How do I read a specified line in a text file?

Question

Given a text file, how would I go about reading an arbitrary line and nothing else in the file?

Say, I have a file test.txt. How would I go about reading line number 15 in the file?

All I've seen is stuff involving storing the entire text file as a String array and then using the value of the line number as the number of the String to use from the array... but there are some complications: The text file is enormously huge and the machine that the application I'm coding isn't exactly a top-notch system. Speed isn't the top priority, but it is definitely a major issue.

Are there any ways to ONLY read a specific line of a text file and store the result as a string?

Thanks for your responses: The file is KINDA structured. It's got 25 lines of info and then X lines of numbers but line 17 of the first 25 has the value of X.

But then, there's 1 blank line and it repeats itself all over as a second record in the file and X can have a different value for each record.

What I want to do is read and store the first 25 lines as independent values and then store the next X (usually around 250) lines as an array. Then I'm going to store it in an SQL database and repeat with the NEXT record until I reach the Yth record (the number of records in the file is in line 3)

EDIT 2: Alright, I think I've gotten to a solution based on a combination of your alls' responses.

I'm going to read the first 25 lines and store it as an array. I'll copy the pertinent contents of the array to local variables then I'll delete the first 25 lines. Then, I can use the info to store the next X lines (the value of item 13 in the array) as an array, serialize it, store it in a database then delete the lines that I just read.

I could then repeat the process for each subsequent record.

Of course, this relies on one assumption I'm making, which to be honest, I'm not sure is true. Is it possible to delete the first n lines from a text file from within C# without having to read the entire thing and re-write it without the first n lines?

Answer 1

A variation. Produces an error if line number is greater than number of lines.

string GetLine(string fileName, int lineNum)
{
    using (StreamReader sr = new StreamReader(fileName))
    {
        string line;
        int count = 1;
        while ((line = sr.ReadLine()) != null)
        {
            if(count == lineNum)
            {
                return line;
            }
            count++;
        }
    }
    return "line number is bigger than number of lines";  
}

Answer 2

Unless you have fixed sized lines, you need to read every line until you reach the line you want. Although, you don't need to store each line, just discard it if it's not the line you desire.

Edit:

As mentioned, it would also be possible to seek in the file if the line lengths were predictable -- that is to say you could apply some deterministic function to transform a line number into a file position.

Answer 3

If the lines are all of a fixed length you can use the Seek method of a stream to move to the correct starting positiion.

If the lines are of a variable length your options are more limited.

If this is a file you will be only using once and then discarding, then you are best off reading it in and working with it in memeory.

If this is a file you will keeping and will be reading from more than writing to, you can create a custom index file that contains the starting positions of each line. Then use that index to get your Seek position. The process of creating the index file is resource intensive. Everytime you add a new line to the file you will need to update the index, so maintenance becomes a non-trivial issue.

Answer 4

You could read line by line so you don't have to read the entire all at once (probably at all)

int i=0
while(!stream.eof() && i!=lineNum)
    stream.readLine()
    i++
line = stream.readLine()

Answer 5

.NET 4.0 edit

Since .NET 4.0, it is possible to access a single line of a file directly. For instance, to access line 15:

string line = File.ReadLines(FileName).Skip(14).Take(1).First();

This will return only the line required

---

Since you can't predict the location (can you?) of the i-th line in the file, you'll have to read all previous lines too. If the line number is small, this can be more efficient than the ReadAllLines method.

string GetLine(string fileName, int line)
{
   using (var sr = new StreamReader(fileName)) {
       for (int i = 1; i < line; i++)
          sr.ReadLine();
       return sr.ReadLine();
   }
}

Answer 6

Late Answer but worth it .

You need to load the lines in array or list object where each line will be assign to an index ,then simply call any range of lines by their index in for loop .

Solution is pretty Good ,but there is a memory consumption in between .

give it try ...Its worth it