How do I display and calculate average ? And how do I write the value to the text file only once?

Question

This is the code:

double value = Convert.ToDouble(sensor.Value);
if (value < minTemp) minTemp = value;
if (value > maxTemp) maxTemp = value;

label8.Text = maxTemp.ToString() + "c";
label8.Visible = true;
label9.Text = minTemp.ToString() + "c";
label9.Visible = true;
temperature_label.Text = sensor.Value.ToString() + "c";
int t = temperature_label.Text.Length;
if (t > 3)
{
   temperature_label.Location = new Point(238, 200);
}

label8 display maximum temperature label9 display the minimum and temeprature_label display the current. I want to add a label that will display in real time the current average. How can i do it ?

And in this part of the code im writing the current temeprature to a logger(text) file if its above a temperature the user set:

if (sensor.Value > float.Parse(textbox3_value))
{
     Logger.Write("The current temperature is ===> " + sensor.Value);
     button1.Enabled = true;
}

The problem is that all this codes are in a timer tick that is updating every second. And its writing to the Logger file its adding each time a new line.

The logger file for example look like this:

1/1/2014--12:50 PM ==> The current temperature is ===> 71
1/1/2014--12:50 PM ==> The current temperature is ===> 71
1/1/2014--12:50 PM ==> The current temperature is ===> 71
1/1/2014--12:50 PM ==> The current temperature is ===> 71
1/1/2014--12:50 PM ==> The current temperature is ===> 71
1/1/2014--12:50 PM ==> The current temperature is ===> 72
1/1/2014--12:50 PM ==> The current temperature is ===> 72
1/1/2014--12:50 PM ==> The current temperature is ===> 73
1/1/2014--12:50 PM ==> The current temperature is ===> 74
1/1/2014--12:50 PM ==> The current temperature is ===> 73
1/1/2014--12:50 PM ==> The current temperature is ===> 73

And i want to make the logger file format to be something like this:

1/1/2014--12:50 PM ==> Session Started
1/1/2014--12:50 PM ==> The current temperature is ===> 71
1/1/2014--12:50 PM ==> The last highest temperature was ===> 80
1/1/2014--12:50 PM ==> The last minimum temperature was ===> 40
1/1/2014--12:50 PM ==> The average temperature is ===> 75

And when there is a new value of one of this lines it will update this line value and won't add a new one.

This is the Write method in the Logger class:

public static void Write(string str)
{
    if (mut.WaitOne() == false)
    {
        return;
    }
    else
    {
        using (StreamWriter sw = new StreamWriter(full_path_log_file_name, true))
        {
            sw.Write(DateTime.Now.ToShortDateString() + "--" + DateTime.Now.ToShortTimeString() + " ==> " + str);
            sw.WriteLine();
            sw.Close();
        }
    }
    mut.ReleaseMutex();
} 

Answer 1

First define a list to store all values, and a variable to store average:

List<int> values = new List<int>();
double average = 0.0;

And in here :

double value = Convert.ToDouble(sensor.Value);
values.Add(value);
average = values.Average();
Logger.Write("The average temperature is ===> " + average);