Searching for specific numbers in an array

Question

I am writing a program that needs to search for specific range of numbers in am Array. The number that I need to search for are actually for grades that student would have received, ie As, Bs, Cs, Ds, Es, and Fs. This is my code:

Imports System.IO Imports System.Math

Public Class frmClassStatistics

'Arrays to hold the name and marks read from file 
Private strName() As String
Private dblMark() As Integer
Private intNumStudents As Integer = 0
Private Average As Double
Private StandardDeviation As Double
Private arrHighestMark() As Double

Private Sub frmClassStatistics_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    'Reads each student name and final mark from the mark file. It them separates each name from the mark
    'and places the student names and the final mark into two, one-dimensional arrays. 
    'One array will be an array of string (for the names) and the other array will be an array of real numbers (for the marks).

    Dim strFilename As String
    Dim i As Integer
    Dim intLastBlank As Integer
    Dim strInput As String
    Dim StudentMarks = "C:\StudentMarks.txt"

    strFilename = "StudentMarks.txt"

    If File.Exists(strFilename) Then
        Dim srReader As New StreamReader(strFilename)

        Try

            i = 0

            While Not srReader.EndOfStream

                strInput = srReader.ReadLine()

                'find location of TAB between the name and the mark.
                intLastBlank = strInput.IndexOf(vbTab)

                'take all characters before the TAB and place them into the name array
                strName(i) = strInput.Substring(0, intLastBlank)

                intNumStudents = intNumStudents + 1

                'take all characters after the TAB, convert them into a real number and
                'place them into the mark array
                dblMark(i) = Double.Parse(strInput.Substring(intLastBlank + 1))

                i = i + 1

            End While

        Catch ex As Exception
            MessageBox.Show("Problem reading data from the file")
        End Try

        srReader.Close()  ' Close the file

    End If

End Sub


Private Sub btnShowStatistics_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnShowStatistics.Click

    'Finds the dblMark array to find the class avarage, highest Mark and number of students above the class avarage

  'I earased it for simplicity

    Grades()

End Sub

Private Sub Grades()

    Dim blnFound As Boolean = False
    Dim intCount As Integer = 0
    Dim intPosition As Integer

' At the moment when I run the program I get an error here saying that a null Reference Exception was unhandled

    Do While blnFound And intCount < dblMark.Length
        If dblMark(intCount) >= 98 Then
            blnFound = True
            intPosition = intCount
        End If
        intCount += 1
    Loop
    'check to see if value found
    If blnFound Then
        txtAs.Text = intPosition
    Else
        txtAs.Text = 0
    End If

End Sub

Am I complete off base here using this kind of loop? Is there some easier way to program this?

If you debug, you should notice that dblMark is equal to nothing. — the_lotus, May 28 '15 at 15:24
possible duplicate of [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — the_lotus, May 28 '15 at 15:25

score 1 · Answer 1 · answered May 28 '15 at 17:36

First of all, you never resized dblMark(). So I'm surprised that you don't get errors within the loop that loads dblMark. This makes me think that your If FileExists() did not find the file and so all of that code is skipped.

The error is ultimately happening because your variable dblMark is nothing. If you assigned it a value or declared it as dblMark(0), your error would go away, but your program still probably won't do what you want it to. Try using a full path for strFilename (like you declared two lines above: strFilename = "c:\studentmarks.txt)

Other observations: your variable dblMark() is declared as an array of integer, but you are parsing as a double (and the name implies double, instead of integer).

Searching for specific numbers in an array

1 Answers1