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Hi I am trying to make a program which ; takes an input txt file then shows its lines in reverse order

I was using File.ReadAllLines so I get a array of strings then I used indexes of the strings and used for loop to reverse the order.

But I did some research and found the File.ReadAllLines is not good for very long txt files so they say I should use File.ReadLines.

But I couldn't understand how to access the lines indexes as I did in string array

My working code is below ( which is written by using File.ReadAllLines) I want to do this with File.ReadLines How can I do this how can I access indexes if I use ReadlLines

    private void button1_Click(object sender, EventArgs e)
    {
        openFileDialog1 = new OpenFileDialog();
        openFileDialog1.Filter = "Txt Files Only |*.txt";
        openFileDialog1.InitialDirectory = @"C:\";
        openFileDialog1.Title = "Load Log File";           
        openFileDialog1.ShowDialog();
        textBox1.Text = openFileDialog1.FileName;
        checkBox1.Checked = true;

    }

    private void checkBox1_CheckedChanged(object sender, EventArgs e)
    {

    }

    private void button2_Click(object sender, EventArgs e)
    {
        textBox1.Clear();
        checkBox1.Visible = false;
        button1.Visible = false;
        textBox1.Multiline = true;
        textBox1.Height = 200;
        textBox1.Width = 600;
        button2.Visible = false;
        textBox1.ScrollBars = ScrollBars.Vertical;
        string[] lines = File.ReadAllLines(openFileDialog1.FileName);
        int n = lines.Length;
        for (int i = 0; n > i; n--)
        {
            textBox1.Text += lines[n - 1] + Environment.NewLine;
        }

    }
U.Deniz A.
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3 Answers3

2

To reverse the lines, you need to load all of them into memory anyway (unless you read from the bottom), so File.ReadAllLines is the correct choice.

But I couldn't understand how to access the lines indexes as I did in string array

You can't. File.ReadLines returns an IEnumerable<String>. IEnumerables can't be indexed. You could call ToArray or ToList to transform it to an array or list. In alternative, you can use ElementAt on the IEnumerable to get the value at the specified index. This is very inefficient, though.

Summary: Stick to File.ReadAllLines

marc_s
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tixina3684
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  • Yes... _for this purpose_. For many other purposes, `ReadLines()` does have a meaningful performance advantage over `ReadAllLines()`. – Joel Coehoorn Jul 25 '22 at 13:25
  • @tixina3684 But how can I access the specific line if I use ReadLines. For example if I want to store the line number of some specific line how can I do that since ReadLines doesn't store index of the line (line number). Or if I don't want to read all lines in txt but only some specific numbered lines for example I want to read (or print) 345. lines to 350. lines. I believe I can not do that with Readlines since it doesn't know line number (row number). I think there is only foreach option to read the file so it reads all of the file. Can you help me on that – U.Deniz A. Jul 28 '22 at 07:50
2

You could use

IEnumerable<string> lines = File.ReadLines(openFileDialog1.FileName);
foreach (string line in lines.Reverse()) {
    ...
}

However, you won't get any performance benefit in this case, as the Reverse() operation will have to buffer all the lines. And this is precisely what ReadLines avoids doing by reading the lines lazily as you are looping through them.

Using ReadAllLines and a reverse for-loop is better in this case. But you can get a performance benefit by using a StringBuilder:

var sb = new StringBuilder();
string[] lines = File.ReadAllLines(openFileDialog1.FileName);
for (int i = lines.Length - 1; i >= 0; i--)
{
    sb.AppendLine(lines[i]);
}
textBox1.Text = sb.ToString();

The reason for this is that creating a string with repeated += operations will create a new, lager string each time and copy the old value into the new one.

See also: Use Visual C# to improve string concatenation performance.

To increase the speed further, you could calculate the size of the resulting text in advance and use it to initialize the StringBuilder with an optimal buffer size.

string[] lines = File.ReadAllLines(openFileDialog1.FileName);
var sb = new StringBuilder(lines.Sum(s => s.Length + 2));

Note: You can expand the forr code snippet in Visual Studio to get a reverse for loop.

Olivier Jacot-Descombes
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  • firstly thanks, But how can I access the specific line if I use ReadLines. For example if I want to store the line number of some specific line how can I do that since ReadLines doesn't store index of the line (line number). Or if I don't want to read all lines in txt but only some specific numbered lines for example I want to read (or print) 345. lines to 350. lines. I believe I can not do that with Readlines since it doesn't know line number (row number). I think there is only foreach option to read the file so it reads all of the file. Can you help me on that – U.Deniz A. Jul 29 '22 at 07:21
  • You can use LINQ extension methods: `lines.Skip(344).Take(6)`. In a foreach you can keep and update an index manually. Or you can have LINQ do it for you, see: [How to get index using LINQ?](https://stackoverflow.com/a/2471611/880990). – Olivier Jacot-Descombes Jul 29 '22 at 11:37
1

I don't think what you are trying to do is possible with File.ReadLines, since it returns an IEnumerable which is in this case implemented as a ReadLinesIterator. The iterator is only able to read one line at a time going forward. Anything that requires knowledge about all the lines (reversing them, counting them...) cannot be done without iterating through all lines first.

You could find lower-level approaches to do that, but it might be tricky. Anyway, since you are displaying all the lines inside a textbox, you are keeping all the file's content in memory anyway. Looking for a more efficient way to read the file doesn't seem really useful in this case.

piadina
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