Difference between revisions of "String Functions and Manipulation in VB6"

Latest revision as of 16:46, 15 February 2008

String Functions and Manipulation in VB6 including common functions such as Asc, Chr, Mid, Join, LSet, StrConv, StrReverse, Filter, Replace

Functions and Description

Asc (function)

Returns an Integer representing the character code corresponding to the first letter in a string.

Syntax

Asc(string)

Chr (function)

Returns a String containing the character associated with the specified character code.

Syntax

Chr(charcode)

InStr (function)

Returns a Variant (Long) specifying the position of the first occurrence of one string within another.

Syntax

InStr([start, ]string1, string2[, compare])

Example

Dim SearchString, SearchChar, MyPos
SearchString ="XXpXXpXXPXXP"   ' String to search in.
SearchChar = "P"   ' Search for "P".

' A textual comparison starting at position 4. Returns 6.
MyPos = Instr(4, SearchString, SearchChar, 1)    

' A binary comparison starting at position 1. Returns 9.
MyPos = Instr(1, SearchString, SearchChar, 0)

' Comparison is binary by default (last argument is omitted).
MyPos = Instr(SearchString, SearchChar)   ' Returns 9.

MyPos = Instr(1, SearchString, "W")   ' Returns 0.

InStrRev (function)

New in VB6.

Similar to InStr but searches from end of string. Returns the position of an occurrence of one string within another, from the end of string.

Syntax

InstrRev(stringcheck, stringmatch[, start[, compare]])

Join (function)

New in VB6

Used to join arrays elements.

Mid (function)

Returns a Variant (String) containing a specified number of characters from a string.

Syntax

Mid(string, start[, length])

Replace (function)

New in VB6.

Returns a string in which a specified substring has been replaced with another substring a specified number of times.

Syntax

Replace(expression, find, replace[, start[, count[, compare]]])

Example

Debug.print Replace("abababa", "a", "")

Example

public Function StripOut(From as string, What as string) as string
  Dim i as Integer
  StripOut = From
  for i = 1 to len(What)
    StripOut = Replace(StripOut, mid$(What, i, 1), "")
  next i
End Function

Reverse (function)

New in VB6.

To reverse a string.

Split (function)

New in VB6

Split a string into a variant array.

StrComp (function)

Returns a Variant (Integer) indicating the result of a string comparison.

Syntax

StrComp(string1, string2[, compare])

Return Values

If                                  StrComp returns
string1 is less than string2        -1
string1 is equal to string2 	     0
string1 is greater than string2     1
string1 or string2 is Null          Null

Performance Rating

table 1

How to read the table: Calling Len takes 1 unit of time, while Asc takes 7 units. You can call Len 7 times in the same time you call Asc once.

table 2

Reference: http://www.aivosto.com/vbtips/stringopt2.html

Null, 0, and Empty Strings

In VB, Null is a subtype of a Variant, in the same way the Empty is also. In other words, Null and Empty are both special states of a Variant variable.

vbNull and vbEmpty are the codes returned from the VarType function when applied to a value of that type, e.g.

   VarType(Empty) = vbEmpty
   VarType(Null) = vbNull

This provides one method of testing whether a value is Null or Empty. The other is by calling the IsNull and IsEmpty functions.

In a database, NULL has a similar status to Null is VB Variants, although it's defined by the SQL standard rather than the VB language.

Use the NULL keyword to assign Null to a recordset field, use the IsNull function to test if a recordset fields contains Null.

• vbNullString: The same as "", as in an empty string. It is more efficient to use vbNullString though. use it instead of ""

• vbNull: This is supposed to be NULL but actually equals 1.

• vbNullChar:

• Null: Used for writing NULL, however, will appear as a 0 in a database numeric field after write.

• Empty: Also appears as a 0 in a database numeric field after write.

• Nothing:

The Differences among Empty, Nothing, vbNull, vbNullChar, vbNullString and the Zero-Length String

quoted from somewhere:

"": A zero-length string (commonly called an "empty string") is technically a zero-length BSTR that actually uses six bytes of memory. In general, you should use the constant vbNullString instead, particularly when calling external DLL procedures.

Empty: A variant of VarType 0 (vbEmpty) that has not yet been initialized. Test whether it is "nil" using the IsEmpty function.

Nothing: Destroys an object reference using the Set statement. Test whether it is "nil" using the Is operator:

If obj Is Nothing Then...

Null: A variant of VarType 1 (vbNull) that means "no valid data" and generally indicates a database field with no value. Don't confuse this with a C NULL, which indicates zero. Test whether it is "nil" using the IsNull function.

vbNullChar: A character having a value of zero. It is commonly used for adding a C NULL to a string or for filling a fixed-length string with zeroes:

Path = String(255, vbNullChar)

vbNullString: A string having a value of zero, such as a C NULL, that takes no memory. Use this string for calling external procedures looking for a null pointer to a string. To distinguish between vbNullString and "", use the VBA StrPtr function: StrPtr(vbNullString) is zero, while StrPtr("") is a nonzero memory address. [Note: the StrPtr() function may not be a part of the VBA members shown in the Object Browser so it will not AutoComplete. Enter the following line in the Immediate Window to determine if this function is available: ?StrPtr(vbNullString). This should return 0 immediately.]