C language selection type supplement

Last time we talked about c In language if,else if ,else,switch Use of statements ,

Although more comprehensive , But there are still deficiencies , So I will do a little this time Add .

Let's start with grammar ,

Grammatical structure ：
  if（ expression ） sentence ;
     if（ expression ） sentence 1;else sentence 2;
     if（ expression ） sentence 1;else if sentence 2;else sentence 3;
  expression coca && || ！（ or And Not ）
If the expression holds And execute multiple statements ： Use -{} Generate code blocks  ->  { sentence };
  

Then let's talk about Dangling code The problem of ：

For example, the following two codes ：

int main() {
    int a = 0;
    int b = 2;
    if (a == 1)
        if (b == 2)
            printf("hehe\n");
        else printf("haha\n");
    return 0;// Nothing there?
}

int main() {
    int a = 0;
    int b = 2;
    if (a == 1)
        if (b == 2)
            printf("hehe\n");
    else 
        printf("haha\n");
    return 0;// Still not
}
// The second code puts else With the first if alignment , Try with the first if Form a selection statement , however , It's not right ,
// The real situation of the code is that of the first code , It's the second one. if And else Form statement ,
// In this code , Structure in order , From the top down , In judging a Is it equal to 1 when , because a=0, So the statement doesn't hold ,
// So the statement will not run any further , So nothing is printed .
// change a Value ; Make it established , as follows ;
int main() {
    int a = 0;
    int b = 2;
    if (a == 0)
        if (b == 2)
            printf("hehe\n");// Print  hehe 
        else printf("haha\n");
    return 0;
}
// Conclusion ：else There is no match with the nearest one if Statement matching

The third point , about Optimization of writing method .

//if Contrast of writing forms ：
// Code 1：
if (condition) {
    return x;
}
return y; The code is established ： Conditions established return x, Don't set up , return y.
// But the code is not readable ,
// Can be optimized downwards ：
// Code 2：
if (condition) {
    return x;
}
else {
    return y;
}

And finally , About switch sentence .

switch sentence ： It has to be integer ,case Must be followed by a constant . Start where the integer constant expression satisfies .
/*switch（ Integer expression ）{
Statement item
}*/
// About statement items ：
//case Integral constant expression ： sentence ;break;
// default ： sentence ;（ except case Other conditions ）

Special form ：

/*int main() {
    int s;
    scanf("%d", &s);
    switch (s) {
    case 1:
    case 2:
        printf("nihao");
        break;// A kind of
    case 3:
    case 4:
        printf("buhao");
        break;// A kind of
    }
    return 0;
}*/

This kind of writing is OK .
//case And default There is no order . And there can be if sentence , Allow nesting