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The relational and equality operators are left-associative in C. Consequently, C, unlike many other languages, allows chaining of relational and equality operators. Subclause 6.5.8, footnote 107, of the C Standard [ISO/IEC 9899:2011], says:

The expression a<b<c is not interpreted as in ordinary mathematics. As the syntax indicates, it means (a<b)<c; in other words, "if a is less than b, compare 1 to c; otherwise, compare 0 to c."

These operators are left-associative, which means the leftmost comparison is performed first, and the result is compared with the rightmost comparison. This syntax allows a not non-associative as they often are in other languages. This allows a C++ programmer to write an expression (particularly an expression used as a condition) that can be easily misinterpreted.

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Noncompliant Code Example

Although this noncompliant code example compiles correctly, it is unlikely that it means what the author of the code intended:

int a = 2;
int b = 2;
int c = 2;
//* ... */
if ( a < b < c ) // condition #1, misleading,* Misleading; likely bug */
//* ... */
if ( a == b == c ) // condition #2, misleading, * Misleading; likely bug
 */

The expression a < b < c evaluates to true rather than, While the code in the Non-Compliant Code Example compiles correctly, it is unlikely that it means what the author of the code intended. Condition #1 will evaluate to true, rather than false as its author probably intended, and condition #2 will evaluate to false, rather than true , to false, and the expression a == b == c evaluates to false rather than, as its author probably intended, to true.

Compliant Solution

Treat relational and equality operators as if it were invalid to chain them.:

if ( (a < b) && (b < c) ) //* clearer,Clearer and probably what was intended */
//* ... */
if ( (a == b) && (a == c) ) /* Ditto */ ditto

Risk Assessment

Incorrect use of relational and equality operators can lead to incorrect control flow.

Other Languages

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Automated Detection

Related Guidelines

Bibliography

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