我有一个变量v在我的程序中，可能需要any值集中的值

"a", "b", "c", ..., "z"

我的目标是仅在以下情况下执行某些语句v is not "x", "y", or "z".

我努力了，

• 对于类 C 语言（其中相等运算符比较actual字符串值；例如c#, javascript, php)

  if (v != "x" || v != "y" || v != "z")
  {
      // the statements I want to be executed
      // if v is neither "x", nor "y", nor "z"
  }
  

• 对于类似 Pascal 的语言（例如plsql)

  IF (v != 'x' OR v != 'y' OR v != 'z') THEN
      -- the statements I want to be executed
      -- if v is neither "x", nor "y", nor "z"
  END IF;
  

里面的语句if状况总是被处决。我做错了什么吗？

Use &&/AND/and, not ||/OR/or:

v != "x" && v != "y" && v != "z"

Problem

If an if块总是被执行，if 块的条件总是评估为true。逻辑表达式一定是错误的。

让我们考虑一下v != "x" || v != "y" || v != "z"对于每个值v.

• When v = "x",

  v != "x"变成"x" != "x"，即false.
  v != "y"变成"x" != "y"，即true.
  v != "z"变成"x" != "z"，即true.

  该表达式的计算结果为false || true || true，即true.

• When v = "y"，表达式变为

    "y" != "x" || "y" != "y" || "y" != "z"
  

  or true || false || true，即true.

• When v = "z"，表达式变为

    "z" != "x" || "z" != "y" || "z" != "z"
  

  or true || true || false，即true.

• 对于任何其他值v，表达式的计算结果为true || true || true，即true.

或者，考虑真值表：

       │     A          B          C      │
  v    │  v != "x"   v != "y"   v != "z"  │  A || B || C
───────┼──────────────────────────────────┼──────────────
 "x"   │    false      true       true    │     true
 "y"   │    true       false      true    │     true
 "z"   │    true       true       false   │     true
other  │    true       true       true    │     true

正如你所看到的，你的逻辑表达always评估为true.

Solution

你想要做的是，找到一个计算结果为的逻辑表达式true when

(v is not "x")and(v is not "y")and(v is not "z").

正确的构造是，

• 对于类 C 语言（例如c#, javascript-（可能需要严格相等运算符!==), php)

    if (v != "x" && v != "y" && v != "z")
    {
        // the statements I want to be executed
        // if v is neither "x", nor "y", nor "z"
    }
  

• 对于类似 Pascal 的语言plsql

    IF (v != 'x' AND v != 'y' AND v != 'z') THEN
        -- the statements I want to be executed
        -- if v is neither "x", nor "y", nor "z"
    END IF;
  

德摩根定律

By 德摩根定律，表达式也可以重写为（使用类似 C 的语法）

!(v == "x" || v == "y" || v == "z")

meaning

not((v is "x")or(v is "y")or(v is "z")).

这使得逻辑更加明显。

特定语言

某些语言具有用于测试集合中的成员资格的特定构造，或者您可以使用数组/列表操作。

• sql: v NOT IN ('x', 'y', 'z')

• javascript: !["x", "y", "z"].includes(v)

• python: v not in {"x", "y", "z"}

• java: !Arrays.asList("x", "y", "z").contains(v)

• java-9（以上）：!Set.of("x", "y", "z").contains(v)