Question

The compiler gives an error on this statement.
i>=3?b=10:b=5;

error: lvalue required as left operand of assignment

Not able to figure out why. Compiler being used is GCC.

Answer 1

The problem is explained by operator precedence. In accordance with C grammar your

 i >= 3 ? b = 10 : b = 5;

is interpreted by C compiler as

 (i >= 3 ? b = 10 : b) = 5;

Firstly, this is probably not what you intended. Secondly, in C language the result of ?: operator is not an lvalue. You cannot assign anything to it. Hence the error message.

In order to correct the problem (assuming I understand your intent correctly), you either have to use braces

 i >= 3 ? (b = 10) : (b = 5);

or simply rewrite it in a more conventional way

 b = i >= 3 ? 10 : 5;

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As a side note, this happens to be one of the differences between C and C++ grammar. In C++ your original statement would be interpreted as i >= 3 ? (b = 10) : (b = 5) even without explicit braces. On top of that, in C++ the result of ?: can be an lvalue.

In other words, in C++ your original statement would compile and work “as intended”, but not in C.