Branch and loop statements (including goto statements) -part1

If you will It's better to start than to give up .     Roman.

May we all have our own goals and are making unremitting efforts for them .

---------------------------------------------------- Split line

C Language is 【 structured 】 The programming language of , Its structure is reflected in ： Sequential structure 、 Selection structure （if sentence 、switch sentence ）、 Loop structure （for sentence 、while sentence 、do...while sentence ）

-------------------------------------------------------

The classification of sentences ：

sentence ：C There is a semicolon in language ; What separates is a statement .

① Expression statement ： expression ＋ ;

② Function call statements ： function ＋ （）

③ Control statement ： Control program execution process , In order to realize various structural ways of the program . It contains ： Conditional statements , Also known as branch statements （if sentence 、switch sentence ）; Loop statement （while sentence 、for sentence 、do...while sentence ）; Turn to statement （goto sentence 、break sentence 、continue sentence 、return sentence ）

④ Compound statement

⑤ Empty statement

-------------------------------------------------------

One 、 Conditional statements , Also known as branch statements （ Select structure ）

  One ）if sentence

 

 1、 Grammatical structure ：

（ If the expression is true, execute , If it is not true, execute the next ）

// Single branch

if（ expression ）

  sentence ;

// Double branch

if（ expression ）

  sentence 1;

else

  sentence 2;

// Multiple branches

if（ expression 1）

  sentence 1;

else if（ expression 2）

  sentence 2;

...

else

  sentence n;

Attention review ： The basis for judging whether the expression is true or false （0 For false , Not 0 It's true ）

 2、 Be careful ：

if 、else if 、 else Statement executes a statement by default , If you want to execute multiple statements , You need to use { }

Writing habits ： Generally used { }

notes ： One { } It's just a code block

3、 In the air else sentence ：

Example ：

What is the output result ？

Put the code in VS2019 Run the following ：

Pay attention to the code layout and output results , This code layout is not intentionally modified , But the system has its own typesetting effect ; You can see that there is no output .

* The explanation is as follows ：

else It's always the closest if Match ;

if...else Together, it is called a statement , So when if Followed by if...else sentence , Even if you don't add {  } Under the condition of , The effect is equivalent to adding {  }

--------------------------- Code style is important , References 《 High-quality C/C++ Programming 》

 4、if Contrast of writing forms ：

// Code 1

if (condition)

{

    return x;

}

return y;

// Code 2

if(condition)

{

    return x;

}

else

{

    return y;

}

* The two writing effects are the same , But the second way is usually used , It's quite clear

Pay attention to the effect of the first writing , Pay attention to the use of return！！

// Code 3

int num = 1;

if(num == 5)

{

    printf("hehe\n");

}

// Code 4

int num = 1;

if(5 == num)

{

    printf("hehe\n");

}

* The two effects are the same , But the second way is usually used , Because writing is easy to miss =, Therefore, the second method is generally used , Avoid the error of assignment . Variables cannot be assigned to constants ！

 5、 practice

1. Judge whether a number is odd

--------------------------------------------- Here is a way to answer ：

2. Output 1-100 Between the odd numbers

------------------------------------------- Here is a way to answer ：

 

Two ）switch sentence ： Often used in multi branch situations

 1、 Grammatical structure

switch（ Integer expression ）

{

    Statement item ;

}

Statement item ： It's some case sentence , form ：

case Integer constant expression ：

  sentence ;

  break;

2、 Be careful ：

1） switch（ expression ） The expression in just determines case Entrance , If there are no export restrictions （break Limit ）, Will go on , Until the end ;

If you want to execute only one case sentence , need break; As an exit （ Jump out of the statement ）

2） Has the same output , Multiple case  ： Just follow one statement

3） Even the last one case It is not recommended to omit break; sentence

3、default：

all case None of the statements can be matched , Then execute the statement , And it also needs to use break;

Its placement order position does not affect the output results , But habit is put last by default

default You don't have to choose , But it is recommended to use

4、switch Run nested use ：

  example ：

// if case There is no break, see default Will it be carried out -- Meeting

 

Two 、 Loop statement （ Loop structure ）

One ）while sentence

1、 Grammatical structure ：

while（ expression ）

  Loop statement ;

（ Expression for genuine talent enters ）

2、break;  Get out of the loop , Early termination of the entire cycle . namely ：break stay while Statement to permanently terminate the loop ！

3.continue; Jump out of current loop , Enter next cycle , namely ： Jump straight to while The judgment part of a sentence

  Once encountered continue, The following statements are not executed ！

4、 Pay attention to add ：

getchar（ ）; Read character , The return type is int, Reading error or reading to the end file returns EOF（ Defined as -1）; But notice ： The final print result is the input character

putchar（ ）; Print character

Press ctrl + z Will stop , Its essence is ctrl + z Will make getchar or scanf return EOF , however VS In the compilation environment, you may not be able to stop by pressing , This is a VS Of itself bug

5、 example ：

1、 Confirm cryptanalysis

// Confirm cryptanalysis 
#include<stdio.h>
int main()
{
	char password[20] = { 0 };
	printf(" Please input a password ：\n");
	scanf("%s", password);
	int tmp = 0;
	while ((tmp = getchar()) != '\n')
	{
		;// Role is ： Clear all... In the buffer '\n'
	}
	printf(" Please confirm the password (Y/N)：\n");
	int ch = getchar();
	//if (strcmp(ch, 'Y') == 0)// error , here ch  yes int type , Not characters , So just make a direct comparison 
	if(ch == 'Y') 
	{
		printf(" Confirm success ！\n");
	}
	else
	{
		printf(" Confirmation failed ！\n");
	}

	return 0;
}

2、 Analyze the following code ：

#include<stdio.h>
int main()
{
	char ch = '\0';
	while ((ch = getchar()) != EOF)
	{
		if (ch < '0' || ch>'9')
			continue;
		// significance ： if ch Of ASCII Code value in 0-9 Between , Then skip continue Output ch
		// from ASCII Code representation ：<'0' or >'9' It's other characters , and '0'--'9' Between them are numeric characters 
		putchar(ch);
	}
	return 0;
}

Enclosed ASCII clock

-------------------------------------------------

buffer ！

scanf（"%s"） Only read the string before the space ！！

while{  ;} It's an empty statement

getchar（ ）; Work once , One character is missing from the buffer ！！

Note that the type is int

Two ）for sentence

1、 Grammatical structure ：

for（ expression 1; expression 2; expression 3）

  Loop statement ;

（ expression 1： initialization ;  expression 2： conditional ;  expression 3： Adjustment part ）

2、 Compare while Circulation and for Difference between cycles ：

int i = 0;
// To achieve the same function , Use while
i=1;// Initialization part 
while(i<=10)// Judgment part 
{
 printf("hehe\n");
 i = i+1;// Adjustment part 
}

// To achieve the same function , Use for
for(i=1; i<=10; i++) {
 printf("hehe\n");
}

3、 Be careful ：

•   stay for There are also... In the loop break and continue
• initialization 、 Judge 、 Adjustment can be omitted , At this point, it falls into an endless loop ：for The judgment part of the loop omits , Just think that judgment is always true

4、 Suggest ：

•   Don't be in for Modify the loop variable in the loop body , prevent for The cycle is out of control
•   Suggest for The loop control variable value of the statement adopts “ Before closed after opening ” How to write it （ namely ： Judge with less than < form ）-- This way of writing makes the number of cycles more definite , But don't be rigid , Can be changed flexibly .eg：100-200 Just use the closed interval

5、 example ：

// How many times does the cycle take ？

#include <stdio.h>

int main()

{

  int i = 0;

  int k = 0;

  for(i =0,k=0; k=0; i++,k++)

        k++;

  return 0;

}

it is to be noted that ： Only when the judgment part is true will it enter the loop

therefore ： The judgment part of the code is ：k=0; the 0 Assign a value to k,0 For false , Therefore, it does not enter the cycle , loop 0 Time .

3、 ... and ）do...while loop

 1、 Grammatical structure ：

do

  The loop body ;

while（ expression ）;

2、 Be careful ：

• The circulatory body is executed at least once
• In the circulatory body break The same is to jump out of the whole cycle
• In the circulatory body continue End the current cycle , The following statements are skipped

3、 ... and 、goto sentence

1、goto Statements can be abused at will , But theoretically there is no need to use

2、 Have any jump function , however You cannot jump across functions

3、 Most common usage ： Terminate the processing of the program in some deep nested structures （ Such as ： Jump two floors at a time or Multilayer circulation ）

4、 Be careful ：goto Statement jump , Is all the statements after the jump , no need { } Results the same

5、 Applicable scenario （ give an example ）：

for(...)

    for(...)

  {

        for(...)

      {

            if(disaster)

                goto error;

      }

  }

    …

error:

if(disaster)

        // Handling error situations

All the anger of a person comes from the pain of his own incompetence .  Go ahead ！

************************************ See the next blog for some exercises and examples