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Compound condition with AND

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• A compound condition (or Boolean expression) is one where different relationships are combined using logical operators

  The logical operators are:

  • or
  • and
  • not

• It is rather trivial to handle not by re-writing a relationship...

  Examples:

  • "not (a < b)" is equivalent to "a >= b"
  • "not (a == b)" is equivalent to "a != b"
  • and so on...

• Short circuit boolean evaluation
  • The "short circuit boolean evaluation" is a technique commonly used in programming languages (such as C, C++, Java) to evalue compound boolean expressions

  • The if-statement with an or logical operator:

       if ( A or "Rest of condition" )
          S1
       else
          S2
    
       Rest of the program
     

    is execute as follows:

    1. evaluate A 2. if TRUE then execute S1 and proceed to "Rest of the program" 3. if FALSE then CONTINUE with the evaluation of the "Rest of condition" 3a. if "Rest of condition" is TRUE, then execute S1 and proceed to "Rest of the program" 3b. if "Rest of condition" is FALSE, then execute S2 and proceed to "Rest of the program"

• Program control of the if-statement with an compound condition "A and theRest":

  if ( A and theRest ) ----> | statement1; V else +--------------+ FALSE statement2; | eval A |-----------------+ +--------------+ | | (TRUE) | | | V | +--------------+ FALSE | | eval theRest |------------+ | +--------------+ | | | (TRUE) | | | | | V | | statement1 | | | | | +---------+ | | | | | | | | | statement2 <-------------+----+ | | | | +-------->+ | | V

• Assembler construct that realizes the control flow of the if-statement with an compound condition "A or theRest" is as follows:

  Evaluate "A" (CMP) FALSE Branch on FALSE (!!!) outcome of "condition" to here (A:) --------+ | (TRUE) | | | V | Evaluate "theRest" (another CMP) | FALSE | Branch on FALSE (!!!) outcome of "theRest" to here (B:) ----------+ | (TRUE) | | | V | A: "statement1" assembler code | | | Branch always to there (B:) -------------+ | | | | | "statement2" assembler code <---------|----------------------+ | | V | C: +<-------------------------------+ | V

• Example:

     int x, y, a; 
  
     if (a <= x and x <= b)
        x = x + 1;
     else
        x = x - 1;
  

  The flow chart of the above program is:

  Assembler program for this compound if-statement: click here