mapping custom object kotlin-CodePudding

I have a custom object:

data class MoneyTransaction(
    val amount: Double,
    val category: String
)

I have a list of MoneyTransaction. I want to create a map out of that list where keys are categories, and the values are the total amount according to the category. Kotlin has functions like groupBy, groupByTo, groupingBy. But there is no tutorial or documentation about those, so I can't figure it out. So far I got this:

 val map = transactionList.groupBy({it.category},{it.amount})

But this doesn't give the total amount, just separate amounts on each category

Any help would be much appreciated.

CodePudding user response：

So first of all you group your transactions by category

transactionList.groupBy { it.category }

this gives you a Map<String, List<MoneyTransaction>> after that you need to sum up the amounts

transactionList.groupBy { it.category }
  .mapValues { (_, transactionsInCategory) -> 
    transactionsInCategory.sumOf { it.amount }
  }

This will give you a Map<String, Double> with the value representing the sum of all transactions in the category.

CodePudding user response：

You can use groupingBy and then fold:

transactions.groupingBy(MoneyTransaction::category)
    .fold(0.0) { acc, next -> acc   next.amount }

groupingBy here would return a Grouping<MoneyTransaction, String>, which is an intermediate collection of the groups. Then you fold each of the groups by starting from 0, and adding the next transaction's amount.

Looking at the implementation, the groupingBy call doesn't actually does any actual "grouping" - it just creates a lazy Grouping object. So effectively, you are going through the collection only once.