Visual basic function for intermittent antibiotic dosing - vb.net

Question

I am a beginner in VB. I wrote a little program to simulate dosing regimens of antibiotics using some exponential equations and pharmacokinetic data.
The problem I have is that I want to display on a graph the following mathematical function:
That simulates the concentration variation at different intervals of time:
Where:
b(t) is the concentration at time t that will be plotted as Y axis, t is time (plotted on the x-axis).
b(0) is the concentration at time 0 and it is a known variable.
u(t-a1) is a function that has the value u(t-a1)=b(o) if t=a1 or 0 if t<>a1
a1 is the time at which a next dose is given.
alpha is the elimination rate constant, a variable that is known.
What I have so far:
Dim y, x As Double
For x = 0 To 24 Step 1
For n As Double = 1 To 24 / tau
y = (1 - test_condition(n * tau, x)) * css * Math.Exp(-ke * x) + test_condition(n * tau, x) * css * Math.Exp(-ke * (x - n * tau))
Chart1.Series("Concentratie1").Points.AddXY(x, y)
Next
Next
The test_condition:
if x=tau then test_condition= 1 else 0
It is close but I don't get an exponential decay after a dose ... don't know how to make that happen.
This works!! for tau (dosing interval) every 4 hours. Can it be rearranged somehow because the tau (dosing interval) will vary (sometimes 4 hours, sometimes every 6 hours)?:
Dim y, x, y2, x2, y3, x3, y4, x4, x5, x6, y5, y6 As Double
For x = 0 To tau Step 1
y = exponential_decay(css, ke, x) + test_condition(tau, x) * (css - Val(mic))
Chart1.Series("Bolus 1").Points.AddXY(x, y)
Next
For x2 = tau To 2 * tau Step 1
y2 = exponential_decay(css, ke, x2 - tau) + test_condition(2 * tau, x2) * (css - Val(mic))
Chart1.Series("Bolus 2").Points.AddXY(x2, y2)
Next
For x3 = 2 * tau To 3 * tau Step 1
y3 = exponential_decay(css, ke, x3 - 2 * tau) + test_condition(3 * tau, x3) * (css - Val(mic))
Chart1.Series("Bolus 3").Points.AddXY(x3, y3)
Next
For x4 = 3 * tau To 4 * tau Step 1
y4 = exponential_decay(css, ke, x4 - 3 * tau) + test_condition(4 * tau, x4) * (css - Val(mic))
Chart1.Series("Bolus 4").Points.AddXY(x4, y4)
Next
For x5 = 4 * tau To 5 * tau Step 1
y5 = exponential_decay(css, ke, x5 - 4 * tau) + test_condition(5 * tau, x5) * (css - Val(mic))
Chart1.Series("Bolus 4").Points.AddXY(x5, y5)
Next
For x6 = 5 * tau To 32 Step 1
y6 = exponential_decay(css, ke, x6 - 5 * tau)
Chart1.Series("Bolus 4").Points.AddXY(x6, y6)
Next
End Sub

Answer 1

I managed to solve the problem:
this function f relates time (t) to dosing interval (tau)
Private Function f(ByVal t As Double, ByVal tau As Double)
Dim n As Integer
For n = 0 To 24 / tau
If t = n * tau Then
f = n * tau
ElseIf t < tau Then
f = 0
ElseIf t > n * tau And t < (n + 1) * tau Then
f = n * tau
ElseIf t >= (n + 1) * tau Then
f = n * tau
End If
Next
End Function
And this is what I draw on the chart:
For x = 0 To 36 Step 0.5
y = exponential_decay(css, ke, x - f(x, tau))
Chart1.Series("Intermitent Dosage").Points.AddXY(x, y)
Next