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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. Section 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 programmer to write an expression (particularly an expression used as a condition) that can be easily misinterpreted.

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int a = 2;
int b = 2;
int c = 2;
/* ... */
if (a < b < c) /* misleadingMisleading, likely bug */
/* ... */
if (a == b == c) /* misleadingMisleading, likely bug */

The expression a < b < c evaluates to true rather than, as its author probably intended, to false, and the expression a == b == c evaluates to false rather than, as its author probably intended, to true.

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if ( (a < b) && (b < c) ) /* clearerClearer, and probably what was intended */
/* ... */
if ( (a == b) && (a == c) ) /* dittoDitto */

Risk Assessment

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

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Bibliography

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