Compare same value in 2 different lists - Kotlin - kotlin

Question

I'm very new to kotlin and I'm doing this exercise where you need 2 lists, one for the invoice and another for the invoice values, for example:
Invoice..........................│ Invoice Values
invoiceNumber = 1........│ itemName = Pepsi
customerName = Alfred│ quantity = 2
TotalValue = $13,00......│ price = 2.50
......................................│ invoiceNumber = 1
......................................│
......................................│ itemName = Cereal
......................................│ quantity = 2
......................................│ price = 4
......................................│ invoiceNumber = 1
and I got it to show only the invoiceNumber that I want, but I can't manage to get the results to do "total += quantity*price" here is what I tried so far:
fun main() {
data class VarInvoice(val numInvoice: Int, val dateInvoice: String, val ssn: String, val total: Double)
data class VarItem(val nameItem: String, val quantity: Int, val unitPrice: Double, val numInvoice2: Int)
val invoice= mutableListOf(
VarInvoice(1, "25/05/1990", "84739572857", 0.00),
VarInvoice(2, "02/09/2009", "38295840284", 0.00),
VarInvoice(3, "13/07/2020", "74959572857", 0.00)
)
val invoiceItem = mutableListOf(
VarItem("Pepsi", 2, 2.50, 1),
VarItem("Cereal", 2, 4.00, 1),
VarItem("Coke", 4, 3.5, 2),
VarItem("Chicken", 1, 13.50, 3)
)
val itemInvoice = invoiceItem.groupBy(VarItem::numInvoice2)
val invoiceEqual = itemInvoice.getValue(1)
println(invoiceEqual)
//but this returns me: [VarItem(nameItem=Pepsi, quantity=2, unitPrice=2.5, numInvoice2=1), VarItem(nameItem=Cereal, quantity=2, unitPrice=4.0, numInvoice2=1)]
//and i have no idea how to do like: if invoiceNumber2 == invoiceNumber { (total += price * quantity)}

Answer 1

Map to total
Assuming you are trying to get the total of a invoice, here is something you could do:
val totals = invoiceItem.groupBy { it.numInvoice2 }.mapValues { item ->
item.value.sumByDouble { it.unitPrice * it.quantity }
}
The totals is a map of invoice number to the total price of all associated items. (assuming you are trying to join invoice to invoiceItem on invoiceNumber2 = invoiceNumber
Extension Property
Or you can slap an extension property onto the VarInvoice, instead of declaring it on the data class as a property (just beware that you can't do this in a function (e.g. your main function))
val VarInvoice.total get() = invoiceItem
.filter { it.numInvoice2 == numInvoice }
.sumByDouble { it.unitPrice * it.quantity }
It would be nice you can provide more information on how you are going to build your application. There could be better solution to this.
P.S. 1
after your code, I got: {1=13.0, 2=14.0, 3=13.5} and I'm trying to figure a way of it beeing sorted and to include the ssn of the person, like this: {1=13.0 "by person with ssn: "84739572857, 3=13.5 "by person with ssn: "74959572857, 2=14.0 "by person with ssn: "38295840284 Sorry, english is not my main language and I can't thank you enough!
Technically you can try something like this:
val totaledInvoice = invoiceItem.groupBy { it.numInvoice2 }.mapValues { item ->
invoice.first { it.numInvoice == item.value.first().numInvoice2 }.copy(
total = item.value.sumByDouble { it.unitPrice * it.quantity }
)
}
You can return whatever you want in the mapValues lambda, but the code is getting ridiculous and inefficient. I would suggest trying out the extension property solution.