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!
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
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.
'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
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"
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
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:
Line Input
VBA statementGet
VBA statement for the whole file and then parsing the string read as described in posts hereEach test case consists of three steps:
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:
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 byBinary 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).
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
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.
Source: Stackoverflow.com