Help with formula for my project

Hello I am a beginner in visual basic and I had to do my first project to calculate a monthly payment using principal, annual rate in decimal, and time in years, which are all controlled by horizontal scroll bars. I have everything set up except I can not figure out the formula for the monthly payment. Also I am required to have option explicit and strict on.
The formula that i was given is this Attachment 135323
where c is the monthly payment,
r is the monthly interest rate, expressed as a decimal, not a percentage
N is the number of monthly payments, called the loan's term,
P is the amount borrowed, known as the loan's principal.

The way that i calculated the annual rate is this:
lblAnnualPercent.Text = CStr(0.1 * hsbAnnualRate.Value)
The way that i calculated the principal is this:
lblPrincipalAmount.Text = "$" & CStr(100 * hsbPrincipal.Value)
And the way that I calculated the time is this:
lblTimeYears.Text = CStr(hsbTime.Value)

I tried a bunch of different ways to come up with the formula to calculate the monthly payment and I kept getting errors. Can anybody please help me?

vb.net Code:

Private Function CalculateMonthlyPayment()
 
        Dim r As Decimal = 0.15 '' intrest rate
        Dim N As Int16 = 1     '' installment
        Dim P As Single = 100 '' or may be double (@ @) borrowed amount 
        Dim C As Single = 0      '' Monthly payment 
 
 
        Dim Pr As Single = P * r
        Dim PrRn As Single = (1 + r) ^ N '' r + 1 to the power of N
        Dim Rn As Single = ((1 + r) ^ N) - 1 '' 1 + R tothe power of N -1
 
 
 
 
        C = Pr * PrRn / Rn
 
 
        Return C
 
 
    End Function

Here I have used Decimal as the data type since this is about money. The one common term, (1 + r) ^ N is calculated once.

Code:

---

        Dim r As Decimal = 0.05D / 12D '5% yearly as monthly rate
        Dim N As Decimal = 30D * 12D '30 years in months
        Dim P As Decimal = 100000D '$ 100,000
        Dim C As Decimal

        Dim OnePlusRN As Decimal = CDec((1D + r) ^ N) 'needed twice, calculate once

        C = (P * r * OnePlusRN) / (OnePlusRN - 1D)

        Dim cs As String = C.ToString("c2") 'payment as string

---

I am so sorry guys I should have said in the first post but we are not to use any variables! We can only use object properties
So for monthly payment I created a label called lblMPAmount so I guess my formula would start with lblMPAmount.Text = ...and then i have to use the Value properties from the scrollbars and also additional things and calculations which I am not sure what they would be.

Quote:

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We can only use object properties

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Means that
you must have a class & properties such as amount , intrest rate , Installment and an OUT readonly property as what ever the EMI
still you need variables in your class

What kind of programming are you trying to do with out data , types and variables :confused:

I don't know it is a project for school and our instructor specifically told us to not use variables I am not sure why. I know that it would be a lot easier that way. I don't know it's due tonight so I have to figure something out soon. I guess I will wait and see if anybody else has any solutions

definitely you must have mistaken,
your instructor must have been told you to use the properties, as i have mentioned above

lets have a class with takes & exposes the properties ,

vb.net Code:

Public Class EmiCalculater
 
 
    Private _IntrestRate As Decimal
    Private _Installments As Decimal
    Private _PrincipleAmount As Decimal
 
 
    Public WriteOnly Property IntrestRate() As String
        '' the value is string because its from text box
 
 
        Set(value As String)
            _IntrestRate = Convert.ToDecimal(value)
        End Set
 
 
    End Property
 
 
    Public WriteOnly Property Installments() As String
        '' the value is string because its from text box
 
 
        Set(value As String)
            _Installments = Convert.ToDecimal(value)
        End Set
 
 
    End Property
 
 
    Public WriteOnly Property PrincipleAmount() As String
        '' the value is string because its from text box
 
 
        Set(value As String)
            _PrincipleAmount = Convert.ToDecimal(value)
        End Set
 
 
    End Property
 
 
    Private Function CalculateMonthlyPayment()
 
 
        Dim C As Decimal = 0      '' Monthly payment 
 
 
        Dim Pr As Decimal = _PrincipleAmount * _IntrestRate
        Dim PrRn As Decimal = (1 + _IntrestRate) ^ _Installments  '' Thanks to dbas (@-@) here
 
 
        C = Pr * PrRn / PrRn - 1  '' Check your formula implementation here
 
 
        Return C
 
 
    End Function
 
 
    Public ReadOnly Property WriteOutPutToLable() As String
 
 
        Get
            Return CalculateMonthlyPayment().ToString
        End Get
 
 
    End Property
 
 
End Class

now assign & write the output to your lables

vb.net Code:

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
 
        Dim Ecm As New EmiCalculater
 
 
        With Ecm
            .PrincipleAmount = 100 '' may be this value from any text box
            .IntrestRate = 0.15      '' .... do ....
            .IntrestRate = 1          '' 
            Me.Label1.Text = .WriteOutPutToLable  '' write the output to your lable
        End With
 
 
 
 
    End Sub

Quote:

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Originally Posted by xtreme1

I don't know it is a project for school and our instructor specifically told us to not use variables I am not sure why. I know that it would be a lot easier that way. I don't know it's due tonight so I have to figure something out soon. I guess I will wait and see if anybody else has any solutions

---

You should tell us exactly what the instructor told you. Writing a method without the use of variables is, IMHO, stupid, if not impossible. Hopefully your instructor is not that and you have just missed something.

Okay here are all the instructions for the project. Look at step number 5 specifically

1. Name your project PaymentCalculator.
2. Using this formula, you are to write a Visual Basic program that accepts as input the principal, rate, and time and uses the monthly payment formula to calculate the monthly loan payment.
3. Additionally, you will calculate the total amount of interest. You do NOT have to look up a formula for this; simply use the values already given or calculated to determine this amount!
4. As seen below, place your initials in the form's title bar. (Note: you will ALWAYS put your initials or name in title of each form. Follow the format, initials in this one, shown in each assignment.) This is the final message about this!
5. You are NOT to use any variables - only use object properties! Be sure you name your objects using the Object Naming Convention discussed in the first PowerPoint slide show. This is the final message about this!
6. Do NOT use any TextBoxes - only labels that look like TextBoxes.
7. When executed, the Payment form appears in the center of the screen, and your form cannot be resized. (Note: This is the final message about this!)
8. Attach your zipped VB project, including the executable file in an e-mail message with your name and section number at the top of the message, and sent it to the e-mail address used for this course

Are the objects in step 5 form controls? How long have you been in this class?

I am not sure what exactly he means in step 5 which is why I am so confused.. I have been in the class for about 3 weeks

Create a form as specified. On it create three textboxes named txtPrin, txtRate, txtTime. Also create two labels named lblPay, and lblTotInt. Double click the form and replace all of the code with this:

Code:

---

Public Class Form1
    Private Sub txtChanged(sender As Object, e As EventArgs) Handles _
        txtPrin.TextChanged, txtRate.TextChanged, txtTime.TextChanged

        Dim r As Decimal 'monthly rate
        Dim N As Decimal 'number of months
        Dim P As Decimal 'principal
        Dim C As Decimal 'monthly payment
        Dim err As Boolean = False
        If Decimal.TryParse(txtPrin.Text, P) AndAlso P > 0D Then
            'principal good
            If Decimal.TryParse(txtRate.Text, r) AndAlso r > 0D Then
                'rate present - if the user input 5, instead of .05 for % fix it
                If r > 1 Then r = r / 100D
                r = r / 12D
                If Decimal.TryParse(txtTime.Text, N) AndAlso N > 0D Then
                    'time good, have good values
                    N = N * 12 'years to months
                    Dim OnePlusRN As Decimal = CDec((1D + r) ^ N) 'needed twice, calculate once
                    C = (P * r * OnePlusRN) / (OnePlusRN - 1D)
                    lblPay.Text = C.ToString("c2")
                    lblTotInt.Text = (C * N - P).ToString("c2")
                Else
                    'input error
                    err = True
                End If
            Else
                'input error
                err = True
            End If
        Else
            'input error
            err = True
        End If
        If err Then
            lblPay.Text = ""
            lblTotInt.Text = ""
        End If
    End Sub
End Class

---

Enter values into the textboxes.

Do me a favor. When you get the instructors code come back here and show it. I might also want their email. This is the final message about this! LOL

Well the only problem is, I don't know how to integrate my scrollbars into your code.. The values of each one - principal, annual rate, time, etc.. they are supposed to change as you are scrolling. I already have those figured out the only output that I need is the one for monthly payment amount

I think he wants the formula to look something like this because I remember him saying in class that we are going to be using a lot of parenthesis for the formula.. I came up with this but it doesn't seem to be working

Code:

---

lblMPAmount.Text = ((lblPrincipalAmount.Text*(lblAnnualPercent.Text/12))*(1+(lblAnnualPercent.Text/12))^(lblTimeYears.Text/12))/((1+(lblAnnualPercent.Text/12))^((lblTimrYears.Text/12))-1)

---

There's no rule on this forum against doing people's homework for them. We tend not to because we recognise that it really doesn't do them (or our industry) any favours in the long run. Also, we do put them at risk of being accused of plagiarism and cheating even if that wasn't their intention. But if a user feels that a student is putting in the effort but needs a prod in the right direction there's nothing wrong with giving them such a prod.

Xtreme1, I don't get the impression you came here looking for handouts. From your posts it's clear that you've already constructed your form with sliders etc. and are already outputting some of the simpler formulae. The technique to calculate and output the extra formulae is exactly the same as you're already doing.

So this:-

Code:

---

lblAnnualPercent.Text = CStr(0.1 * hsbAnnualRate.Value)

---

...is taking the value of your annual rate slider, multiplying it by 0.1, converting the result to a string and outputting that string to you annual percent label.

You know how to take values from sliders - just take the values you need and plug them into your formula. then convert the result to a string and output it to the appropriate label as you are with other values.

Have a punt at it and see how you get on. You probably won't get it right first time but if you get stuck, post your code back here and we'll see if we can tell you what went wrong.

I've deleted a bunch of posts because they were just argument. For clarification of our forum rules on what help we should or shouldn't give out here see my prior post.

We all like to offer help in different ways but let's try not to derail someone thread arguing about it.

To be honest, it does look like the hard part, here, is trying to figure out what the instructor is asking in the narrow confines of the outlines steps. It does sound like you haven't learned about classes, is this correct?

What you could do, is build the equation using variables in separate lines, and construct the formula;
Then, you would 'compress' or refactor, the formula back into a one liner that you stated isn't working in post #16.

I tried so many different ways and nothing is giving me the correct output which makes me think that either the formula is wrong or I am just not typing it correctly. The last thing I tried was to create a function and then just call that function every time I slide a scrollbar but it is not giving me the correct value.. Here's what I have

Code:

---

Function calculate() As Double
        Dim r As Double = (0.1 * hsbAnnualRate.Value) / 12
        Dim N As Double = hsbTime.Value * 12
        Dim P As Double = hsbPrincipal.Value * 100
        Dim c As Double = (P*r*(1+r)^N)/((1+r)^N-1)
        Return c
    End Function

---

It is close to the correct value but a little bit off

you mentioned lots of brackets, but you have lost a number in your attempts so far

the order by which the machine calculates your expressions is called the precedence BODMAS is the numonic for simple maths precedence Brackets Or Division, Multiplication, Addition then Subtraction this is the order you do a calculation.

your (p*r*(1+r)^N) might be better as (p*r*((1+r^N)) also the ((1+r)^N-1) might be better as (((1+r)^N)-1)

a simple way to test these is to use the immediate window in the IDE/SDK environment typr print followed by the expression, putting your actual values in the brackets.

if you need to test while running place stop before and after the lines you want to test and then cursor over the values that you are wanting to test when running the program

you can also add debug.print whatever to your code and see what is being produced when the program runs ( seen in the immediate window or msgbox whatever and a message box will pop up ( annoying but usefull)


I had suggested you build a function, but was expecting something like this...

Code:

---

Function calculate(r,N,P) As Double
              Return  (P*r*(1+r)^N)/((1+r)^N-1)
          End Function

---

called as follows

Code:

---

result=calculate( 0.1 * hsbAnnualRate.Value) / 12, hsbTime.Value * 12, hsbPrincipal.Value * 100)

---

although I am not at all sure of your units in that call - if you try this pass in the real values in the immediate window as a way of testing it

NOTE I have not corrected any error here!

here to help

@incidentals, I think you might have got your own brackets confused. Shouldn't this: (p*r*((1+r^N)) be this: (p*r*((1+r)^N) ? I'm assuming you added the extra parentheses to force the power to be applied before the multiplication.

I don't think the parentheses are the problem, though. They look right to me but I'll freely admit I haven't been very thorough with my checking on that. Adding the extra pars might add clarity but I don't think it will change the result in this case. I suggest you check them thoroughly, though, as that will be an easy area to make a mistake on.

I think my suggestion would be much the same as incidentals: step through the code and make sure each of the values (r, N, P) is what you're expecting. Other than that, do some research and check the formula is correct (it probably is) and that your expected answer is correct. From what I'm looking at I believe you've applied the formula correctly.

you are quite correct I added the parameters kepping his for clarity and then lost one in the process

I am also not sure of the precedence, but would always want to ensure the order that i wanted by using too many brackets!

nice catch anyway thanks

I hope the remarks about stops,debug and print/msgbox make sense to the OP

here to help

Okay I finally got everything to work as my instructor emailed me with some more help. Unfortunately I did it using variables which I was not supposed to but it's better than nothing. Thank you everyone for the help!

I would still like to see how it is supposed to have been done.

Code written like this (p*r*((1+r)^N) / ((1+r)^N-1) confuses me and is inefficient unless the compiler factors out the (1+r)^n. You might have less lines of code but the compiler wouldn't care if you did this one step at a time.

Yeah that is the only way that I could think of so I just used the formula that I posted up there but instead of 12 I divided by 1200 and it worked.. If he posts his solution then I will post it here!

Quote:

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I did it using variables

---

I think more than one of us probably breathed a sigh of relief when we read that. I can understand why your instructor might not have wanted to introduce them yet but to ban them seemed a little crazy.

Quote:

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instead of 12 I divided by 1200 and it worked

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Ah, so you were pulling the value for the monthly interest rate wrong. By the way, that probably means you could just take the formula as you had it in post 16 and change the 12 to a 1200 and get the right answer. That would mean you wouldn't be using variables. I personally think it would be a backward step but it might keep your instructor happy.

Quote:

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Originally Posted by FunkyDexter

I think more than one of us probably breathed a sigh of relief when we read that. I can understand why your instructor might not have wanted to introduce them yet but to ban them seemed a little crazy...

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In one of the posts I asked how long the class has been in session. Call me crazy but three weeks in and variables haven't been introduced seems odd.

I'm inclined to agree but I could envisage someone wanting to teach the visual controls first. I guess you could spend 3 weeks + exploring all the options and properties. It's not the approach I would use and it does seem a little bass ackwards but it might not be completely crazy.

Quote:

---

Originally Posted by FunkyDexter

I'm inclined to agree but I could envisage someone wanting to teach the visual controls first. I guess you could spend 3 weeks + exploring all the options and properties. It's not the approach I would use and it does seem a little bass ackwards but it might not be completely crazy.

---

True, but spend a couple of lessons on the basics of EVERY language might be a better idea - I really don't know where they get these teachers from, or the course curriculum. That's why I would personally advocate C as the first step to any programming course. Or, at the very least, a console program first.

Okay he posted the solution here is what the whole homework is supposed to look like

Code:

---

Option Explicit On
Option Strict On

Public Class frmMain

        Private Sub hsbPrincipal_ValueChanged(sender As System.Object, e As System.EventArgs) Handles hsbPrincipal.ValueChanged
                lblPrincipal.Text = FormatCurrency(hsbPrincipal.Value * 100, 0)
                FindValues()
        End Sub

        Private Sub hsbRAte_ValueChanged(sender As System.Object, e As System.EventArgs) Handles hsbRate.ValueChanged
                lblRate.Text = CStr(hsbRate.Value * 0.1)
                FindValues()
        End Sub

        Private Sub hsbTime_ValueChanged(sender As System.Object, e As System.EventArgs) Handles hsbTime.ValueChanged
                lblTime.Text = Cstr(hsbTime.Value)
                FindValues()
        End Sub

        Private Sub FindValues()
                lblPayment.Text = FormatCurrency((Cdbl(lblPrincipal.Text) * CDbl(lblRate.Text) /1200 * (1 + CDbl(lblRate.Text) / 1200) ^ (12 * CDbl(lblTime.Text))) / ((1+ CDbl(lblRate.Text) / 1200) ^ (12 * CDbl(lblTime.Text)) -1))
                lblInterest.Text = FormatCurrency(CDbl(lblPayment.Text) * 12 * CDbl(lblTime.Text) - CDbl(lblPrincipal.Text))
        End Sub
End Class

---

What I expected. Did the instructor tell you what would happen if the users weren't perfect? Have the instructor show you what happens if the user accidentally enters letters into these 'numeric' fields.

I think they're scroll bars rather than text boxes so they're guaranteed to be numeric.

I do know what you're driving at, though, and whole-heartedly agree. It would be nice if FindValues was two separate functions and was separated from the UI. Of course that would have involved not just variables but also parameters! Heaven forbid!

xtreme1, I hope you get a decent mark because, from what I saw you made a pretty god fist of it. And your instructors solution is... troubling. If he's just introducing you to basic controls it might make sense but it is full of bad habits. Hopefully he's way ahead of us and will address some of those habits in future lessons. If not just keep hanging around here and we'll start drilling some better practices for you.

Here's my deal about that code.

I don't like it. I don't like the concept of teaching new developers that there's anything positive about it at all. If it came to one of my code reviews, I'd reject it with half a page of details for improvement. And it would lessen my opinion of whoever tried to submit it.

A lot of times, people dismiss this kind of code as "good enough for examples" or "good enough for homework". Please don't treat it as the kind of code you should write. When you are in school, you get exposed to one of the worst truths of our industry: we're paid to write code that works, not code that is pretty.

But ugly code is hard to maintain. We've spent 50 years of software development learning if we don't think hard about how to write our code elegantly, we usually end up having problems working on that code further down the road.

If nothing else, pay attention to this:

I don't like code that directly uses control values within formulas. No one should like it, and I will stubbornly declare anyone wrong that advocates for it. Let's pretend this is the smallest bit of the formula:

Code:

---

Cdbl(lblPrincipal.Text) * CDbl(lblRate.Text)

---

This is a much better way to write that:

Code:

---

Dim principal = Double.Parse(lblPrincipal.Text)
Dim rate = Double.Parse(lblRate.Text)

Dim result = principal * rate

---

Now, people can and will bicker about CDbl() vs. Double.Parse() vs. Double.TryParse(), but that's ignoring what's really important here. The second technique makes it explicit that we want a TextBox's text to be converted to a Double, and separates that bit of logic from the logic that performs calculations. Separating bits of code that do two different things is the most important skill you can learn.

People can and will bicker about whether it's better to write fewer lines of code. I will again stubbornly declare them wrong, and I hold that opinion very strongly. It is always a good idea to write two lines of code if you want to do two things. When you are an expert with several years of experience, you will understand when that rule can be bent a little. To get there, you have to break it the wrong way a lot of times and get angry at yourself when it costs you an hour of debugging.

So pick up this trick, and for now only break the rule when a professor asks you to do it. School isn't the last place your code will be graded for style: it will happen again in job interviews. But the expectations there are much higher. Look for a book called Clean Code by Robert C. Martin for some guidance there!

@Sitten - Amen! When the OP first posted I had a bad feeling for them. The 'no variables' set all kind of warning bells off.

The teachers coding is suseptable to empty strings in the text or labels used. the divisor can be made zero value and the calculation will explode under "division by zero error"

under the line (1+0)^n-1 is zero therefore the calculation fails!

here to suggest you take teacher to task for not checking for this problem.

Still here to suggest you don't. He deserves it, but you don't want to be the messenger who get's shot for delivering bad news.:eek:

Yeah, I don't recommend telling your teacher a bunch of people on an internet forum say his code's bogus. We're right, but he has a lot of power over you and it's in your best interests to make friends with him. True story: I got an A in a class I should've had a C in because I was the only student who didn't openly voice contempt for the teacher. Diplomacy's sometimes more useful than real knowledge.

Here's what I recommend: if you can, shop around for a better school. You're probably spending real money on this, you may as well get your money's worth. If you don't have many options, whatever. My school classes only taught C++ and Java, and here I am 9 years later writing C#, VB, Swift, and sometimes HTML. If you don't mind reading books, you want a career in programming, and you want to look smarter than your classmates, I can suggest half a dozen books that'll get you there.

It makes me sad how common this is of VB curriculums. It feels like a lot of schools are more worried about generating tuition than useful programmers.