Vba excel повторить символ

VBA has built-in functions for repeating a single character:

Function String$(Number As Long, Character) As String

Function Space$(Number As Long) As String

But neither are of any use when you need to repeat a string that has more than one character.

You could repeat a string "abcde" 5 times by doing something crafty like:

?Join(Split(String$(5,"."),"."),"abcde")

But that is neither intuitive nor performant.

In Excel, there is also WorksheetFunction.Rept, but it is painfully slow, and only available in Excel.

So I made a custom function that builds the string, while minimizing the concatenations. In fact, it doesn’t use any concatenation, but instead uses a buffer and CopyMemory to fill the buffer. And rather than filling the buffer one instance at a time, the code fills the buffer using a lookback that reduces the number of buffer writes exponentially:

Given a string "abcde" that repeats 5 times:

Create a buffer of 25 spaces

"                         "

1st buffer write — assign the string to the first buffer position

"abcde                    "
 [NEW]

2nd buffer write — copy the existing populated buffer (5 characters) into the next buffer position

"abcdeabcde               "
      [NEW]

3rd buffer write — copy the existing populated buffer (10 characters) into the next buffer position

"abcdeabcdeabcdeabcde     "
           [  NEW   ]`

4th buffer write — copy the lesser of the existing populated buffer (20 characters) and the remaining buffer (5 characters) into the next buffer position.

"abcdeabcdeabcdeabcdeabcde"
                     [NEW]

StringRepeat

Private Declare Sub CopyMemory Lib "kernel32.dll" Alias "RtlMoveMemory" (ByVal Destination As Long, ByVal source As Long, ByVal Length As Long)

Public Function StringRepeat(number As Long, expression As String) As String

  Dim copyBufferLength As Long
  copyBufferLength = LenB(expression)

  'Create a buffer
  StringRepeat = Space$(number * Len(expression))

  Dim bufferLengthBytes As Long
  bufferLengthBytes = LenB(StringRepeat)

  Dim bufferPointer As Long
  bufferPointer = StrPtr(StringRepeat)

  'Copy the original expression to the start of the buffer
  CopyMemory bufferPointer, StrPtr(expression), copyBufferLength

  Do While copyBufferLength < bufferLengthBytes
    Dim remainingByteCount As Long
    'Check we're not going to overflow the buffer
    remainingByteCount = bufferLengthBytes - copyBufferLength
    If copyBufferLength > remainingByteCount Then
      CopyMemory bufferPointer + copyBufferLength, bufferPointer, remainingByteCount
    Else
      CopyMemory bufferPointer + copyBufferLength, bufferPointer, copyBufferLength
    End If
    copyBufferLength = copyBufferLength * 2
  Loop

End Function

The performance varies by the number of repeats, and the number of characters in the string to be repeated. I tried handling special cases like repeating a string 1 time (just return the string), and/or repeating a single character (return the result of String$ instead), but while that speeds up the special cases, it slows down all other cases.

I’m not checking whether the number input is positive, and I’m not checking that the string to repeat is at least 1 character long, as for now, I’m focusing on performance.

In some instances (small values of number, short expression lengths), avoiding the Exponential lookback approach is not as fast as a straight-up loop and copy:

RepeatString Simple

Function StringRepeatSimple(number As Long, expression As String) As String

  Dim expressionLengthBytes As Long
  expressionLengthBytes = LenB(expression)

  'Create a buffer
  StringRepeatSimple= Space$(number * Len(expression))

  Dim bufferPointer As Long
  bufferPointer = StrPtr(StringRepeatSimple)

  Dim expressionPointer As Long
  expressionPointer = StrPtr(expression)

  Dim copyCounter As Long
  For copyCounter = 0 To number - 1
    CopyMemory bufferPointer + copyCounter * expressionLengthBytes, expressionPointer, expressionLengthBytes
  Next copyCounter 

End Function

String

String(Number,Character)

Функция String используется для создания строки из одинаковых символов

Возвращаемое значение

Возвращает значение типа Variant (String), содержащее повторяющуюся строку указанной длины

Параметры

Функция содержит именованные аргументы

Number

Обязательный аргумент — число типа Long, определяющее длину возвращаемой строки. Если аргумент имеет значение Null, то возвращаемое значение тоже Null

Character

Обязательный аргумент — значение типа Variant. Код символа или строковое выражение, первый символ которого используется при создании возвращаемой строки. Если аргумент имеет значение Null, возвращается значение Null. Значения аргумента, превышающие 255, преобразуются функцией String в допустимые коды символов по следующей формуле:

character Mod 256

Пример

' В данном примере функция String
' используется для генерации строк,
' содержащих указанное число
' повторяющихся символов

Dim MyString
MyString = String(3, "M") ' Возвращает "MMM"
MyString = String(5, 65) ' Возвращает "ААААА".Возвращает 5 раз символ,код которого 65
MyString = String(6, "Русский_Проект") ' Возвращает "РРРРРР".Повторяет 6 раз первый символ исходной строки

Категория
Функции обработки строк

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