（赋值中无效的左值）我运行它时会发生此错误。这是什么意思？

Question

this is the code and the compiler says there is sth wrong with line 7.

include<stdio.h>
main()
{
char m;
 int a,b,n=0;
scanf("%c%d%d",&m,&a,&b);
m=='A' || m=='B' || m=='C' ? n=(3*a)+(5*b) : n=(5*a)+(3*b);
printf("%d\n",n);
}

Answer 1

Use instead

m=='A' || m=='B' || m=='C' ? n=(3*a)+(5*b) : ( n=(5*a)+(3*b));

Otherwise the statement looks like

( m=='A' || m=='B' || m=='C' ? n=(3*a)+(5*b) : n)=(5*a)+(3*b);

Or you could write

n = m=='A' || m=='B' || m=='C' ? (3*a)+(5*b) : (5*a)+(3*b);

The conditional operator in C is defined the following way

conditional-expression:
    logical-OR-expression
    logical-OR-expression ? expression : conditional-expression

As the assignment operator has lower priority then the compiler issues an error because the assignment is excluded from the conditional operator for the third operand

The used by you expression would be valid in C++ because in C++ the operator is defined differently

conditional-expression:
    logical-or-expression
    logical-or-expression ? expression : assignment-expression
                                         ^^^^^^^^^^^^^^^^^^^^^

Answer 2

There is no need to use a complicated statement that confuses everyone, including the compiler. This is just as effective, and a lot easier to read:

if (m=='A' || m=='B' || m=='C')
   n=(3*a)+(5*b);
else
   n=(5*a)+(3*b);