[C language - zero foundation lesson 7] sequential structure and selection structure

Preface

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This issue is about C Language order structure and choice structure , If there is any mistake, you are welcome to put forward your views .

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Catalog

  Import

Sequential structure

Selection structure

Relational operator

Logical operators

Logical expression

If Conditional statements

Switch Conditional statements

---

  Import

   Life comes in order , No dress rehearsal . From our birth to our school 、 Then we started working 、 marry 、 Have children and move towards middle age 、 Finally, we bid farewell to the world in old age. We come step by step in order. Generally, there will be no newly born old people ,C The same is true of sequential structures in languages .

Life is made up of many choices , Some choices are our active choices, of course, some choices are also choices we are forced to make . Whether you want it or not, you have to make a choice . for example ： You buy a house in a big city in order to stand out , You can choose to do

Do business to make money 、 You can choose to be a programmer to make money 、 You can also choose to study hard and make money with your knowledge 、 You can also run a taxi 、 delivery 、 As long as delivery is not illegal . This is the selection structure in the sequential structure of our chapter . I want to ask you ？ You learn C Language is also a choice you make on your own initiative or forced to make .

Just like life ,C There are also choices in the program ,C Language provides two kinds of choice statements .

If and  switch

Sequential structure

The following is a code with sequential structure , From top to bottom .

int a,b,c;
a=1;
b=2;
c=3;

Selection structure

Before learning the choice structure, we need to understand the following C Logical values in language

stay C Language in Africa 0 Treat as positive ,0 Be false

Relational operator

>、<、== be equal to 、!= It's not equal to 、>=、<=

If it is true , If not, it is false

printf("%d",3 > 5);

result 0

printf("%d",3 <5);

result 1

Logical operators

！ Not && And || or

priority ！>&&>||

associativity ：! From left to right  &&|| From right to left

Logical expression

&& 

0&&1=0    1&&0=0   0&&0=0  1&&1=0

||

0||1=1    1||0=1   0||0=0  1||1=0

!

!0=1   !1=0

printf("%d\n",!3);
	printf("%d\n", !0);
	printf("%d\n", -1&&0);
	printf("%d\n", -1 && 1);
	printf("%d\n", 5 && 0);
	printf("%d\n", -1 || 0);
	printf("%d\n", -1 || 1);
	printf("%d\n", 5 ||  0);
	printf("%d\n", 0 || 0);

  result  

0
1
0
1
0
1
1
1
0

If Conditional statements

Single branch

Format ：if( Conditions ) {

sentence ;

}

if (5>2) {
		printf(" having dinner ");
	}

   result ： having dinner

---

if (0>2) {
		printf(" having dinner ");
	}

   result ：     

Conditions not established

If the condition is true, execute the statement in the statement block , If it is not established, it will not be implemented

Double branch

Format ：if( Conditions ) {

sentence 1;

}else{

sentence 2;

}

if (0>2) {
		printf(" having dinner ");
	}
	else {
		printf(" sleep ");
	}

   result ： sleep

If the condition is true, execute the statement in the statement block , If the condition does not hold, execute else Statement block in

Multiple branches

Format ：if( Conditions ) {

sentence 1;

}else if（）{

sentence 2;

} else if（）{

sentence 2;

} else if（）{

sentence n;

}

else{

}

int a = 2;
	if (a==0) {
		printf(" having dinner ");
	}
	else if(a == 1) {
		printf(" sleep ");
	}
	else if (a == 2) {
		printf(" Blogging ");
	}else{
        printf(" Don't blog ");
}

    result ： Blogging

Execute the statement according to the judgment condition

If Sentences can be omitted else

Switch Conditional statements

Format ：

switch（ expression ）{

case Constant 1： sentence ; （break;)

case Constant 2： sentence ;（break;)

case Constant n： sentence ;（break;)

default： Constant n+1 sentence ;

} 

switch（ expression ） The inside of the expression must be character and integer expressions .

case Constant 1、2、3 Equal sum default They all serve as labels .

break; Jump out of switch Code block .

default： If the expression judges case If all constants are not satisfied, execute default The statement in . There can be no default, If case Constant If there is no match with the condition form, it will jump out switch Code block .

First judge the value in the expression , Is it equal to case Constant in . If equal to, execute case The statement in ; After execution, check whether the right break The statement ends with , No, break Then proceed until you encounter break Statement or execution completed switch Everything in the code block .

int a = 0;
	switch (a) {
	case 0: printf(" zero \n");
	case 1: printf(" One \n");
	case 2: printf(" Two \n");
	case 3: printf(" 3、 ... and \n");
	default: printf(" other \n");
	}

result ：

zero
One
Two
3、 ... and
other

int a = 0;
	switch (a) {
	case 0: printf(" zero \n");break;
	case 1: printf(" One \n");
	case 2: printf(" Two \n");
	case 3: printf(" 3、 ... and \n");
	default: printf(" other \n");
	}

result ：

zero
 

int a = 0;
	switch (a) {
	case 0: printf(" zero \n");
	case 1: printf(" One \n");
	case 2: printf(" Two \n");
	case 3: printf(" 3、 ... and \n");break;
	default: printf(" other \n");

result ：

zero
One
Two
3、 ... and

int a = 5;
	switch (a) {
	case 0: printf(" zero \n");
	case 1: printf(" One \n");
	case 2: printf(" Two \n");
	case 3: printf(" 3、 ... and \n");
	default: printf(" other \n");

result ：

other

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