当前位置：网站首页>[branch and cycle] | | super long detailed explanation + code analysis + a trick game

[branch and cycle] | | super long detailed explanation + code analysis + a trick game

2022-07-03 05:38:00 【Sobtemesa】

Stay hungry. Stay foolish. ——Steve Jobs

Stay Hungry , stay foolish .

c Language is a structured programming language , It has three basic structures , Namely ： Sequential structure , Selection structure , Loop structure .

C Language order structure is to let the program execute each item in order from beginning to end C The language code , Do not repeat any code , And don't skip any code .
C Language choice structure is also called branch structure , It's about letting the program “ Round the corner ”, Selective execution code ; let me put it another way , You can skip useless code , Only execute useful code .
C The language loop structure is to make the program “ Kill a rifle ”, Repeatedly execute the same piece of code .

The sequence structure will not be repeated , Today, let's focus on learning branch sentences （ Selection structure ） And loop statements .

First , Let's make clear two concepts first .

What is? sentence ？

C In language , Separated by a semicolon is a sentence .eg:

printf(" you are beautiful ");
a++;
; ------- Empty statement

What is? Sentence block ？

C In language , By a {} One or more statements enclosed , It is called a statement block . If there is only one statement in the statement block , You can omit it { }.

int main()
{
return 0;
}

Catalog

while Statement break and continue

Branch statement

if sentence

We need to do something when certain conditions are met , When the conditions are not met, no operation is carried out , At this time we can just use if sentence .

For example, judge whether a number is even .

int n=0;
scanf("%d",&n);
if ( n % 2 == 0)
{
printf( " It's even "）;
}
return 0;

if-else sentence

if and else It's two new keywords ,if Meaning for “ If ”,else Meaning for “ otherwise ”, Used to judge conditions , And execute different statements according to the judgment results . Execute different code in different situations , thus Implement multi branch .

if( Judge the condition )
sentence 1;
else if( Judge the condition )
sentence 2;
else
sentence 2;

application ：

Convert student grades .

In the air else

if When statements are nested , it is to be noted that if and else The problem of pairing .C Language policy ,else It's always the closest to it if pairing , for example ：

int main()
{
        int a = 0;
        int b = 2;
        if (a == 1)
               if (b == 2)
                       printf("hehe");
        else
               printf("haha");
        return 0;
}

What do you think is the output ？

analysis ：else It's always the closest to it if pairing ,a=0, first if The statement does not enter at all , So no output .

Here we need to pay attention to our writing form , We need to develop a good code style . Revise it .

if Contrast of writing forms

Code 1 ：
if(condition)
{
return x;
}
return y;

Code 2 ：
if（condition）
{
return x;
}
else
{
return y;
}

Do these two codes express the same meaning ？ If it's you, which code do you choose ？

Code three ：

int num=1;
if(num==5)
{
printf("haha");
}

Code four ：

int num=1;
if(5==num）
{
printf("haha");
}

If it's you, which code do you choose ？

Code 2、4 more intuitive , Good logic , It's not easy to make mistakes. , High readability .

practice ：

1： Judge whether a number is odd

2. Print 1-100 Between the odd numbers

switch sentence

C Although there are no restrictions on language if else Number of branches that can be processed , But when there are too many branches , use if else It will be inconvenient to deal with , And it's prone to if else Pairing error . In this case , In general use switch Sentence instead .

switch（ expression ）
{
case Integer constant expression 1: sentence 1;
case Integer constant expression 2: sentence 2;
case Integer constant expression 3: sentence 3;
case Integer constant expression 4: sentence 4;
case Integer constant expression n: sentence n;
default: sentence n+1;

Operation process ：

1. First, execute switch The expression after , Let its value be a

2. If a== Integer constant expression 1, Then follow All statements ; That is from “ sentence 1” Until “ sentence n+1”, and No matter what's in the back case Whether the match is successful .

3. If “ Integer constant expression 1” and a It's not equal , Just skip the... After the colon “ sentence 1”, Continue to compare the second one case、 Third case…… Once it is found to be equal to an integer value , Will execute all the following statements . hypothesis m and “ Integer constant expression 1” equal , Then we will “ sentence 5” Until “ sentence n+1”.

4) If you go to the last one “ Integer values n” No equal value was found , Then execute default After “ sentence n+1”.

What needs to be emphasized is , After matching with an integer value successfully , The statement of this branch and all subsequent branches will be executed .

break yes C A key word in a language , Designed to jump out of switch sentence . So-called “ Jump out of ”, Refer to Once encountered break, I'm not going to execute it anymore switch Any statement in , Include statements in the current branch and statements in other branches ; in other words , Whole switch Execution is over , Then the whole... Will be executed switch Later code .

practice ：

int main()

{

        int n = 1;

        int m = 2;

        switch (n)

        {

        case 1:

               m++;

        case 2:

               n++;

        case 3:

               switch (n)

               {

               case 1:

                       n++;

               case 2:

                       m++;

                       n++;

                       break;

               }

        case 4:

               m++;

               break;

        default:

               break;

        }

        printf("m=%d n=%d\n", m, n);

}

loop

So-called loop （Loop）, Is to execute the same piece of code repeatedly .

while loop

while( expression )
{
Sentence block ;
}

Operation process ： To calculate “ expression ” Value , True value is true. （ Not 0） when , perform “ Sentence block ”; After execution “ Sentence block ”, Evaluate the expression again , If it is true , Carry on “ Sentence block ”…… This process will be repeated all the time , Until the value of the expression is false （0）, Just exit the loop , perform while Later code .

while The whole idea of circulation ： Set a loop condition with variables , That is, an expression with variables ; Add an additional statement to the loop body , Let it change the value of the variable in the loop condition . such , As the cycle goes on , The value of the variable in the loop condition will also change , There will be a moment , The cyclic condition no longer holds , The whole cycle is over .

while Statement break and continue

break: Permanent termination of cycle .

continue: Terminate the loop .

int main()
{
        int i = 1;
        while (i <= 10)
        {
               if (i == 5)
                       continue;
               printf("%d", i);
               i++;
        }
        return 0;
}

What is the output of the code ？

practice ：

1. Look at two codes

notes ：1.EOF------ End of file flag
2.getchar When reading fails, it will return EOF
3. Characters returned , Essentially, ASCII Code value , Is an integer , It can be stored in ch in
4.getchar Functions do more than just return normal characters , And return to EOF, Its value is -1

2. Analyze the code below

scanf/getchar It's all in Input buffer Get the input value in ,scanf When getting the input value , Take away \0 The value of the former , Assigned to an array , So the next one getchar when , Take the rest from the input buffer \0, So no longer wait for input .

Modify the code ：

This operation is still flawed ：

modify ：

for loop

for The general form of the cycle is ：

for( expression 1; expression 2; expression 3)
{
Sentence block
}

Its operation process is ：

1) Execute first “ expression 1”.

2) Re execution “ expression 2”, If it's true （ Not 0）, Then execute the loop body , Otherwise end the cycle .

3) Execute after the loop body “ expression 3”.

4) Repeat steps 2) and 3), until “ expression 2” The value of is false , Just close the loop .

In the above steps ,2) and 3) It's a cycle , Will repeat ,for The main function of a statement is to continuously execute steps 2) and 3).

continue problem ：

continue Skip the following code , Went to the adjustment part , Make the loop variable adjust , It is not easy to cause a dead cycle .

do-while loop

do-while The general form of the cycle is ：

do
{
Sentence block
}while( expression );

do-while Circulation and while The difference with cycles is that ： It will be executed first “ Sentence block ”, Then judge whether the expression is true , If it is true, it continues to loop ; If it is false , Then stop the cycle . therefore ,do-while The loop must be executed at least once “ Sentence block ”.

continue problem ：

Code practice ：

1. Calculation n The factorial ：

2. Calculation 1！+2！+3!+.....+n!

3. Find numbers in an ordered array n

Two points search

3. Write code , Demonstrate that multiple characters move from both ends to converge in the middle

4. Write code to achieve , Simulate user login scenarios , And can only log in three times .

5. Guess the number game

brief introduction ：
rand() function
function ： Returns a pseudo-random number . It's a pseudo-random number , Because in the absence of other operations , Run the same program every time , call rand The random number sequence is fixed ( Not really “ Random ”).
In order to make rand The results are more “ really ” some , That is to make the return value more random ( uncertainty ),C Language in stdlib.h It is also provided srand function , Through this function, you can set a random number seed , Generally, the number of milliseconds of the current time is used as the parameter . adopt time(NULL) You can get the millisecond value of the current time ( This function is located in time.h) in . If there is no random number of seeds ,rand() Function is called , Automatically design random number seeds as 1. Same random seed , The random number generated each time will also be the same .
Use rand The process can be summarized as ：
1 call srand(time(NULL)) Set random number seed .
2 call rand Function to get one or a series of random numbers .
It should be noted that ,srand Just in all rand Before calling , It can be called once , There's no need to call it more than once .

void menu()

{

        printf("**********************************\n");

        printf("****1.paly************************\n");

        printf("****0.exit************************\n");

        printf("**********************************\n");

        printf("**********************************\n");

}

void play()

{

        int input = 0;

        int random_num = rand() % 100 + 1;

          while (1)

        {

               printf(" Please enter a number ：");

               scanf("%d", &input);

               if (input > random_num)

               {

                       printf(" Guess the \n");

               }

               else if (input < random_num)

               {

                       printf(" Guess a little \n");

               }

               else

               {

                       printf(" congratulations , Guessed it ");

                       break;

               }

        }     

}

int main()

{

        int input = 0;

        int random_num = rand() %100 + 1;

        do

        {

               menu();

               printf(" Please select （0/1):");

               scanf("%d", &input);

               switch (input)

               {

               case 0:

                       break;

               case 1:

                       play();

                       break;

               default:

                       printf(" Input error ");

               }

        } while (input);

        return 0;

}

A trick game

( Send the document to your friends , Feel the happiness of life ~）

goto sentence

goto The most common use of statements is Jump out of two or more loops . But due to the goto Statements can easily cause code confusion , Maintenance and reading difficulties , much-maligned , Not recommended , and goto Loops can be completely replaced by other loops .

Now let's use goto Play a little game