Asc Function

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

Syntax

Asc(string)

Parameters

• string - Required. Any valid string expression. If the string contains no characters, a run-time error occurs.

Return Value

Returns an Integer representing the ANSI character code of the first character in the string.

• For ANSI characters (0-127), returns standard ASCII values
• For extended ANSI characters (128-255), returns extended ASCII values
• Unicode characters are converted to ANSI before the code is returned

Remarks

The Asc function returns the numeric ANSI character code for the first character in a string. This is useful for: - Validating input characters - Performing character-based operations - Converting characters to their numeric representations - Character range checking

Important Notes

1. Only First Character: Only the first character of the string is examined
2. Empty String Error: Passing an empty string results in a run-time error (Error 5: Invalid procedure call or argument)
3. ANSI vs. Unicode: In VB6, Asc returns ANSI codes; AscW returns Unicode values
4. Return Type: Returns Integer (16-bit signed), range -32,768 to 32,767, but character codes are 0-255
5. Case Sensitive: Upper and lowercase letters have different codes (e.g., "A" = 65, "a" = 97)

Character Code Ranges

• 0-31: Control characters (non-printable)
• 32: Space
• 48-57: Digits '0' through '9'
• 65-90: Uppercase letters 'A' through 'Z'
• 97-122: Lowercase letters 'a' through 'z'
• 128-255: Extended ANSI characters

Examples

Basic Usage

Dim code As Integer
code = Asc("A")          ' Returns 65
code = Asc("Apple")      ' Returns 65 (first character only)
code = Asc("a")          ' Returns 97
code = Asc("0")          ' Returns 48
code = Asc(" ")          ' Returns 32 (space)

Character Validation

Function IsDigit(ch As String) As Boolean
Dim code As Integer
code = Asc(ch)
IsDigit = (code >= 48 And code <= 57)
End Function

Function IsUpperCase(ch As String) As Boolean
Dim code As Integer
code = Asc(ch)
IsUpperCase = (code >= 65 And code <= 90)
End Function

Case Conversion Offset

' Calculate offset between upper and lower case
Dim offset As Integer
offset = Asc("a") - Asc("A")  ' Returns 32

Character Range Checking

Function IsPrintable(ch As String) As Boolean
Dim code As Integer
code = Asc(ch)
IsPrintable = (code >= 32 And code <= 126)
End Function

String Encoding

Function EncodeString(s As String) As String
Dim i As Integer
Dim result As String
For i = 1 To Len(s)
If result <> "" Then result = result & ","
result = result & CStr(Asc(Mid(s, i, 1)))
Next i
EncodeString = result
End Function

Common Patterns

1. Input Validation

If Asc(userInput) >= 48 And Asc(userInput) <= 57 Then
' First character is a digit
End If

2. Alphabetic Checking

Dim code As Integer
code = Asc(UCase(letter))
If code >= 65 And code <= 90 Then
' It's a letter
End If

3. CSV Parsing Helper

If Asc(field) = 34 Then  ' 34 is double quote
' Handle quoted field
End If

4. Character Comparison

If Asc(char1) < Asc(char2) Then
' char1 comes before char2 in ASCII order
End If

5. Special Character Detection

Select Case Asc(ch)
Case 9      ' Tab
Case 10     ' Line feed
Case 13     ' Carriage return
Case 32     ' Space
End Select

6. Keyboard Input Processing

Private Sub Text1_KeyPress(KeyAscii As Integer)
If KeyAscii = 13 Then  ' Enter key
' Process input
End If
End Sub

7. Character Class Testing

Function IsControl(ch As String) As Boolean
Dim code As Integer
code = Asc(ch)
IsControl = (code < 32 Or code = 127)
End Function

8. Simple Encryption

Function ROT13Char(ch As String) As String
Dim code As Integer
code = Asc(UCase(ch))
If code >= 65 And code <= 90 Then
code = ((code - 65 + 13) Mod 26) + 65
ROT13Char = Chr(code)
Else
ROT13Char = ch
End If
End Function

Common Character Codes

Error Handling

On Error Resume Next
code = Asc(inputString)
If Err.Number = 5 Then
' Empty string error
MsgBox "String cannot be empty"
End If

Related Functions

• AscB: Returns the first byte of a string
• AscW: Returns the Unicode character code
• Chr: Returns the character for a given character code (inverse of Asc)
• ChrB: Returns a byte containing the character
• ChrW: Returns a Unicode character
• InStr: Finds the position of a character in a string
• Mid: Extracts a substring
• Left: Gets leftmost characters
• Right: Gets rightmost characters

Performance Notes

• Asc is a very fast operation (direct character code lookup)
• More efficient than string comparison for single character checks
• Use Asc for character-based validation instead of multiple string comparisons
• In tight loops, cache Asc results if checking the same character repeatedly

Parsing Notes

The Asc function is not a reserved keyword in VB6. It is parsed as a regular function call (CallExpression). This module exists primarily for documentation purposes and to provide a comprehensive test suite that validates the parser correctly handles Asc function calls in various contexts.