What is a superfast way to read large files line-b

问题:

I believe I have come up with a very efficient way to read very, very large files line-by-line. Please tell me if you know of a better/faster way or see room for improvement. I am trying to get better at coding, so any sort of advice you have would be nice. Hopefully this is something that other people might find useful, too.

It appears to be something like 8 times faster than using Line Input from my tests.

'This function reads a file into a string.                        '
'I found this in the book Programming Excel with VBA and .NET.    '
Public Function QuickRead(FName As String) As String
    Dim I As Integer
    Dim res As String
    Dim l As Long

    I = FreeFile
    l = FileLen(FName)
    res = Space(l)
    Open FName For Binary Access Read As #I
    Get #I, , res
    Close I
    QuickRead = res
End Function

'This function works like the Line Input statement'
Public Sub QRLineInput( _
    ByRef strFileData As String, _
    ByRef lngFilePosition As Long, _
    ByRef strOutputString, _
    ByRef blnEOF As Boolean _
    )
    On Error GoTo LastLine
    strOutputString = Mid$(strFileData, lngFilePosition, _
        InStr(lngFilePosition, strFileData, vbNewLine) - lngFilePosition)
    lngFilePosition = InStr(lngFilePosition, strFileData, vbNewLine) + 2
    Exit Sub
LastLine:
    blnEOF = True
End Sub

Sub Test()
    Dim strFilePathName As String: strFilePathName = "C:\Fld\File.txt"
    Dim strFile As String
    Dim lngPos As Long
    Dim blnEOF As Boolean
    Dim strFileLine As String

    strFile = QuickRead(strFilePathName) & vbNewLine
    lngPos = 1

    Do Until blnEOF
        Call QRLineInput(strFile, lngPos, strFileLine, blnEOF)
    Loop
End Sub

Thanks for the advice!

回答1:

You can use Scripting.FileSystemObject to do that thing. From the Reference:

The ReadLine method allows a script to read individual lines in a text file. To use this method, open the text file, and then set up a Do Loop that continues until the AtEndOfStream property is True. (This simply means that you have reached the end of the file.) Within the Do Loop, call the ReadLine method, store the contents of the first line in a variable, and then perform some action. When the script loops around, it will automatically drop down a line and read the second line of the file into the variable. This will continue until each line has been read (or until the script specifically exits the loop).

And a quick example:

Set objFSO = CreateObject("Scripting.FileSystemObject")
Set objFile = objFSO.OpenTextFile("C:\FSO\ServerList.txt", 1)
Do Until objFile.AtEndOfStream
 strLine = objFile.ReadLine
 MsgBox strLine
Loop
objFile.Close

回答2:

My two cents…

Not long ago I needed reading large files using VBA and noticed this question. I tested the three approaches to read data from a file to compare its speed and reliability for a wide range of file sizes and line lengths. The approaches are:

1. Line Input VBA statement
2. Using the File System Object (FSO)
3. Using Get VBA statement for the whole file and then parsing the string read as described in posts here

Each test case consists of three steps:

1. Test case setup that writes a text file containing given number of lines of the same given length filled by the known character pattern.
2. Integrity test. Read each file line and verify its length and contents.
3. File read speed test. Read each line of the file repeated 10 times.

As you can notice, Step #3 verifies the true file read speed (as asked in the question) while Step #2 verifies the file read integrity and therefore simulates real conditions when string parsing is needed.

The following chart shows the test results for the File read speed test. The file size is 64M bytes for all tests, and the tests differ in line length that varies from 2 bytes (not including CRLF) to 8M bytes.

CONCLUSION:

1. All the three methods are reliable for large files with normal and abnormal line lengths (please compare to Graeme Howard’s answer)
2. All the three methods produce almost equivalent file reading speed for normal line lengths
3. “Superfast way” (Method #3) works fine for extremely long lines while the other two don’t.
4. All this is applicable to different Offices, different PCs, for VBA and VB6

回答3:

Line Input works fine for small files. However, when file sizes reach around 90k, Line Input jumps all over the place and reads data in the wrong order from the source file. I tested it with different filesizes:

49k = ok
60k = ok
78k = ok
85k = ok
93k = error
101k = error
127k = error
156k = error

Lesson learned - use Scripting.FileSystemObject

回答4:

With that code you load the file in memory (as a big string) and then you read that string line by line.

By using Mid$() and InStr() you actually read the "file" twice but since it's in memory, there is no problem.
I don't know if VB's String has a length limit (probably not) but if the text files are hundreds of megabyte in size it's likely to see a performance drop, due to virtual memory usage.

回答5:

I would think , in a large file scenario using a stream would be far more efficient, because memory consumption would be very small.

But your algorithm could alternate between using a stream and loading the entire thing in memory based on the file size. I wouldn't be surprised if one is only better than the other under certain criteria.

回答6:

'you can modify above and read full file in one go and then display each line as shown below

Option Explicit

Public Function QuickRead(FName As String) As Variant
    Dim i As Integer
    Dim res As String
    Dim l As Long
    Dim v As Variant

    i = FreeFile
    l = FileLen(FName)
    res = Space(l)
    Open FName For Binary Access Read As #i
    Get #i, , res
    Close i
    'split the file with vbcrlf
    QuickRead = Split(res, vbCrLf)
End Function

Sub Test()
    ' you can replace file for "c:\writename.txt to any file name you desire
    Dim strFilePathName As String: strFilePathName = "C:\writename.txt"
    Dim strFileLine As String
    Dim v As Variant
    Dim i As Long
    v = QuickRead(strFilePathName)
    For i = 0 To UBound(v)
        MsgBox v(i)
    Next
End Sub

回答7:

My take on it...obviously, you've got to do something with the data you read in. If it involves writing it to the sheet, that'll be deadly slow with a normal For Loop. I came up with the following based upon a rehash of some of the items there, plus some help from the Chip Pearson website.

Reading in the text file (assuming you don't know the length of the range it will create, so only the startingCell is given):

Public Sub ReadInPlainText(startCell As Range, Optional textfilename As Variant)

   If IsMissing(textfilename) Then textfilename = Application.GetOpenFilename("All Files (*.*), *.*", , "Select Text File to Read")
   If textfilename = "" Then Exit Sub

   Dim filelength As Long
   Dim filenumber As Integer
   filenumber = FreeFile
   filelength = filelen(textfilename)
   Dim text As String
   Dim textlines As Variant

   Open textfilename For Binary Access Read As filenumber

   text = Space(filelength)
   Get #filenumber, , text

   'split the file with vbcrlf
   textlines = Split(text, vbCrLf) 

   'output to range
   Dim outputRange As Range
   Set outputRange = startCell
   Set outputRange = outputRange.Resize(UBound(textlines), 1)
   outputRange.Value = Application.Transpose(textlines)

   Close filenumber
 End Sub

Conversely, if you need to write out a range to a text file, this does it quickly in one print statement (note: the file 'Open' type here is in text mode, not binary..unlike the read routine above).

Public Sub WriteRangeAsPlainText(ExportRange As Range, Optional textfilename As Variant)
   If IsMissing(textfilename) Then textfilename = Application.GetSaveAsFilename(FileFilter:="Text Files (*.txt), *.txt")
   If textfilename = "" Then Exit Sub

   Dim filenumber As Integer
   filenumber = FreeFile
   Open textfilename For Output As filenumber

   Dim textlines() As Variant, outputvar As Variant

   textlines = Application.Transpose(ExportRange.Value)
   outputvar = Join(textlines, vbCrLf)
   Print #filenumber, outputvar
   Close filenumber
End Sub

回答8:

Be careful when using Application.Transpose with a huge number of values. If you transpose values to a column, excel will assume you are assuming you transposed them from rows.

---

Max Column Limit < Max Row Limit, and it will only display the first (Max Column Limit) values, and anithing after that will be "N/A"

回答9:

I just wanted to share some of my results...

I have text files, which apparently came from a Linux system, so I only have a vbLF/Chr(10) at the end of each line and not vbCR/Chr(13).

Note 1:

• This meant that the Line Input method would read in the entire file, instead of just one line at a time.

From my research testing small (152KB) & large (2778LB) files, both on and off the network I found the following:

Open FileName For Input: Line Input was the slowest (See Note 1 above)

Open FileName For Binary Access Read: Input was the fastest for reading the whole file

FSO.OpenTextFile: ReadLine was fast, but a bit slower then Binary Input

Note 2:

• If I just needed to check the file header (first 1-2 lines) to check if I had the proper file/format, then FSO.OpenTextFile was the fastest, followed very closely by Binary Input.

• The drawback with the Binary Input is that you have to know how many characters you want to read.

• On normal files, Line Input would also be a good option as well, but I couldn't test due to Note 1.

 

Note 3:

• Obviously, the files on the network showed the largest difference in read speed. They also showed the greatest benefit from reading the file a second time (although there are certainly memory buffers that come into play here).