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Grammatical structure ： The way 1： if( expression )

sentence ;

The way 2：

if( expression )

sentence 1;

else

sentence 2;

The way 3：

if( expression 1)

sentence 1;

else if( expression 2)

sentence 2;

else

sentence 3;

if Sentence example

int main()
{
 int age = 0;
    scanf("%d", &age);
    if(age<18)
   {
        printf(" A minor \n");
   }
}

Enter an age , If the age is less than 18 Print minor

if else Sentence example

#include <stdio.h>
int main()
{
 int age = 0;
    scanf("%d", &age);
    if(age<18)
   {
        printf(" A minor \n");
   }
    else
   {
        printf(" adult \n");
   }
}

Enter an age , If the age is less than 18 Print minors , If you are older than 18 Print adult

if else if else give an example

#include <stdio.h>
int main()
{
 int age = 0;
    scanf("%d", &age);
    if(age<18)
   {
        printf(" juvenile \n");
   }
    else if(age>=18 && age<30)
   {
        printf(" youth \n");
   }
    else if(age>=30 && age<50)
   {  
        printf(" middle-aged \n");
   }
    else if(age>=50 && age<80)
   {
        printf(" The elderly \n");
   }
    else
   {
        printf(" Old birthday star \n");
   }
    
}

Enter the age , If age <18, Print juvenile

Age >=18 And age <30, Print youth

Age >=30 And age <50, Print middle age

Age >=50 And age <80, Print old

If the age is not in the above number field , Print old birthday

matters needing attention ： If if or else or else if The following expression statement , More than one sentence , We need to use it. {} Will belong to if Or of else Or of else if Enclose the sentence

Judge the condition

stay C In language ,0 For false , Not 0 It's true , When selecting a statement （） When the content is true , The expression of the current statement will be executed .

else and if Matching problems

#include <stdio.h>
int main()
{
    int a = 0;
    int b = 2;
    if(a == 1)
        if(b == 2)
            printf("hehe\n");
    else
        printf("haha\n");
    return 0;
}

We found nothing printed at this time , This is because ,a=0, Can't get into if(a==1) expression , here if(b==2) It belongs to if（a==1） The expression of , So you can't execute if（b==2）, there else with if（b==2） A match , namely else That is closest to and within the same scope if Want to match

Correct input method

#include <stdio.h>
int main()
{
    int a = 0;
    int b = 2;
    if(a == 1)
   {
        if(b == 2)
       {
            printf("hehe\n");
       }
   }
    else
   {
         printf("haha\n");
   }       
    return 0;
}

switch sentence

switch Statement is also a branch statement . It is often used in the case of multiple branches .

such as ： from 1-7 For each number entered in the, the corresponding week will be printed . If used if The sentence is useful 7 Time , A little bit of a problem , At this point, we can use another statement called switch sentence

switch Sentence format ,switch( Integer expression )

{ case Integer constant expression : sentence ; }

from 1-7 Enter numbers in , Print the corresponding week

#include <stdio.h>
int main()
{
    int day = 0;
    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;
}

break The role of ： If input 1, Print no after Monday break, The program will automatically go down , Get into case2 Until I met break Jump out of the program or until switch End of sentence .

But if the value entered exceeds case What about the scope after the statement ？C In language , When switch（） The value in does not correspond to case When , We can use defalut sentence .

default Sentence function ： When switch（） The value entered in case When there is no corresponding condition in , That is, beyond case The scope of time , This statement will be unified into default sentence ,default stay switch Position in statement , It has no effect on the output results , But most people put it in by default switch{} At the end of .

matters needing attention ：switch（ Shaping expression ）,（） The content in can be an integer expression , It can also be char type , Because the character type itself is also plastic , This is because it is still ASCII Code value to save ,case Integer constant expression : sentence ; case The expression after must be a constant , It can't be a variable , If it is a variable, an error will be reported .

switch nesting

switch Statements allow nested use of

#include <stdio.h>
int main()
{
    int n = 1;
        int m = 2;
    switch (n)
    {
    case 1:
        m++;
    case 2:
        n++;
    case 3:
        switch (n)
        {//switch Allow nesting 
        case 1:
            n++;
        case 2:
            m++;
            n++;
            break;
        }
    case 4:
        m++;
        break;
    default:
        break;
    }
    printf("m = %d, n = %d\n", m, n);
    return 0;
}

Loop statement

while（） loop

while( expression )

Loop statement ;

Print... On the screen 1-10

#include <stdio.h>
int main()
{
 int i = 1;
 while(i<=10)
 {
 printf("%d ", i);
 i = i+1;
 }
 return 0;
}

while( expression )

Loop statement ;

Use while When the sentence is , First, we will judge the expression , If the expression is true , Then execute the loop statement , If the expression is false , Do not enter while loop

break and continue

When we use in a loop break after

It was originally printed 1-10, But use break After that, only 1-4, This is because entering while After the cycle, there is a if（i==5）, Here, judge i Is it 5, if 5 execute break sentence , When i==5 when , Yes break sentence , After that, no numbers were printed , This is because break The function of the statement is to jump out of the current loop

When we use in a loop continue after

The cursor is still flashing

When used here continue after , Print ;1-4, not used continue Print 5, This is because... Is not used continue when , There is a judgment condition （i==5） Judge i Is it equal to 5, If i be equal to 5, Just print , Use continue after , The program will automatically jump to while（） Then move on , When you meet again continue after , Still jump to while Then move on , So at this time i Always equal to 5

for loop

for( expression 1; expression 2; expression 3)

Loop statement ;

expression 1 expression 1 For the initialization part , Used to initialize loop variables .

expression 2 expression 2 For the conditional judgment part , Used to determine when a loop ends .

expression 3 expression 3 For the adjustment part , For adjustment of cyclic conditions .

use for Loop printing 1-10

for Cycle flow chart

for The expression is executed at the beginning of the loop 1. And then we do the expression 2 Judge , If the second is true, execute the loop statement , Then execute expression three , Know the expression 2 Stop the cycle for false

continue stay for The role in the cycle

stay for Use in statement continue after , When the statement is executed continue after , The program will then execute the expression 3

break stay for The role in the cycle

stay for Use in statement continue after , When the statement is executed break after , The program will jump directly out for loop

for Cycle characteristics

for The loop can omit the expression 1,2,3, But when the expression 2 Omit , expression 2 The default condition is constant , As shown in the figure below, it will enter a dead cycle .

for Loops can also be in statements 1 Build variables , And assign a value to it , However, this method is generally not recommended for loop

for Circulating lightning protection

#include <stdio.h>
    int main()
    {
        int i = 0;
        int k = 0;
        for (i = 0, k = 0; k = 0; i++, k++)
            k++;
        return 0;
    }

This program will not enter for loop , Because of the expression 2 For an assignment statement instead of a judgment statement , take 0 Assign to k,k This is false , So the program will never enter for loop

do...while() loop

do

{

Loop statement ;

}while( expression );

do...while A loop is to execute the statements in the loop first , And then judge the conditions , If the condition is true , Then continue to execute the loop , Otherwise jump out of the loop

continue stay do...while The role of

When i be equal to 5 when , perform continue sentence , At this point, the program will always execute if（i==5） and continue, So the cursor is flashing all the time

break stay do...while The role of

here 4 The following cursor stops flashing , Explain that the program has been executed , When i==5 when , perform break, The program will jump directly out of the loop

goto sentence

C The language provides a language that can be abused at will goto Statement and mark the label of jump . In theory, goto Statements are not necessary , In practice, there is no goto Statements can also easily write code . But on some occasions goto Statements are still useful , The most common use is to terminate the processing of structures nested in some depth cheng . for example ： Jump out of two or more loops at a time . Multilayer loops are used in this case break You can't achieve your goal . It can only exit from the innermost loop to the upper loop .

goto A statement is when a statement is executed , encounter goto After statement , Will automatically jump to goto Where indicated by the statement , The program then goes on , Now, yes goto Correct use of statements

for(...)
    for(...)
   {
        for(...)
       {
            if(disaster)
                goto error;
       }
   }
    …
error:
 if(disaster)
         //  Handling error situations

give an example

We found the command prompt in the computer , And enter the shutdown -s -t 60, It means that the computer starts after this statement is executed 60s Countdown shutdown , When you type... Again shutdown -a when , Will cancel the shutdown command

however goto Statements have certain limitations , We try to use less goto sentence , Replace... With other statements goto sentence ,stdlib.h yes system The header file ,strcmp The header file for is string.h

#include <stdio.h>
#include <stdlib.h>
int main()
{
    char input[10] = {0};
    system("shutdown -s -t 60");
    while(1)
   {
        printf(" The computer will be in 1 Shut down in minutes , If input ： I am a pig , Just cancel the shutdown !\n Please enter :>");
        scanf("%s", input);
        if(0 == strcmp(input, " I am a pig "))
       {
            system("shutdown -a");
            break;
       }
   }
    return 0;
}