How to sum items in a List in Dart - dart

Question

What's the best way to sum all the "amounts" of all "expenses", for each user?
I've tried a few different things but I can't quite get it right. It should return 2 values: 20.0 and 24.90
[
{
"id": 3,
"company": {
"id": 2
},
"user": {
"id": 3,
"first_name": "Fred",
"last_name": "Smith",
"email": "asdfasf",
"is_suspended": false,
"vendor_id": "FS-100",
"username": "etytyurtyu",
"expense": [
{
"id": 7,
"date": "2019-12-14T00:00:00.000Z",
"amount": 20.0,
"payment_type": "companyAccount",
"last_modified": "2019-12-16T23:50:00.459064Z",
"receipt_uri": [
"URL8",
"URL9"
],
"user": {
"id": 3
},
"expense_type": {
"id": 4
},
"booking": {
"id": "HI-3565346"
}
}
]
}
},
{
"id": 2,
"company": {
"id": 2
},
"user": {
"id": 2,
"first_name": "Pierre",
"last_name": "XXXMar",
"email": "asdfasdfads",
"is_suspended": false,
"vendor_id": "PM-100",
"username": "asdfas",
"expense": [
{
"id": 2,
"date": "2019-12-16T00:00:00.000Z",
"amount": 12.45,
"payment_type": "provided",
"last_modified": "2019-12-16T19:01:37.092932Z",
"receipt_uri": [
"URL1"
],
"user": {
"id": 2
},
"expense_type": {
"id": 6
},
"booking": {
"id": "MU-123414"
}
},
{
"id": 5,
"date": "2019-12-08T00:00:00.000Z",
"amount": 12.45,
"payment_type": "provided",
"last_modified": "2019-12-16T23:50:00.459064Z",
"receipt_uri": [
"URL1"
],
"user": {
"id": 2
},
"expense_type": {
"id": 6
},
"booking": {
"id": "MU-123414"
}
},
{
"id": 3,
"date": "2019-12-17T00:00:00.000Z",
"amount": 20.0,
"payment_type": "companyCard",
"last_modified": "2019-12-16T19:01:37.092932Z",
"receipt_uri": [
"URL5",
"URL6"
],
"user": {
"id": 2
},
"expense_type": {
"id": 12
},
"booking": {
"id": "HI-3565346"
}
}
]
}
}
]
Thanks

Answer 1

You would first map the expenses amounts, then fold it into a single value, like this :
double sum = expenses.map((expense) => expense.amount).fold(0, (prev, amount) => prev + amount);

Answer 2

List item
For the sake of completeness: In 2021 you can replace the .fold(0, (prev, amount) => prev + amount) part by just calling .sum on the list. Just import import 'package:collection/collection.dart'; first –
tmaihoff