Select the type of statement

There are two kinds of selection statements in c language

1. if else statement
2. switch statement

As the name suggests, we will face some choices. If the selection is correct, it will be executed, otherwise it will not be executed

c language is a structured programming language
There are sequential structure, selective structure and cyclic structure

The selection statement is the selection structure

Meaning of statement

What is a statement?
A semicolon in c language; Separated is a statement
For example:

1|3+5;
2|;//No content is an empty statement    

True and false judgment

How does c language express true and false?
Non 0 is true, 0 is false

What are statement items

//Some case statements
case Integer constant expression:
    sentence;
//switch allows nested use, that is, multiple case statements can be used

###Syntax structure of if statement

Writing method 1:

if(expression)//If the expression result is true, the statement will be executed, otherwise it will not be executed
    sentence;

Code example:

#include <stdio.h>
int main()
{
	int age = 10;
	if (age >= 18)//The result is false and the statement is not executed
		printf("adult");
    return 0；
}//So the print result is empty

Method 2:

if(expression)//If the expression is true, statement 1 is executed, otherwise statement 1 is executed
    Statement 1;
else
    Statement 2;

Code example:

#include <stdio.h>
int main()
{
	int age = 10;
	if (age >= 18)//Statement is false, execute statement 2
		printf("adult");
	else
		printf("under age");//Statement 2 is a minor, so the printed result is a minor
    return 0；
}

Writing method 3:

//Multi branch
if(Expression 1)//If expression 1 is true, statement 1 will execute; otherwise, if expression 2 is true, statement 2 will execute; otherwise, statement 3 will execute
    Statement 1;
else if(Expression 2)
    Statement 2;
else
    Statement 3;

Code example:

#include <stdio.h>
int main()
{
	int age = 20;
	if (age >18)//Expression 1 is true
		printf("adult\n");//The print result is adult
	else if(age = 18)
		printf("Just grown up");
	else
		printf("under age\n");
    return 0;
}

Common problems in if statements

Output does not match imagination, {} usage

for example

#include <stdio.h>
int main()
{
	int age = 20;
	if (age >= 18)
		printf("adult\n");
	else
		printf("under age\n");
		printf("Can't fall in love");
    return 0；
}//The printed result is adult and can't fall in love

Why does this happen?
Because each if and else can only control one statement

If we want else to control multiple statements
At this time, we will use {} to put multiple statements together

#include <stdio.h>
int main()
{
	int age = 20;
	if (age >= 18)
		printf("adult\n");
	else
	{
		printf("under age\n");
		printf("Can't fall in love");//Use braces to put the two statements together
	}
    return 0;
}//Printed out as adult, the result is correct

Judgment condition error, use of logical operator

for example

#include <stdio.h>
int main()
{
	int age = 40;
	if (age <18)
		printf("juvenile\n");
	else if(18<=age<=25)//When the judgment enters this expression, the result is true when 18 < = 40, and 1,1 < = 30 is also true
		printf("youth\n");
	else
		printf("other");
	return 0;
}//So the result is youth

If we want to get the right result
At this time & & is needed to connect multiple judgments

#include <stdio.h>
int main()
{
	int age = 40;
	if (age <18)
		printf("juvenile\n");
	else if(18<=age&&age<=25)//Use & & to connect multiple judgments
		printf("youth\n");
	else
		printf("other");
	return 0;
}//The printed results are other, and the results are correct

Syntax structure of switch statement

switch(Integer expression)//Integer expressions can also be characters because the ASCII value of a character is an integer
{
        Statement item;
}

Code example

#include <stdio.h>
int main()
{
    int day = 1;
    switch (day)//Parentheses cannot be omitted
    {
    case 1:
        printf("Monday");
    }
    return 0;
}//The printed result is Monday

Common errors in switch statements

Statements after multiple case s are printed consecutively (the role of break in switch)

for example

#include <stdio.h>
int main()
{
	int day = 0;
	scanf("%d", &day);
	switch (day)
	{
	case 1:
		printf("Monday\n");
	case 2:
		printf("Tuesday\n");
	case 3:
		printf("Wednesday\n");
	case 4:
		printf("Thursday\n");
	case 5:
		printf("Friday\n");
	case 6:
		printf("Saturday\n");
	case 7:
		printf("Sunday\n");
	}
	return 0;
}//When we enter 6, we print Saturday and Sunday

This is because if we do not stop the sixth instruction after it is executed, it will continue to execute until the statement is executed. If there is a default expression, the statement after default will also be executed

#include <stdio.h>
int main()
{
	int day = 0;
	scanf("%d", &day);
	switch (day)
	{
	case 1:
		printf("Monday\n");
		break;
	case 2:
		printf("Tuesday\n");
		break;
	case 3:
		printf("Wednesday\n");
		break;
	case 4:
		printf("Thursday\n");
		break;
	case 5:
		printf("Friday\n");
		break;
	case 6:
		printf("Saturday\n");
		break;
	case 7:
		printf("Sunday\n");
		break;
	}
	return 0;
}//When we add break and enter 6, the printed result is Saturday, and break prevents the program from continuing

**Note: * * case: it is not necessary to add break later, just add it as required

Usage of default in switch statement

When we enter a value, there is always no value in the case statement, when we want to give people a feedback
At this time, we can use default

for example

#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
int main()
{
	int day = 0;
	scanf("%d", &day);
	switch (day)
	{
	case 1:
	case 2:
	case 3:
	case 4:
	case 5:
		printf("weekdays\n");
		break;
	case 6:
	case 7:
		printf("Rest Day\n");
		break;
	default:
		printf("Input error");
		break;//When we print 8, 9 and 10, the printed result is an input error
	}
	return 0;
}

**Note: only one default statement can appear in each switch statement, but it can appear anywhere in the statement list (switch {}) * *