Don't use a GoTo. There's no need. Also, remove the user interaction from the method that does the actual work. Read the data in one place, pass it to a method (usually a Function rather than a Sub) that does the work and returns a result, and then show the result to the user after the function ends.
That makes this question tricky, because the only result we see in the original code is writing "Hello" to the Console, and that's clearly just a debugging statement. What do you want this code to do? (I'm gonna assume you mean this)
Function Bisect(a as Double, b As Double) As Double
Dim c As Double = (a + b) / 2
Dim d As Double = func(c)
While Math.Abs(d) >= 0.0001
If Math.Sign(d) = Math.Sign(func(a)) Then
a = c
Else
b = c
End If
c = (a + b) / 2
d = func(c)
End While
Return c
End Function
Function func(x As Double) As Double
Return x ^ 2 - 2
End Function
And really, it should look like this:
Function Bisect(a as Double, b As Double, f As Function(Of Double, Double)) As Double
Dim c As Double = (a + b) / 2
Dim d As Double = f(c)
While Math.Abs(d) >= 0.0001
If Math.Sign(d) = Math.Sign(f(a)) Then
a = c
Else
b = c
End If
c = (a + b) / 2
d = f(c)
End While
Return c
End Function
and be called like this:
Bisect(a, b, Function(x) x ^ 2 - 2)
Also, the algorithm here is slightly off based the wikipedia article. This is more precise:
Function Bisect(a as Double, b As Double, f As Function(Of Double, Double)) As Double
Dim TOL As Double = 0.0001
Dim MaxSteps As Integer = 1000
Dim c As Double = (a + b) / 2
While Math.Abs(f(c)) >= TOL AndAlso (b-a)/2 >= TOL AndAlso MaxSteps > 0
If Math.Sign(f(c)) = Math.Sign(f(a)) Then
a = c
Else
b = c
End If
MaxSteps -= 1
c = (a + b) / 2
End While
If MaxSteps = 0 Then
Throw New ArgumentException("The bisection fails to converge within the allowed time for the supplied arguments.")
End If
Return c
End Function
I bring it up, because the complaint in the original question is this:
the program loops infinently[sic]
and one of the tenants of the algorithm is it's not guaranteed to converge, hence the step counter.
Finally, this looks to me like it might do well as a recursive function. Recursion can improve things here because we can rely on the call stack overflowing rather than needing to implement a step counter:
Function Bisect(a as Double, b As Double, f As Function(Of Double, Double)) As Double
Dim c As Double = (a + b) / 2
If Math.Abs(f(c)) < TOL OrElse (b-a)/2 < TOL Then Return c
If Math.Sign(f(c)) = Math.Sign(f(a)) Then
Return Bisect(c, b, f)
Else
Return Bisect(a, c, f)
End If
End Function
Of course, catching that StackOverflowException is a trick in itself, so you may still want that step counter. But I need to leave something for you to do yourself.
This also helps demonstrate part of why I recommend removing all user I/O from the Bisect() method. If this method were also responsible for talking to the end user, it would not be possible to even consider the recursive option, where the code is clearly far shorter and simpler than any of the others.
