Using the .SubString Function to Reverse the string

[Roxide]

I have an assignment where we have to a number of things such as count the vowels, words, and reverse strings. I was able to write the code for the vowels and the words but am lost on the reverse. Here is the section of the assignment the professor asked.

6) Write the code for the “Reverse” button click event. The subroutine should get the text from txtSentence and pass it to a function called ReverseString. The ReverseString function should reverse the string and return it. For example ReverseString( “ABC” ) should return “CBA”. The “Reverse” button click subroutine should then update the text in txtSentence with the reversed string. You cannot use the StrReverse function. Use .Substring to extract each letter but assemble the characters back together using concatentation so that the first character ends up as the last.

I understand the concept of what it is doing and but don't know how to start it. I have used the .substring in the past to output part of the string. Can anyone give me some advice on how to go about it or even a little example? Here is the code I have written for the vowels, words, as well as the overall defined variables so far.
Thanks again.

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim str As String
        Dim total As Integer

        total = 0
        str = txtSentence.Text
        'look for each vowel and add the count to total
        total = total + CharCount(str, "A")
        total = total + CharCount(str, "E")
        total = total + CharCount(str, "I")
        total = total + CharCount(str, "O")
        total = total + CharCount(str, "U")
        total = total + CharCount(str, "Y")

        'display results
        MsgBox("Total Vowels = " & total)

    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Dim str As String
        Dim i, l, words As Integer
        str = txtSentence.Text
        str = LTrim(str) 'removes leading blank spaces
        str = RTrim(str) ' removes trailing blank spaces
        l = str.Length
        i = 0
        words = 0
        While (i < l)
            If str(i) = " " Then
                words = words + 1
                i = i + 1
                While str(i) = " " ' removes continuous blank spaces
                    i = i + 1
                End While
            Else
                i = i + 1
            End If

        End While
        words = words + 1 ' adds the last word
        MessageBox.Show("WORDS =" & words)
    End Sub
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim random As String = "asfdgasfdgasfdgasfd11"
        Dim length As Integer = Nothing
        length = Random.Length
        Console.WriteLine(random.Length)
        Console.WriteLine(length)
        Console.WriteLine()
        Console.WriteLine()
        Console.ReadLine()

        Dim curStart = 0
        Dim loopCounter = 0
        While (curStart < random.Length)
            Console.WriteLine(random.Substring(curStart, System.Math.Min(20, length - 20 * loopCounter)))
            curStart = curStart + 20
            loopCounter = loopCounter + 1
        End While
    End Sub

    Private Function CharCount(ByVal str As String, ByVal findCH As Char) As Integer

        Dim idx, Ln, cnt As Integer
        Dim ch As String

        Ln = str.Length
        cnt = 0

        findCH = Char.ToUpper(findCH) 'make sure char is an uppercase

        For idx = 0 To Ln - 1
            'read individual letters from string and convert to uppercase
            ch = CChar(str.Substring(idx, 1).ToUpper) 'convert to char
            If ch = findCH Then
                cnt = cnt + 1 'found a match increment the count
            End If
        Next
        Return cnt

    End Function


    Private Sub txtSentence_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtSentence.TextChanged

    End Sub
End Class

[dbasnett]

Hint:

    Private Function ReverseIt(someString As String) As String
        Dim rv As String
        For x As Integer = someString.Length - 1 To 0 Step -1
            rv &= 'complete this statement
        Next
        Return rv
    End Function

[Roxide]

Hint:

    Private Function ReverseIt(someString As String) As String
        Dim rv As String
        For x As Integer = someString.Length - 1 To 0 Step -1
            rv &= 'complete this statement
        Next
        Return rv
    End Function

Sorry I am still confused. This is my first semester of this. So do I then type the txtbox I am getting the string from then .substring followed by the numbers in the parenthesis?

[dbasnett]

Sorry, I wrote a Function to do the work of reversing. See if this helps.

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        txtSentence.Text = ReverseIt(txtSentence.Text)
    End Sub

    Private Function ReverseIt(someString As String) As String
        Dim rv As String
        For x As Integer = someString.Length - 1 To 0 Step -1
            rv &= 'complete this statement
        Next
        Return rv
    End Function

[Roxide]

Sorry, I wrote a Function to do the work of reversing. See if this helps.

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        txtSentence.Text = ReverseIt(txtSentence.Text)
    End Sub

    Private Function ReverseIt(someString As String) As String
        Dim rv As String
        For x As Integer = someString.Length - 1 To 0 Step -1
            rv &= 'complete this statement
        Next
        Return rv
    End Function

Ok So I have been trying and can't get it. I tried

rv &= txtsentence.Substring(0, 0)

But obviously that is wrong. I tried changing the numbers in the parenthesis but that didn't do anything. Can someone walk me through the statement so I can understand it or have an easy way of explaining it. Sorry again for my asking I just can't figure it out.

[dunfiddlin]

SomeString = "0123456789"
rv = ""
x = 9
rv = SomeString.SubString(9, 1) = "9"
x = 8
rv = rv & SomeString.SubString(8,1) = "98"

etc.

Now just do it without using literal numbers!

[Shaggy Hiker]

Walk you through what? Do you know what substring does, and what the arguments are for? If you do, then consider the steps that you are trying to perform:

For every character in the string, starting with the rightmost character, add it to a new string. Therefore, the steps would be these:

1) Get the last character in the string.
2) Concatenate it onto the new string.

You are doing #2 fine, so the problem would have to be with step #1, which is entirely based on the arguments in the line you just posted. If you don't know what those arguments are, we could explain them, but we'd be doing no more than paraphrasing what is written in MSDN on the subject of Substring. Sometimes that would help, since MSDN isn't always written in a way that is clear to everybody, but I think that the writing is fairly clear for the Substring method.

However, if after reading what MSDN has to say about Substring, you are still unclear on the arguments, then I think that you would benefit the greatest by putting a breakpoint on the line where you use Substring, and when execution stops on the breakpoint, highlight the Substring(0,0) part and press Shift+F9 to see what it returns. When you do that, you can try changing the arguments and reevaluating, so you can see for yourself what Substring does, which should be far more informative than anything we can say. Besides, if you haven't used breakpoints, there is no more useful tool you can have, and they are super easy, so just learning that would be worth the minor time involved.

[MattP]

I tried changing the numbers in the parenthesis but that didn't do anything.

Here's the documentation for String.Substring(Int32, Int32). Have a look at it and then try to see what dbasnett has set up for you in the loop.

[stanav]

Try something like this:

        Dim original As String = "0123456789"
        Dim reversed As String = original
        Dim length As Integer = original.Length
        If length > 1 Then
            reversed = String.Empty
            For i As Integer = length To 1 Step -1
                reversed &= original.Substring(i - 1, 1)
            Next
        End If
        MessageBox.Show(reversed)

[Roxide]

Sorry, I wrote a Function to do the work of reversing. See if this helps.

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        txtSentence.Text = ReverseIt(txtSentence.Text)
    End Sub

    Private Function ReverseIt(someString As String) As String
        Dim rv As String
        For x As Integer = someString.Length - 1 To 0 Step -1
            rv &= 'complete this statement
        Next
        Return rv
    End Function

AHA I think I finally figured it out. Would this be correct?

   rv &= txtSentence.Text.Substring(x, 1)

[Evil_Giraffe]

Not sure whether you'll get extra credit for doing it correctly, but you should probably know that iterating through every Char in the String backwards (or every one-character long substring, which is essentially the same thing) does not reverse a string reliably in .NET. It fails to account for multi-character glyphs (for example where a letter has an accent, this can be formed by the base character followed by a combining character - when you reverse these characters, the combining character ends up following the character that was after the accented character in the original string, so the accent changes which letter it is on when the string is reversed) and characters that exist outside the Basic Multilingual Plane that are represented as two Chars that form a Surrogate Pair - reverse these two characters and you've corrupted the string.

The last time I came across this assignment (note, there have been no reports of needing to do this for a real program) I did find a class that split up a string into singly glyphs, keeping all combining characters together with their base character, and not splitting up surrogate pairs, then all you had to do was iterate over it and stitch the strings together in reverse, however, I can't remember what it was called, don't know if I actually posted it because the search on here is a bit wonky, and don't have the test code I wrote on this computer.

[Evil_Giraffe]

Ah, it's StringInfo.GetTextElementEnumerator()
Pass it a string and get back an iterator of strings. There doesn't seem to be an Enumerable version you can pipe directly to Linq's Reverse method, so you'd need to wrap it as this SO answer (C#) does: http://stackoverflow.com/a/15111719

[dbasnett]

AHA I think I finally figured it out. Would this be correct?

   rv &= txtSentence.Text.Substring(x, 1)

No. It should be one of these

            rv &= someString(x) 'complete this statement
            'or
            rv &= someString.Substring(x, 1) 'complete this statement

EG's solution

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        txtSentence.Text = ReverseIt(txtSentence.Text) 'has çbā̈ in text field as test
    End Sub

    Private Function ReverseIt(someString As String) As String
        Dim its As Globalization.TextElementEnumerator = Globalization.StringInfo.GetTextElementEnumerator(someString)
        Dim sb As New System.Text.StringBuilder
        Do While its.MoveNext
            sb.Insert(0, its.GetTextElement)
        Loop
        Dim rv As String = sb.ToString
        Return rv
    End Function