[RESOLVED] Math function on big numbers

[Troy Lundin]

I am making a calculator that accepts large number (represented as strings) and performs basic math functions on them. So far I have Add and Multiply figured out.

Here is addition

    Private Function Add(ByVal Number1 As String, ByVal Number2 As String) As String
        Dim num1, num2, r, total As New Integer
        Dim sum As String = ""

        ' Make the numbers the same length, padding zeroes were necessary.
        If Number1.Length > Number2.Length Then
            Number2 = Number2.PadLeft(Number1.Length, Chr(48))
        Else
            Number1 = Number1.PadLeft(Number2.Length, Chr(48))
        End If

        ' This loop adds each column of numbers together, along with the remainder from the previous column.
        For i As Integer = Number1.Length - 1 To 0 Step -1
            num1 = Val(Number1(i))
            num2 = Val(Number2(i))

            total = num1 + num2 + r

            r = 0 ' Reset the remainder

            Do While total > 9
                total -= 10
                r += 1
            Loop

            sum = total & sum ' Adds the new column answer in front of the previous one.

        Next

        If r <> 0 Then        ' If there is a remainder, put it on the front.
            Return r & sum
        Else
            Return sum
        End If
    End Function

And Multiply

    Private Function Multiply(ByVal Number1 As String, ByVal Number2 As String) As String
        Dim num1(Number1.Length - 1), num2(Number2.Length - 1), r, temp As Integer
        Dim total(num1.Count - 1) As String

        ' Fill the arrays with the digits of each number
        For a As Integer = Number1.Length - 1 To 0 Step -1
            num1(a) = Val(Number1(a))
            num2(a) = Val(Number2(a))
        Next

        ' Multiply each lower digit by each upper digit
        For b As Integer = num2.Count - 1 To 0 Step -1
            For c As Integer = num1.Count - 1 To 0 Step -1
                temp = num1(c) * num2(b) + r         ' Multiply the two number together along with any remainder.
                r = 0                                ' Reset the remainder to 0
                Do While temp > 9 And Not c = 0      ' If the number is greater than 9 then we need to get the remainders.
                    temp -= 10
                    r += 1
                Loop
                total(b) = temp & total(b)           ' Store the answer
            Next
        Next

        ' Pad following zeroes
        For d As Integer = 2 To total.Count
            For f As Integer = 2 To d
                total(total.Count - d) = total(total.Count - d) & "0"
            Next
        Next

        ' Add the totals together
        Dim sum As String = ""
        For e As Integer = total.Count - 1 To 0 Step -1
            sum = Add(sum, total(e))
        Next

        ' Trim leading zeroes and commas
        TrimZero(sum)

        ' Add the remainder to the front of the sum, if it exists.
        ' Return the number
        If r <> 0 Then
            Return r & sum
        Else
            Return sum
        End If
    End Function

How would I do subtract and divide?

I saw something like this in the code bank but the examples there were from VB6(?) and were hugely inefficient (compared his Add and Multiply examples to mine, his having way more code and loops.)

[jmcilhinney]

Big numbers are not an issue in .NET 4.0 because it includes the BigInteger type.

[Troy Lundin]

Wow, I did not know that. On top of not knowing about it, it took me 20 minutes to figure out I have to add it as a reference manually.

Haha, late night. Thanks.

[dbasnett]

I so want to stop fooling around with SQL and put BigInteger into my code timer to see how it performs.

[Vectris]

Along the lines of actually coding a calculator I suggest you take a different approach. Check out the calculator in my sig.

I approached the problem by splitting the input into 3 parts, part 1 is the first number, part 2 is the function, and part 3 is the second number. Then just use a select case for part 2 (each function) and perform the function with part 1 and 3, the numbers.

If you do it that way there will be a lot less code, it will be easier to read, and a lot easier to add new functions do unlike your method now where your addition/subtraction and multiplication functions are completely different bodies of code.

[Troy Lundin]

Well, with the addition of BigInteger I can just use

BigInteger.Multiply(num1, num2)

It has the functions built in. I messed with it for a while using massive numbers (over 200,000 digits each). Fun stuff.

[jmcilhinney]

Well, with the addition of BigInteger I can just use

BigInteger.Multiply(num1, num2)

It has the functions built in. I messed with it for a while using massive numbers (over 200,000 digits each). Fun stuff.

You don't have to use methods. The BigInteger type supports standard mathematical operators:

Dim bi1 As BigInteger = 5
Dim bi2 As BigInteger = 10
Dim bi3 As BigInteger = bi1 + bi2
Dim bi4 As BigInteger = bi1 * bi2