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Display only Even numbers in C#, using Windows Forms

I think this is a very easy answer and I understand that you enter something like this get an even number:
if (i % 2 == 0)
But I am just struggling to figure out how to slot it into my current code that I have here...
I have a form like so:
I am double clicking the 'Show Numbers' button
And I want the user to click the show numbers button and it only spits out even numbers, regardless if the box is checked or not.
namespace CHECK_BOXES
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
textBox1.Text = "";
if (checkBox1.Checked)
{
for (int i = 1; i <= 20; i++)
{
textBox1.Text += i.ToString() + "\r\n";
}
}
else
{
for (int i = 20; i >= 1; i--)
{
textBox1.Text += i.ToString() + "\r\n";
}
}
}
}
}
Would appreciate any help.
Thank you

Try with this function:
public bool IsEven(int value)
{
return value % 2 == 0;
}
and then update your for each loop with the following statement:
for (int i = 1; i <= 20; i++)
{
if (IsEven(i))
{
textBox1.Text += i.ToString() + "\r\n";
}
}
Or just easily increment i in for loop with 2.

Just do "double"-steps.
for (int i = 0; i <= 20; i += 2) { // <- pay attention, i will be incremented by 2
textBox1.Text += i.ToString() + "\r\n";
}

could be one line with Linq
textBox1.Text = string.Join("\r\n", Enumerable.Range(0, 20)
.Where((_, index) => index % 2 == 0).Select(x => x));

C# Clear button to se all my int to 0 and strings to " "

I would like to make a button that sets all my int's and string's back to nothing (0 and "").
Example of string's and int's being used:
private void btn_H_Click(object sender, EventArgs e)
{
H++;
h = "H";
txt_Chemical.Text = h + li + na + k + rb + cs + fr;
if (H > 1)
{
h = "";
h = "H" + H;
//hm = hm * H;
//string strhm = Convert.ToString(hm);
// txt_Mass.Text = strhm + strlim;
txt_Chemical.Text = h + li + na + k + rb + cs + fr;
}
}
What I tried to do:
private void btn_Clear_Click(object sender, EventArgs e)
{
int H = 0;
string h = "";
int Li = 0;
string li = "";
txt_Chemical.Clear();
txt_Mass.Clear();
}

CheckedListBox get selected index

How can I get the value of the selected index in a CheckedListBox. I tried via through an if condition and switch cases, but it is not working as expected.
private void checkedListBox1_SelectedIndexChanged(object sender, EventArgs e)
{
if (checkedListBox1.GetItemCheckState(0)==CheckState.Checked)
{
richTextBox1.Font = new Font(richTextBox1.Font, FontStyle.Bold);
}
}

i think you can use
checkedListBox1.CheckedIndices
Something like
foreach(int index in checkedListBox1.CheckedIndices)
{
if(index == 1)
{
//do something
}
}

Try this:
if(checkedListBox1.CheckedItems.Count != 0)
{
// If so, loop through all checked items and print results.
string s = "";
for(int x = 0; x <= checkedListBox1.CheckedItems.Count - 1 ; x++)
{
s = s + "Checked Item " + (x+1).ToString() + " = " + checkedListBox1.CheckedItems[x].ToString() + "\n";
}
MessageBox.Show (s);
}

How can I solve an index out of range error?

I hava a combo box that contains students from a studentList. When I select a student it should populate a text field of the students name. Whenever a student is selected from the combo box I get the following error
ArgumentOutOfRangeException was unhandled
Index was out of range. Must be non-negative and less than the size of the collection.
I think the problem may be in my loop but I'm having trouble finding out how to fix the error, any help would be appreciated
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
int i;
for (i = 0; i < Main.studentList.Count; i++)
{
if (comboBox1.SelectedItem == Main.studentList[i].StudentName + " " + Main.studentList[i].StudentId)
{
break;
}
}
txtName.Text = Main.studentList[i].StudentName; //where the error occurs
}
public void ChangeStudent_Load(object sender, EventArgs e)
{
//loading combobox from studentList
foreach (var student in Main.studentList)
{
comboBox1.Items.Add(student.StudentName + " " + student.StudentId);
}
}

The reason it's throwing an error is after the break, i gets incremented. If i was the last item in the list, it's now out of bounds. If it wasn't, it's now pointing to the next item.
The simple solution would be to move the line that is throwing the error above the break;
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
int i;
for (i = 0; i < Main.studentList.Count; i++)
{
if (comboBox1.SelectedItem == Main.studentList[i].StudentName + " " + Main.studentList[i].StudentId)
{
txtName.Text = Main.studentList[i].StudentName;
break;
}
}
}
Also, consider using a foreach loop. Here's the exact same logic with a foreach loop. It makes it a bit more readable.
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
foreach (var student in Main.studentList)
{
if (comboBox1.SelectedItem == student.StudentName + " " + student.StudentId)
{
txtName.Text = student.StudentName;
break;
}
}
}

The error occurs because on the last loop in the for loop i is incremented by 1:
i++
It then evaluates to false in the expression:
i < Main.studentList.Count
So when you get to the line where the error occurs I is equal to Main.studentList.Count and so an index out of range error occurs.
If you want to access the last element of the list you can do:
Main.studentList[Main.studentList.Count - 1].StudentName
Alternatively if you want to evaluate the statement on each loop just move it inside of your for loop:
for (int i = 0; i < Main.studentList.Count; i++)
{
if (comboBox1.SelectedItem == Main.studentList[i].StudentName + " " + Main.studentList[i].StudentId)
{
txtName.Text = Main.studentList[i].StudentName;
break;
}
}
This also has the advantage of keeping the variable i local to the scope of the loop.

Change your code like that and please try again
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
int i;
for (i = 0; i < Main.studentList.Count; i++)
{
if (comboBox1.SelectedItem == Main.studentList[i].StudentName + " " + Main.studentList[i].StudentId)
{
txtName.Text = Main.studentList[i].StudentName;
break;
}
}
}

You are using the same variable(i) for range calculation and also assignment.
Try this.
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
int j;
for (int i = 0; i < Main.studentList.Count; i++)
{
if (comboBox1.SelectedItem == Main.studentList[i].StudentName + " " + Main.studentList[i].StudentId)
{
j=i;
break;
}
}
txtName.Text = Main.studentList[j].StudentName; //where the error occurs
}
public void ChangeStudent_Load(object sender, EventArgs e)
{
//loading combobox from studentList
foreach (var student in Main.studentList)
{
comboBox1.Items.Add(student.StudentName + " " + student.StudentId);
}
}

cannot display a list ?kam

I want to dispaly a list on a button click.I have added a list box in .xaml file and want to add 10 text boxes in list.The following code shows errors.
private void listbutton_C(object sender, RoutedEventArgs e)
{
String str = "thumb_";
TextBox[] name = new TextBox[20];
for (int i = 1; i < 11; i++)
{
if (i == 10)
{
strPath = str + "0" + i + ".jpg";
}
else
{
strPath = str + "00" + i + ".jpg";
}
name[i].Text = strPath;
listBox1.Items.Add(name[i]);
}
ContentPanel2.Visibility = Visibility.Collapsed;
listBox1.Visibility = Visibility.Visible;
}
name[i].text=strpath show nullreferenceExceptions .Can somebody explain what is the problem?

I think you need to instantiate every textbox, you have only created the array.
for (int i = 1; i < 11; i++)
{
name[i] = new TextBox(); // insert this line
if (i == 10)
{
strPath = str + "0" + i + ".jpg";
}
else
{
strPath = str + "00" + i + ".jpg";
}
name[i].Text = strPath;
listBox1.Items.Add(name[i]);
}