Errors using ternary operator in c

Question

I have a piece of code in C given as follows :

main()
{
    int a=10, b;
    a>=5 ? b=100 : b=200 ;
    printf("%d" , b);
}

running the code on gcc compiler in unix generates the compile-time error as 'lvalue required as left operand of assignment' and points the error at b = 200 whereas in windows compiling using Turbo C gives 200 as output.

Can anybody please explain what exactly is happening in this case ?

Answer 1

In C the ternary operator is defined like

logical-OR-expression ? expression : conditional-expression

where conditional expression is defined like

logical-OR-expression

The assignment operator has a lower priority than the OR operator. Thus you have to write

a >= 5 ? b = 100 : ( b = 200 );

Otherwise the compiler consideres the expression like

( a >= 5 ? b = 100 :  b ) = 200;

As the result of the ternary operator in C is not an lvalue then the above expression is invalid and the compiler issues an error.

From the C Standard:

the result is the value of the second or third operand (whichever is evaluated), converted to the type described below

and footnote:

110) A conditional expression does not yield an lvalue.

Take into account that there is an essential difference between the operator definition in C and C++. In C++ it is defined as

logical-or-expression ? expression : assignment-expression

In C++ the same GCC compiles the code successfully

#include <iostream>

int main() 
{
    int a = 10, b;

    a >= 5 ? b = 100 : b = 200;

    std::cout << "b = " << b << std::endl;

    return 0;
}