Here's what I want this code to do.

1. Read the textfile random.txt into a List
2. For each line of the textfile read into the list I want to determine if it is positive or negative using .RemoveAll with a lambda expression.
3. RemoveAll should remove each line containing a negative number
4. I want to display the changed list into a listbox to display.

I cannot use Linq, and I must use the ListMethod RemoveAll.

'''''

 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Data;
 using System.Drawing;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using System.Windows.Forms;
 using System.IO;

 namespace meade_13_1
 {
  public partial class Form1 : Form
 {

    public Form1()
    {
        InitializeComponent();

    }
    private void btnFindAll_Click(object sender, EventArgs e)
    {
       
    }

    private void btnRemoveNeg_Click(object sender, EventArgs e)
    {
        List<int> list = new List<int>();
        using (StreamReader reader = new StreamReader("random.txt"))
        {
            string line;
            while ((line = reader.ReadLine()) != null)
            {
                list.Add(Int32.Parse(line));
                
                
            }
        }
        list.RemoveAll(x => x > 0);
        listBox1.Items.Add(list);

         }
     }
 }

'''''

CodePudding user response：

Instead of working with reader, list directly, I suggest quering with a help of Linq:

using System.Linq;

...

private void btnFindAll_Click(object sender, EventArgs e) {
  var items = File
    .ReadLines("random.txt")
    .Where(line => !string.IsNullOrWhiteSpace(line))
    .Select(line => double.Parse(line)) //TODO: int.Parse if item is int
    .Where(item => item > 0); 

  foreach (var line in items) 
    listBox1.Items.Add(line);        
}

Here I've added .Where(line => !string.IsNullOrWhiteSpace(line)) to be on the safe side of the rode - I ignore possible empty lines

CodePudding user response：

It doesn't remove negative numbers simply because the lamda expression you pass to RemoveAll is used to filter positive numbers. For negatives it should be x => x < 0.

So if you need the version without Linq, I can propose two options:

1. Without list at all (just add to the listbox only positive numbers)

using (StreamReader reader = new StreamReader("random.txt"))
{
   string line;
   while ((line = reader.ReadLine()) != null)
   {
    int number = int.Parse(line);

    if (int.TryParse(line, out int number) && number >= 0)
       listBox1.Items.Add(number);
   }
}

2. With list and RemoveAll

var numbers = new List<int>();
using (StreamReader reader = new StreamReader("random.txt"))
{
   string line;
   while ((line = reader.ReadLine()) != null)
   {
     if (int.TryParse(line, out int number)
       numbers.Add(number);
   }
   numbers.RemoveAll(n => n < 0);
   foreach (int num in numbers)
     listBox1.Items.Add(num);
}

And by the way for this kind of task the File.ReadLines is more natural (because even the method name says that this does exactly what we need):

foreach (string line in File.ReadLines("random.txt")
{
  if (int.TryParse(line, out int num) && num > 0)
     listBox1.Items.Add(num);
}