假设我们想在data.table的{}内使用j对data.table执行“错综复杂的”操作系列，la：
data(mtcars)
carName <- row.names(mtcars)
mtcars <- data.table(mtcars)
mtcars[, car_name := carName]
mtcars[, {
num_cars <- length(unique(car_name))
avg_disp <- mean(disp)
avg_hp <- mean(hp)
eff <- mpg/wt
data.table(num_cars, avg_disp, avg_hp,
avg_eff = mean(eff), median_eff = median(eff))
}, keyby = cyl]
我知道这可以通过几个步骤执行：
mtcars[, `:=`(num_cars = length(unique(car_name)),
avg_disp = mean(disp),
avg_hp = mean(hp),
eff = mpg/wt),
keyby = cyl]
mtcars[, `:=`(avg_eff = mean(eff), median_eff = median(eff)), keyby = cyl]
mtcars[, .SD[1,list(num_cars, avg_disp, avg_hp, avg_eff, median_eff)],
keyby = cyl]
但我有兴趣知道最常用的方式（阅读：最佳实践）是如何执行此类事情的。
在数据表格的{}中，j内的<!DOCTYPE html>
<html>
<head>
<title>Angular JS table sort and filter example </title>
<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
<script src="http://code.angularjs.org/1.2.16/angular.js"></script>
<script src="http://code.angularjs.org/1.2.16/angular-resource.js"></script>
<script src="http://angular-ui.github.io/bootstrap/ui-bootstrap-tpls-0.11.0.js"></script>
<script>
var app = angular.module('MyForm', ['ui.bootstrap', 'ngResource']);
app.controller('myCtrl', function ($scope) {
$scope.predicate = 'name';
$scope.reverse = true;
$scope.currentPage = 1;
$scope.order = function (predicate) {
$scope.reverse = ($scope.predicate === predicate) ? !$scope.reverse : false;
$scope.predicate = predicate;
};
$scope.students = [
{ name: 'Kevin', age: 25, gender: 'boy' },
{ name: 'John', age: 30, gender: 'girl' },
{ name: 'Laura', age: 28, gender: 'girl' },
{ name: 'Joy', age: 15, gender: 'girl' },
{ name: 'Mary', age: 28, gender: 'girl' },
{ name: 'Peter', age: 95, gender: 'boy' },
{ name: 'Bob', age: 50, gender: 'boy' },
{ name: 'Erika', age: 27, gender: 'girl' },
{ name: 'Patrick', age: 40, gender: 'boy' },
{ name: 'Tery', age: 60, gender: 'girl' }
];
$scope.totalItems = $scope.students.length;
$scope.numPerPage = 5;
$scope.paginate = function (value) {
var begin, end, index;
begin = ($scope.currentPage - 1) * $scope.numPerPage;
end = begin + $scope.numPerPage;
index = $scope.students.indexOf(value);
return (begin <= index && index < end);
};
});
</script>
<style>
.odd {
background-color: antiquewhite;
color: #008b8b;
}
td th {
height: 30px;
min-width: 100px;
}
thead {
background-color: darkgray;
color: white;
height: 30px;
}
</style>
</head>
<body ng-app="MyForm">
<div ng-controller="myCtrl">
<h3>List students</h3>
<div class="container-fluid">
<pre>Click header link to sort, input into filter text to filter</pre>
<hr />
<table class="table table-striped">
<thead>
<tr>
<th>Edit</th>
<th>
<a href="" ng-click="order('name')">Name</a>
</th>
<th><a href="" ng-click="order('age')"> Age</a> </th>
<th><a href="" ng-click="order('gender')">Gender</a> </th>
</tr>
</thead>
<tbody>
<tr>
<td>Filter =>></td>
<td> <input type="text" ng-model="search.name" /></td>
<td> <input type="text" ng-model="search.age" /> </td>
<td><input type="text" ng-model="search.gender" /> </td>
</tr>
<tr ng-repeat="user in students | orderBy:predicate:reverse | filter:paginate| filter:search" ng-class-odd="'odd'">
<td>
<button class="btn">
Edit
</button>
</td>
<td>{{ user.name}}</td>
<td>{{ user.age}}</td>
<td>{{ user.gender}}</td>
</tr>
</tbody>
</table>
<pagination total-items="totalItems" ng-model="currentPage"
max-size="5" boundary-links="true"
items-per-page="numPerPage" class="pagination-sm">
</pagination>
</div>
</div>
</body>
</html> 内工作是不是很不受欢迎？