R提供了两种不同的方法来访问list或data.frame中的元素:[]和[[]]。

这两者之间的区别是什么，什么时候我应该使用其中一个而不是另一个?

我一直在写一些shell脚本，如果有能力停止所述shell脚本的执行，如果任何命令失败，我会发现它很有用。请看下面的例子:

#!/bin/bash

cd some_dir

./configure --some-flags

make

make install

因此，在这种情况下，如果脚本不能更改到指定的目录，那么如果失败，它肯定不想在之后执行./configure。

现在我很清楚，我可以对每个命令进行if检查(我认为这是一个无望的解决方案)，但是是否存在一个全局设置，以便在其中一个命令失败时使脚本退出?

我需要隐藏所有被拒绝的消息:

find . > files_and_folders

当这样的信息出现时，我正在做实验。我需要收集所有的文件夹和文件，它没有出现。

是否可以将权限级别直接指向files_and_folders文件?

如何同时隐藏错误?

我正在使用Javascript(ES6) /FaceBook反应，并试图获得大小不同的数组的前3个元素。我想做Linq take(n)的等效。

在我的Jsx文件中我有以下内容:

var items = list.map(i => {
  return (
    <myview item={i} key={i.id} />
  );
});

然后得到我尝试的前3个项目

  var map = new Map(list);
    map.size = 3;
    var items = map(i => {
      return (<SpotlightLandingGlobalInboxItem item={i} key={i.id} />);
    });

这并没有起作用，因为map没有一个set函数。

你能帮帮我吗?

这是什么?

这是一些关于在编程PHP时可能遇到的警告、错误和注意事项的答案，而不知道如何修复它们。这也是一个社区维基，所以每个人都被邀请加入和维护这个列表。

为什么会这样?

Questions like "Headers already sent" or "Calling a member of a non-object" pop up frequently on Stack Overflow. The root cause of those questions is always the same. So the answers to those questions typically repeat them and then show the OP which line to change in their particular case. These answers do not add any value to the site because they only apply to the OP's particular code. Other users having the same error cannot easily read the solution out of it because they are too localized. That is sad because once you understood the root cause, fixing the error is trivial. Hence, this list tries to explain the solution in a general way to apply.

我该怎么做呢?

如果您的问题被标记为此问题的副本，请在下面找到您的错误消息并将修复应用于您的代码。答案通常包含进一步的调查链接，以防仅从一般答案中不清楚。

如果您想投稿，请添加您“最喜欢的”错误消息、警告或通知，每个答案一条，简短描述它的含义(即使它只是突出显示手册页的术语)，可能的解决方案或调试方法，以及现有的有价值的问答列表。此外，请随意改进任何现有的答案。

列表

Nothing is seen. The page is empty and white. (also known as White Page/Screen Of Death) Code doesn't run/what looks like parts of my PHP code are output Warning: Cannot modify header information - headers already sent Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean given a.k.a. Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource Warning: [function] expects parameter 1 to be resource, boolean given Warning: [function]: failed to open stream: [reason] Warning: open_basedir restriction in effect Warning: Division by zero Warning: Illegal string offset 'XXX' Warning: count(): Parameter must be an array or an object that implements Countable Parse error: syntax error, unexpected '[' Parse error: syntax error, unexpected T_XXX Parse error: syntax error, unexpected T_ENCAPSED_AND_WHITESPACE Parse error: syntax error, unexpected T_PAAMAYIM_NEKUDOTAYIM Parse error: syntax error, unexpected 'require_once' (T_REQUIRE_ONCE), expecting function (T_FUNCTION) Parse error: syntax error, unexpected T_VARIABLE Fatal error: Allowed memory size of XXX bytes exhausted (tried to allocate XXX bytes) Fatal error: Maximum execution time of XX seconds exceeded Fatal error: Call to a member function ... on a non-object or null Fatal Error: Call to Undefined function XXX Fatal Error: Cannot redeclare XXX Fatal error: Can't use function return value in write context Fatal error: Declaration of AAA::BBB() must be compatible with that of CCC::BBB()' Return type of AAA::BBB() should either be compatible with CCC::BBB(), or the #[\ReturnTypeWillChange] attribute should be used Fatal error: Using $this when not in object context Fatal error: Object of class Closure could not be converted to string Fatal error: Undefined class constant Fatal error: Uncaught TypeError: Argument #n must be of type x, y given Notice: Array to string conversion (< PHP 8.0) or Warning: Array to string conversion (>= PHP 8.0) Notice: Trying to get property of non-object error Notice: Undefined variable or property "Notice: Undefined Index", or "Warning: Undefined array key" Notice: Undefined offset XXX [Reference] Notice: Uninitialized string offset: XXX Notice: Use of undefined constant XXX - assumed 'XXX' / Error: Undefined constant XXX MySQL: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ... at line ... Strict Standards: Non-static method [<class>::<method>] should not be called statically Warning: function expects parameter X to be boolean/string/integer HTTP Error 500 - Internal server error Deprecated: Arrays and strings offset access syntax with curly braces is deprecated

还看到:

这个符号在PHP中是什么意思?

我如何打印错误/异常在except:块?

try:
    ...
except:
    print(exception)

我检查了我的PHP ini文件（PHP.ini），并设置了display_errors，错误报告也是E_ALL。我已重新启动Apache Web服务器。

我甚至把这些行放在了脚本的顶部，它甚至不能捕捉到简单的解析错误。例如，我用“$”声明变量，而不关闭语句“；”。但我的所有脚本都显示了这些错误的空白页面，但我希望在浏览器输出中看到这些错误。

error_reporting(E_ALL);
ini_set('display_errors', 1);

还有什么要做？

考虑到这段代码，我能绝对确定finally块总是执行的吗，不管something（）是什么？

try {  
    something();  
    return success;  
}  
catch (Exception e) {   
    return failure;  
}  
finally {  
    System.out.println("I don't know if this will get printed out");
}