INFO: Precedence Affects Grouping of Operands, Not Evaluation

 Last reviewed: August 26, 1997
Article ID: Q37624
The information in this article applies to:
  • Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
  • Microsoft C/C++ for MS-DOS, version 7.0
  • Microsoft Visual C++ for Windows, versions 1.0. 1.5, 1.51, 1.52
  • Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

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." 

Keywords          : CLngIss kbfasttip
Version           : MS-DOS:5.1,6.0,6.00a,6.00ax,7.0;  WINDOWS:1.0,1.5,1.51,1.52; WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo

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Last reviewed: August 26, 1997
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