Q37624: INFO: Precedence Affects Grouping of Operands, Not Evaluation

Article: Q37624
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 12-DEC-2001

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The information in this article applies to:

- Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52, 2.0, 2.1, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SUMMARY
=======

Operator precedence in C affects how operands are grouped; it does not
necessarily indicate the order in which operands are evaluated. The logical AND
(&&) operator has a higher precedence than the logical OR (||) operator.
Therefore, the statement:

  lvalue = operand1 || operand2 && operand3;

is logically equivalent to the following:

  lvalue = operand1 || (operand2 && operand3);

However, this grouping does not indicate that the subexpression (operand2
&& operand3) will be evaluated first. In fact, this statement is a
logical OR expression with two operands: operand1 and "(operand2 &&
operand3)." Because a logical OR expression evaluates its operands in
left-to-right order, operand1 is evaluated first.

MORE INFORMATION
================

The following example demonstrates this behavior:

Sample Code:
------------

  /*
   * Compile options needed: none
   */ 

  #include <stdio.h>

  int a, b, c, d;

  void main()
  {
     a = (b = 2) || (c = 3) && (d = 4);
     printf("a = %d, b = %d, c = %d, d = %d\n", a, b, c, d);
  }

Program output:

  a = 1, b = 2, c = 0, d = 0

Because "(b = 2)" is not 0, the result of the expression is TRUE (1) and the code
does not perform any further evaluations. Therefore, the code does not perform
the assignments to c and d. To ensure that values are always assigned to the
variables, use separate assignment statements.

As Kernighan and Ritchie note on page 54 of "The C Programming Language" (second
edition), "The moral is that writing code that depends on the order of
evaluation is a bad programming practice in any language."

Additional query words:

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Keywords          : kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbvc150 kbvc100 kbVC500 kbVC600 kbVC151 kbVC200 kbVC210 kbVC32bitSearch kbVC152 kbVC500Search
Version           : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Issue type        : kbinfo

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