[C Advanced] 15, logical operators

Summary

1) The && expression calculated from left to right. When the condition is false, the entire expression is false, and the following expressions are not executed anymore, which is the short-circuit rule;

2) ||The expression calculated from left to right. When the condition is true, the entire expression is true, and the following expressions will not be executed anymore, which is the short-circuit rule;

3) In logical expressions && has a higher priority than ||, which is expressed as: When && and || appear in the same expression, the entire expression is regarded as a || expression. First calculate the && expression, and finally calculate the entire || expression. When an expression is true, the entire || expression is short-circuited.

4) The logical not "!" C language only recognizes 0 (false), and any other value is not 0 (true).
"!" is often used in if statement to determine whether an expression is false; it can also be used to change an integer to 1, such as !!100;

Hidden knowledge points of logical operators

1. The short-circuit rule in && and ||

• || from left to right: stop the calculation when the condition is true, the whole expression is true; the expression is false only when all the conditions are false

  int LogicFunc(int i)
  {
    printf("LogicFunc(int i) : %d\n", i);
    return i;
  }
  
  int main()
  {
  
    if(LogicFunc(1) || LogicFunc(0))
    {
        printf("Logic expression is true");
    }
    else
    {
        printf("Logic expression is false");
  
    }
  }
  输出：
  LogicFunc(int i) : 1
  Logic expression is true
  
  原因：逻辑||表达式的短路规则，第1个表达式LogicFunc(1)返回1，为true，第二个表达式由于短路规则并未执行

• && Start calculation from left to right: stop calculation when the condition is false, the entire expression is false; the expression is true only when all the conditions are true

  int LogicFunc(int i)
  {
    printf("LogicFunc(int i) : %d\n", i);
    return i;
  }
  
  int main()
  {
  
    if(LogicFunc(0) && LogicFunc(1))
    {
        printf("Logic expression is true");
    }
    else
    {
        printf("Logic expression is false");
  
    }
  }
  输出：
  LogicFunc(int i) : 0
  Logic expression is false
  
  原因：逻辑&&表达式的短路规则，第1个表达式LogicFunc(0)返回0，false，第二个表达式由于短路规则并未执行

2. Rules for && and || mixed operations

In logical expressions && has a higher priority than ||, which is expressed as: When && and || appear in the same expression, the entire expression is regarded as a || expression. Calculate && first, then || last.

• Code reading

  // 以下代码的输出是？
    int i = 0;
    int j = 0;
    int k = 0;
  
    int t = ++i || ++j && ++k;
  
    printf("t = %d\n", t);
    printf("i = %d\n", i);
    printf("j = %d\n", j);
    printf("k = %d\n", k);

• Code analysis

  输出：
  t = 1
  i = 1
  j = 0
  k = 0
  
  分析：
  ++i || ++j && ++k;  ==> 等价于一个逻辑或表达式 (1 && ++i) || (++j && ++k);
  左边的逻辑与表示，1为真，继续计算++i，i自增1，然后取值，所以左侧表达式也为真
  这时候遇到了逻辑或表达式的短路规则：左侧表达式已经为真，右侧的表达式不再计算。
  因此，j和k仍然为0

3. "!" What is the magic horse

The logical negation operator in C language only recognizes 0, and returns 1 when it encounters 0; otherwise, it returns 0;

    printf("%d\n", !0);        // 1
    printf("%d\n", !-1);       // 0
    printf("%d\n", !100);      // 0
    printf("%d\n", !!100);     // 1

This article is summarized from "C Language Advanced Course" by Tang Zuolin from "Ditai Software Academy".
If there are any errors or omissions, please correct me.