Why does non-equality check of one variable against many values always return true?

by

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

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

Problem

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

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

  • When v = "x",

    v != "x" becomes "x" != "x", which is false.

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

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

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

  • When v = "y", the expression becomes

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

    or true || false || true, which is true.

  • When v = "z", the expression becomes

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

    or true || true || false, which is true.

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

Alternatively, consider the truth-table:

       │     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

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

Solution

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

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

The correct construction is,

  • 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"
  }

  • for Pascal-like languages 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;

De Morgan’s law

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

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

meaning

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

This makes the logic a bit more obvious.

Specific languages

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

More Related Contents:

  1. What is the best way to organize a lot of data which contains multiple conditions?
  2. Why the logical operators are not working as expected? [closed]
  3. C# Nested IFs OR and condition?
  4. c# home works can’t solve [closed]
  5. Define if statement for property in C# [closed]
  6. If statement in LINQ
  7. IOException: The process cannot access the file ‘file path’ because it is being used by another process
  8. Is “else if” faster than “switch() case”? [duplicate]
  9. Is (4 > y > 1) a valid statement in C++? How do you evaluate it if so?
  10. if statements matching multiple values
  11. Is a double really unsuitable for money?
  12. If vs. Switch Speed
  13. Is “IF” expensive?
  14. Error on if statement – cannot implicitly convert type to ‘bool’
  15. Why the switch statement and not if-else?
  16. What does the word “literal” mean?
  17. Is relying on && short-circuiting safe in .NET?
  18. How to avoid “if” chains?
  19. Why are const parameters not allowed in C#?
  20. What is the difference between IF-ELSE and SWITCH?
  21. Variable declarations following if statements
  22. Why is processing a sorted array slower than an unsorted array?
  23. What is the difference between logical and conditional AND, OR in C#? [duplicate]
  24. Ternary ? operator vs the conventional If-else operator in c# [duplicate]
  25. Is the conditional operator slow?
  26. Execution order of conditions in C# If statement
  27. A clear, layman’s explanation of the difference between | and || in c#?
  28. Are global static classes and methods bad?
  29. Is BitArray faster in C# for getting a bit value than a simple conjuction with bitwise shift?
  30. Database Error: There is no row at position 0

Leave a Comment