为什么针对多个值对一个变量进行非等式检查总是返回 true? [英] Why does non-equality check of one variable against many values always return true?

问题描述

我的程序中有一个变量 v，它可以从一组值中取任何值

I have a variable v in my program, and it may take any value from the set of values

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

我的目标是仅当 v 不是 "x"、"y" 或 时才执行某些语句>z".

And my goal is to execute some statement only when v is not "x", "y", or "z".

我试过了，

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

• for C-like languages (where equality operators compare the actual string values; e.g. 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;

if 条件中的语句总是被执行.我做错了什么吗?

The statements inside the if condition always get executed. Am I doing anything wrong?

推荐答案

使用 &&/AND/and，而不是 <代码>||/OR/or:

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

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

问题

如果 if 块总是被执行，则 if 块的条件 总是评估为 true.逻辑表达式一定是错误的.

Problem

If an if block is always executed, the condition for the if block always evaluates to true. The logical expression must be wrong.

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

Let us consider v != "x" || v != "y" || v != "z" for each value of v.

• 当 v = "x" 时，

v != "x" 变成 "x";!= "x"，这是 false.
v != y" 变成 x";!= "y"，这是 true.
v != "z" 变成 "x";!= "z"，这是真.

v != "x" becomes "x" != "x", which is false.
v != "y" becomes "x" != "y", which is true.
v != "z" becomes "x" != "z", which is true.

表达式计算结果为 false ||真||真，即真.

The expression evaluates to false || true || true, which is true.

当v = "y"时，表达式变为

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

或 true ||假||真，即真.

当v = "z"时，表达式变为

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

或 true ||真||假，这是真.

对于 v 的任何其他值，表达式的计算结果为 true ||真||真，即真.

For any other value for v, the expression evaluates to true || true || true, which is 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

如您所见，您的逻辑表达式总是的计算结果为 true.

As you can see, your logical expression always evaluates to true.

你想要做的是，找到一个逻辑表达式，当

What you want to do is, find a logical expression that evaluates to true when

(v 不是x")and(v 不是y")and(v 不是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)

• for C-like languages (eg. c#, javascript-(may need the strict equality operator !==), 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;

根据德摩根定律，该表达式也可以改写为(使用 C-像语法)

By De Morgan's law, the expression can also be rewritten as (using C-like syntax)

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

意义

not((v 是x")or<代码>(v 是y")or(v 是z")).

这使得逻辑更明显一些.

This makes the logic a bit more obvious.

某些语言具有用于测试集合成员资格的特定构造，或者您可以使用数组/列表操作.

Some languages have specific constructs for testing membership in sets, or you can use array/list operations.

• sql: v 不在 ('x', 'y', 'z')

javascript: [x"，y"，z"].indexOf(v) == -1

python: v 不在 {x"、y"、z"}

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

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

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