := value.IndexOf("An ") + 3
capacity := 0xFFFF

; Instantiate a StringBuilder from a string.
sb1 := new MfText.StringBuilder(value)
SbAppendInfo(sb1, sb)

; Instantiate a StringBuilder from string and define a capacity.
sb2 := new MfText.StringBuilder(value, capacity)
SbAppendInfo(sb2, sb)

; Instantiate a StringBuilder from substring and define a capacity.
sb3 := new MfText.StringBuilder(value, index
                               ,value.Length - index
                               ,capacity)
SbAppendInfo(sb3, sb)

MsgBox % sb.ToString()
/*
Value: An ordinary string
Capacity: 18    MaxCapacity: 2147483647    Length: 18

Value: An ordinary string
Capacity: 65535    MaxCapacity: 2147483647    Length: 18

Value: ordinary string
Capacity: 65535    MaxCapacity: 2147483647    Length: 15


*/

SbAppendInfo(sbIn, sbOut)
{
   sbOut.AppendFormat("Value: {0}{1}", sbIn, MfEnvironment.Instance.NewLine)
   sbOut.AppendFormat("Capacity: {0:i}    MaxCapacity: {1:i}    Length: {2:i}", sbIn.Capacity, sbIn.MaxCapacity, sbIn.Length)
   sbOut.AppendLine()
   sbOut.AppendLine()
}

Calling StringBuilder methods

Most of the methods that modify the string in a StringBuilder instance return a reference to that same instance. This enables you to call StringBuilder methods in two ways:

1) You can make individual method calls and ignore the return value, as the following example does.

sb := new MfText.StringBuilder()
sb.Append("This is the beginning of a sentence, ")
sb.Replace("the beginning of ", "")
sb.Insert(sb.ToString(true).IndexOf("a ") + 2, "complete ")
sb.Replace(",", ".")
MsgBox % sb.ToString() ; This is a complete sentence.

2) You can make a series of method calls in a single statement. This can be convenient if you want to write a single statement that chains successive operations. The following example consolidates three method calls from the previous example into a single line of code.

sb := new MfText.StringBuilder()
MsgBox % sb.Append("This is the beginning of a sentence, ")
 .Replace("the beginning of ", "")
 .Insert(sb.ToString(true).IndexOf("a ") + 2, "complete ").Replace(",", ".")
 .ToString() ; This is a complete sentence.

Performing StringBuilder operations

You can use the methods of the StringBuilder class to iterate, add, delete, or modify characters in a StringBuilder object.

Iterating StringBuilder characters

You can access the characters in a StringBuilder object by using the StringBuilder.Chars property. This enables you to set or retrieve individual characters by using their index only, without explicitly referencing the Chars property. Characters in a StringBuilder object begin at index 0 (zero) and continue to index Length - 1.

The following example illustrates the Chars property. It appends ten random numbers to a StringBuilder object, and then iterates each character. If the character's Unicode category is MfUnicodeCategory.DecimalDigitNumber, it decreases the number by 1 (or changes the number to 9 if its value is 0). The example displays the contents of the StringBuilder object both before and after the values of individual characters were changed.

sb  := new MfText.StringBuilder()
sb1 := new MfText.StringBuilder()

; Generate 10 random numbers and store them in a StringBuilder.
ctr := 0
While (ctr <= 9)
{
   sb1.Append(Mfunc.Random()).Append(",")
   ctr++
}
sb1.Length-- ; remove last ,

sb.AppendLine("The original string:")
sb.AppendLine(sb1.ToString())

; Decrease each number by one.
ctr := 0
While (ctr < sb1.Length)
{
   if (MfChar.GetUnicodeCategory(sb1.Chars[ctr]).Equals(MfUnicodeCategory.Instance.DecimalDigitNumber))
   {
       number := MfChar.GetNumericValue(sb1.Chars[ctr])
       number--
       if (number < 0)
           number := 9
       sb1.Chars[ctr] := new MfString(number, true).Index[0]
   }
   ctr++
}

sb.AppendLine("The new string:")
sb.AppendLine(sb1.ToString())

MsgBox % sb.ToString()
/*
The original string:
26850403,107280301,1550505509,841162532,623723528,1192182565,181914076,799185027,1940502902,2107953480
The new string:
15749392,096179290,0449494498,730051421,512612417,0081071454,070803965,688074916,0839491891,1096842379

*/

Adding text to a StringBuilder object

The StringBuilder class includes the following methods for expanding the contents of a StringBuilder object:

• The Append method appends a string, a substring, a character array, a portion of a character array, a single character repeated multiple times, or the string representation of a primitive data type to a StringBuilder object.
• The AppendLine method appends a line terminator or a string along with a line terminator to a StringBuilder object.
• The AppendFormat method appends a to a StringBuilder object. The string representations of objects included in the result string can reflect the formatting conventions of the current system culture or a specified culture.
• The Insert method inserts a string, a substring, multiple repetitions of a string, a character array, a portion of a character array, or the string representation of a primitive data type at a specified position in the StringBuilder object. The position is defined by a zero-based index.

sb  := new MfText.StringBuilder()
sb1 := new MfText.StringBuilder()

sb1.Append("*", 10).Append(" Adding Text to a StringBuilder Object ").Append("*", 10)
sb1.AppendLine("`r`n")
sb1.AppendLine("Some code points and their corresponding characters:")

;Append some formatted text.
ctr := 50
While (ctr < 60)
{
    sb1.AppendFormat("{0:#12X} {1:12c}", ctr, ctr)
    sb1.AppendLine()
    ctr++
}
pos := sb1.ToString(true).IndexOf("characters:") + 11 + StrLen(MfEnvironment.Instance.NewLine)

; Insert a column header.
sb1.Insert(pos, MfString.Format("{2}{0:12} {1:12}{2}", "Code Unit", "Character", MfEnvironment.Instance.NewLine))
sb.Append(sb1.ToString())
MsgBox % sb.ToString()
/*
********** Adding Text to a StringBuilder Object **********

Some code points and their corresponding characters:

  Code Unit    Character
       0X32            2
       0X33            3
       0X34            4
       0X35            5
       0X36            6
       0X37            7
       0X38            8
       0X39            9
       0X3A            :
       0X3B            ;

*/

Deleting text from a StringBuilder object

The StringBuilder class includes methods that can reduce the size of the current StringBuilder instance. The Clear method removes all characters and sets the Length property to zero. The Remove method deletes a specified number of characters starting at a particular index position. In addition, you can remove characters from the end of a StringBuilder object by setting its Length property to a value that is less than the length of the current instance.

The following example removes some of the text from a StringBuilder object, displays its resulting capacity, maximum capacity, and length property values, and then calls the Clear method to remove all the characters from the StringBuilder object.

sb  := new MfText.StringBuilder()
sb1 := new MfText.StringBuilder("A StringBuilder object")

SbInfo(sb1, sb)

; Remove "object" from the text.
textToRemove := new MfString("object")
pos := sb1.ToString(true).IndexOf(textToRemove)
if (pos >= 0) {
   sb1.Remove(pos, textToRemove.Length)
   SbInfo(sb1, sb)
}

; Clear the StringBuilder contents.
sb1.Clear()
SbInfo(sb1, sb)

MsgBox % sb.ToString()
/*
Value: A StringBuilder object
Capacity: 22    MaxCapacity: 2147483647    Length: 22

Value: A StringBuilder
Capacity: 22    MaxCapacity: 2147483647    Length: 16

Value:
Capacity: 22    MaxCapacity: 2147483647    Length: 0
*/

SbInfo(sbIn, sbOut)
{
   sbOut.AppendFormat("Value: {0}{1}", sbIn, MfEnvironment.Instance.NewLine)
   sbOut.AppendFormat("Capacity: {0:i}    MaxCapacity: {1:i}    Length: {2:i}", sbIn.Capacity, sbIn.MaxCapacity, sbIn.Length)
   sbOut.AppendLine()
   sbOut.AppendLine()
}

Modifying the text in a StringBuilder object

The StringBuilder.Replace method replaces all occurrences of a character or a string in the entire StringBuilder object or in a particular character range. The following example uses the Replace method to replace all exclamation points (!) with question marks (?) in the StringBuilder object.

sb := new MfText.StringBuilder("Hello World!")
sb.Replace("!", "?")
MsgBox % sb.ToString() ; Hello World?

Searching the text in a StringBuilder object

The StringBuilder class does not include methods similar to the MfString.Contains, MFString.IndexOf, and MfString.StartsWith methods provided by the MfString class, which allow you to search the object for a particular character or a substring. Determining the presence or starting character position of a substring requires that you search a String value by using either a string search method or a regular expression method. There are four ways to implement such searches, as the following table shows.

Example

sb := new MfText.StringBuilder()
sb1 := new MfText.StringBuilder("ABC", 50)

; Append three characters (D, E, and F) to the end of the StringBuilder.
lst := MfCharList.FromString("DEF")
sb1.Append(lst)

; Append a format string to the end of the StringBuilder.
sb1.AppendFormat("GHI{0}{1}", "J", MFChar.FromCharCode(107))

sb.AppendLine(MfString.Format("{0} chars: {1}", sb1.Length, sb1.ToString()))

; Insert a string at the beginning of the StringBuilder.
sb1.Insert(0, "Alphabet: ")

; Replace all lowercase k's with uppercase K's.
sb1.Replace("k", "K")

sb.Append(sb1.ToString())
MsgBox % sb.ToString()
/*
11 chars: ABCDEFGHIJk
Alphabet: ABCDEFGHIJK
*/