C Language Operators [closed]

问题:

#include <stdio.h>  

int main() 
{

    int a=-1?2:5 + 8?4:5;
    printf("%d\n",a);
    return 0;
}

The output of above program is 2. But why ? Please explain

回答1:

Write human-readable and understandable code. (Atleast, try to...)

 int a=-1?2:5 + 8?4:5;

is the same as

int a = (-1) ? 2 : ( 5 + ( 8 ? 4 : 5) );

^{Reference: Operator Precedence}

Now, let's compare that with the ternary operator condition, as mentioned in C11, chapter §6.5.15,

The first operand is evaluated; there is a sequence point between its evaluation and the evaluation of the second or third operand (whichever is evaluated). The second operand is evaluated only if the first compares unequal to 0; the third operand is evaluated only if the first compares equal to 0; the result is the value of the second or third operand (whichever is evaluated),

So, in your case,

• First operand is unequal to zero
• So, it evaluates the second operand and the result, the value of the operand, is returned and stored into the LHS variable of assignment operator.

回答2:

The statement :

int a=-1?2:5 + 8?4:5;

or better written with parentheses :

int a = (-1) ? 2 : ( 5 + 8?4:5);

which in turn means :

if (-1)
    a = 2;
else {
    if (8)
        a = 9; //5+4
    else
        a = 10; //5+5
}

Any condition different than 0 is evaluated as true. So the condition if(-1) is different than 0, therefore true. Thus, the if block will be executed and a will get value 2.

回答3:

Because a ? b : c evaluates to b if a is non-zero, and in your code a (-1) is non-zero and b is 2.

回答4:

Lets look it step by step process,

int a=-1?2:5 + 8?4:5;

int a = (-1) ? 2 : ( 5 + 8?4:5);

for ternary operator (condition) ? return true: return false;

0 = False 
Other than 0 = true

Hence -1 = true

  int a = (true) ? 2 : ( 5 + 8?4:5);
  int a = 2;

回答5:

On a general note: the format:

x = a ? b : c;

Means:

if (a) // always results in true unless a = 0
    x = b;
else
   x = c;

Now, your statement when parenthesized:

int a = (-1) ? 2 : ( 5 + ( 8 ? 4 : 5) );

is merely a shortened if-else nested group of statements. When expanded, it looks something like:

if (-1) // since it is not zero, this is true
    a = 2;

else // since the if-block is true, this is ignored by the compiler
{
    if (8)
        a = 5 + 4; // computes to 9
    else
        a = 5 + 5; // computes to 10
}

Hence, since the first if-statement is true, the compiler stores 2 in variable a.

As an aside, you should refrain from writing code that is hard to read. Even though your statement was shorter than writing an entire if-block, reading the if-else nested block is way easier to understand than reading your statement.