我必须创建一个依赖于包 data.table 的R包。但是，如果我要执行一个功能，例如包中的下一个功能
randomdt <- function(){
dt <- data.table(random = rnorm(10))
dt[dt$random > 0]
}
函数 [将使用data.frame的方法而不是data.table，因此错误
Error in `[.data.frame`(x, i) : undefined columns selected
会出现。通常这可以通过使用get('[.data.table')或类似的方法来解决（package::function是最简单的），但似乎不起作用。毕竟，[是一个原始函数，我不知道它的方法是如何工作的。
那么，如何从我的包中调用data.table [函数？
答案 0 :(得分：5)
根据MichaelChirico的一些反馈以及Arun和Soheil的评论进行了更新。
粗略地说，您可以考虑两种方法。第一个是将依赖项构建到包本身中，而第二个是在R代码中包含用于测试data.table是否存在的行（如果找不到则会自动安装它）。
data.table FAQ在6.9中具体说明了这一点，并指出您可以通过以下方式确保data.table已正确加载：
要么i）在DESCRIPTION文件的Depends：字段中包含data.table，要么ii）在描述文件的Imports：字段中包含data.table并在NAMESPACE文件中包含import（data.table）。
如评论中所述，这是许多软件包中常见的R行为。
另一种方法是创建特定的代码行，以测试并导入所需的包作为代码的一部分。考虑到使用上面提供的选项的优雅，我认为这不是理想的解决方案。但是，这在技术上是可行的。
执行此操作的一种简单方法是使用require或library来检查是否存在data.table，如果无法附加，则会引发错误。您甚至可以使用一组简单的条件语句来运行install.packages来安装加载它们时所需的内容。
library和require here之间差异的文章，并强调了在library中使用ListView myList;
Button getChoice, clearAll, selectAll;
SharedPreferences sharedpreferences;
public static final String MyPREFERENCES = "MyUserChoice" ;
ArrayList<String> selectedItems = new ArrayList<String>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
myList = (ListView)findViewById(android.R.id.list);
getChoice = (Button)findViewById(R.id.getchoice);
clearAll = (Button)findViewById(R.id.clearall);
selectAll = (Button)findViewById(R.id.selectall);
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_multiple_choice);
myList.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
myList.setAdapter(adapter);
sharedpreferences = getSharedPreferences(MyPREFERENCES, Context.MODE_PRIVATE);
if(sharedpreferences.contains(MyPREFERENCES)){
LoadSelections();
}
getChoice.setOnClickListener(new Button.OnClickListener() {
@Override
public void onClick(View v) {
String selected = "";
int cntChoice = myList.getCount();
SparseBooleanArray sparseBooleanArray = myList.getCheckedItemPositions();
for (int i = 0; i < cntChoice; i++) {
if (sparseBooleanArray.get(i)) {
selected += myList.getItemAtPosition(i).toString() + "\n";
System.out.println("Checking list while adding:" + myList.getItemAtPosition(i).toString());
SaveSelections();
}
}
Toast.makeText(MainActivity.this, selected, Toast.LENGTH_LONG).show();
}
});
clearAll.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
ClearSelections();
}
});
selectAll.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
SelectAllSelections();
}
});
packageManager = getPackageManager();
new LoadApplications().execute();
}
private void SaveSelections() {
// save the selections in the shared preference in private mode for the user
SharedPreferences.Editor prefEditor = sharedpreferences.edit();
String savedItems = getSavedItems();
prefEditor.putString(MyPREFERENCES.toString(), savedItems);
prefEditor.commit();
}
private String getSavedItems() {
String savedItems = "";
int count = this.myList.getAdapter().getCount();
for (int i = 0; i < count; i++) {
if (this.myList.isItemChecked(i)) {
if (savedItems.length() > 0) {
savedItems += "," + this.myList.getItemAtPosition(i);
} else {
savedItems += this.myList.getItemAtPosition(i);
}
}
}
return savedItems;
}
private void LoadSelections() {
// if the selections were previously saved load them
if (sharedpreferences.contains(MyPREFERENCES.toString())) {
String savedItems = sharedpreferences.getString(MyPREFERENCES.toString(), "");
selectedItems.addAll(Arrays.asList(savedItems.split(",")));
int count = this.myList.getAdapter().getCount();
for (int i = 0; i < count; i++) {
String currentItem = (String) myList.getAdapter()
.getItem(i);
if (selectedItems.contains(currentItem)) {
myList.setItemChecked(i, true);
Toast.makeText(getApplicationContext(),
"Current Item: " + currentItem,
Toast.LENGTH_LONG).show();
} else {
myList.setItemChecked(i, false);
}
}
}
}
private void ClearSelections() {
// user has clicked clear button so uncheck all the items
int count = this.myList.getAdapter().getCount();
for (int i = 0; i < count; i++) {
this.myList.setItemChecked(i, false);
}
// also clear the saved selections
SaveSelections();
}
private void SelectAllSelections() {
// user has clicked clear button so uncheck all the items
int count = this.myList.getAdapter().getCount();
for (int i = 0; i < count; i++) {
this.myList.setItemChecked(i, true);
}
// also clear the saved selections then uncomment the below line.
// SaveSelections();
}
protected void onListItemClick(ListView l, View v, int position, long id){
super.onListItemClick(l, v, position, id);
ApplicationInfo app = applist.get(position);
try{
Intent intent = packageManager.getLaunchIntentForPackage(app.packageName);
/*if(intent != null){
startActivity(intent);
}*/
}catch(ActivityNotFoundException e){
Toast.makeText(MainActivity.this,e.getMessage(), Toast.LENGTH_LONG).show();
}catch(Exception e){
Toast.makeText(MainActivity.this,e.getMessage(), Toast.LENGTH_LONG).show();
}
}
private List<ApplicationInfo> checkForLaunchIntent(List<ApplicationInfo> list){
ArrayList<ApplicationInfo> appList = new ArrayList<ApplicationInfo>();
for(ApplicationInfo info : list){
try{
if(packageManager.getLaunchIntentForPackage(info.packageName)!=null){
appList.add(info);
}
}catch(Exception e){
e.printStackTrace();
}
}
return appList;
}
private class LoadApplications extends AsyncTask<Void, Void, Void>{
private ProgressDialog progress = null;
protected Void doInBackground(Void... params){
applist = checkForLaunchIntent(packageManager.getInstalledApplications(PackageManager.GET_META_DATA));
listadapter = new AppAdapter(MainActivity.this, R.layout.activity_list_app, applist);
return null;
}
protected void onPostExecute(Void result){
setListAdapter(listadapter);
progress.dismiss();
super.onPostExecute(result);
}
protected void onPreExecute(){
progress = ProgressDialog.show(MainActivity.this, null, "Loading apps info...");
super.onPreExecute();
}
}
}
的情况该软件包对于即将推出的代码绝对必不可少。