How do i parse a text file in c#.

Question

How do i parse a text file in c#?

Answer 1

If you have more than a trivial language, use a parser generator. It drove me nuts but I've heard good things about ANTLR (Note: get the manual and read it before you start. If you have used a parser generator other than it before you will not approach it correctly right off the bat, at least I didn't)

Other tools also exist.

Answer 2

Without really knowing what sort of text file you're on about, its hard to answer. However, the FileHelpers library has a broad set of tools to help with fixed length file formats, multirecord, delimited etc.

Answer 3

What do you mean by parse? Parse usually means to split the input into tokens, which you might do if you're trying to implement a programming language. If you're just wanting to read the contents of a text file, look at System.IO.FileInfo.

Answer 4

The algorithm might look like this:

1. Open Text File
2. For every line in the file:
3. Parse Line

There are several approaches to parsing a line.

The easiest from a beginner standpoint is to use the String methods.

System.String at MSDN

If you are up for more of a challenge, then you can use the System.Text.RegularExpression library to parse your text.

RegEx at MSDN

Answer 5

Check this interesting approach, Linq To Text Files, very nice, you only need a IEnumerable<string> method, that yields every file.ReadLine(), and you do the query.

Here is another article that better explains the same technique.

Answer 6

            using (TextReader rdr = new StreamReader(fullFilePath))
            {
                string line;

                while ((line = rdr.ReadLine()) != null)
                {
            // use line here
                }
            }

set the variable "fullFilePath" to the full path eg. C:\temp\myTextFile.txt

Answer 7

A small improvement on Pero's answer:

FileInfo txtFile = new FileInfo("c:\myfile.txt");
if(!txtFile.Exists) { // error handling }

using (TextReader rdr = txtFile.OpenText())
{
     // use the text file as Pero suggested
}

The FileInfo class gives you the opportunity to "do stuff" with the file before you actually start reading from it. You can also pass it around between functions as a better abstraction of the file's location (rather than using the full path string). FileInfo canonicalizes the path so it's absolutely correct (e.g. turning / into \ where appropriate) and lets you extract extra data about the file -- parent directory, extension, name only, permissions, etc.

Answer 8

To begin with, make sure that you have the following namespaces:

using System.Data; using System.IO; using System.Text.RegularExpressions;

Next, we build a function that parses any CSV input string into a DataTable:

public DataTable ParseCSV(string inputString) {

DataTable dt=new DataTable();

// declare the Regular Expression that will match versus the input string Regex re=new Regex("((?[^\",\r\n]+)|\"(?([^\"]|\"\")+)\")(,|(?\r\n|\n|$))");

ArrayList colArray=new ArrayList(); ArrayList rowArray=new ArrayList();

int colCount=0; int maxColCount=0; string rowbreak=""; string field="";

MatchCollection mc=re.Matches(inputString);

foreach(Match m in mc) {

// retrieve the field and replace two double-quotes with a single double-quote
field=m.Result("${field}").Replace("\"\"","\"");

rowbreak=m.Result("${rowbreak}");

if (field.Length > 0) {
  colArray.Add(field);      
  colCount++;
}

if (rowbreak.Length > 0) {

  // add the column array to the row Array List
  rowArray.Add(colArray.ToArray());

  // create a new Array List to hold the field values
  colArray=new ArrayList(); 

  if (colCount > maxColCount)
    maxColCount=colCount;

  colCount=0;
}

}

if (rowbreak.Length == 0) { // this is executed when the last line doesn't // end with a line break rowArray.Add(colArray.ToArray()); if (colCount > maxColCount) maxColCount=colCount; }

// create the columns for the table for(int i=0; i < maxColCount; i++) dt.Columns.Add(String.Format("col{0:000}",i));

// convert the row Array List into an Array object for easier access Array ra=rowArray.ToArray(); for(int i=0; i < ra.Length; i++) {

// create a new DataRow
DataRow dr=dt.NewRow();

// convert the column Array List into an Array object for easier access
Array ca=(Array)(ra.GetValue(i));      

// add each field into the new DataRow
for(int j=0; j < ca.Length; j++)
  dr[j]=ca.GetValue(j);

// add the new DataRow to the DataTable
dt.Rows.Add(dr);

}

// in case no data was parsed, create a single column if (dt.Columns.Count == 0) dt.Columns.Add("NoData");

return dt; }

Now that we have a parser for converting a string into a DataTable, all we need now is a function that will read the content from a CSV file and pass it to our ParseCSV function:

public DataTable ParseCSVFile(string path) {

string inputString="";

// check that the file exists before opening it if (File.Exists(path)) {

StreamReader sr = new StreamReader(path);
inputString = sr.ReadToEnd();
sr.Close();

}

return ParseCSV(inputString); }

And now you can easily fill a DataGrid with data coming off the CSV file:

protected System.Web.UI.WebControls.DataGrid DataGrid1;

private void Page_Load(object sender, System.EventArgs e) {

// call the parser DataTable dt=ParseCSVFile(Server.MapPath("./demo.csv"));

// bind the resulting DataTable to a DataGrid Web Control DataGrid1.DataSource=dt; DataGrid1.DataBind(); }

Congratulations! You are now able to parse CSV into a DataTable. Good luck with your programming.

Sanjay Manju Suman [email protected] www.wix.com/sanjaysumantera/sanju