如何在 Dart 中对列表中的项目求和

Question

为每个用户汇总所有"费用"的所有"金额"的最佳方法是什么？

我尝试了几种不同的东西，但我不能完全正确。它应该返回 2 个值：20.0 和 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"
}
}
]
}
}
]

谢谢

Answer 1

您将首先映射费用金额，然后将其折叠为单个值，如下所示：

double sum = expenses.map((expense) => expense.amount).fold(0, (prev, amount) => prev + amount);

Answer 2

1. 列表项

为了完整起见：在 2021 年，您只需在列表中调用 .sum 即可替换 .fold(0， (prev， amount( => prev + amount( 部分。只需导入导入'package：collection/collection.dart';第一– 特迈霍夫