使用在C三元操作错误 [英] Errors using ternary operator in c

问题描述

我有一块用C code给出如下：

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);
}

运行在UNIX gcc编译器的code生成编译时错误的
需要作为分配的左操作数左值，并在乙组分误差为200，而在使用的Turbo C编译窗户给200作为输出。

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.

有人能解释一下究竟是什么在这种情况下发生？

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

推荐答案

在C中的三元运算符如

逻辑或-EX pression？前pression：有条件-EX pression

logical-OR-expression ? expression : conditional-expression

其中的条件前pression 的的定义如下

where conditional expression is defined like

logical-OR-expression

赋值运算符比OR运算符优先级较低。因此，你必须写

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

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

否则编译器consideres除权pression像

Otherwise the compiler consideres the expression like

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

在C语言中三元运算符的结果不是左值那么上述前pression无效，编译器会发出错误。

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.

从C标准：

结果是第二个或第三个操作数的值（取
  被评估），转换为下面

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

和脚注：

110）有条件的前pression不会产生左值。

110) A conditional expression does not yield an lvalue.

要考虑到有在C和C ++操作定义之间的本质区别。在C ++中，它被定义为

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

逻辑或-EX pression？前pression：分配-EX pression

logical-or-expression ? expression : assignment-expression

在C ++中相同的GCC编译code成功

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;
}

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