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Question 21 → Suppose you enter a number 5, Realization 1+2+3+4+5=16, Similarly, input any number to produce the sum of each number ( Recursive method implementation )

The programming skill of program calling itself is called recursion . As an algorithm, recursion is widely used in programming languages . A procedure or function has a method that directly or indirectly calls itself in its definition or description , It usually converts a large and complex problem into a smaller problem similar to the original problem , Recursive strategy only needs less programs to describe the multiple repeated calculations required in the process of solving problems , Greatly reduces the amount of program code . The main way of thinking for recursive people is ： Turn the big thing into a small one . The reason why recursion can achieve , Because each execution process of the function has its own formal parameters and local variables in the stack , These copies are not related to other execution processes of the function .

Be careful ： When recursion enters the innermost layer , The recursion is over , Began to exit layer by layer , That is, layer by layer  

return

  sentence . When executing the last layer  return  after , Just execute the recursion of the upper layer ！ Let's talk about the conditions of recursion

• Every recursive function should only make a limited number of recursive calls , Otherwise it will enter a dead end , Never quit , Such a procedure is meaningless .

• There are restrictions
  , When this restriction is met , Recursion will not continue .

• After every recursive call
  Getting closer to this limit
  .

Question 22 → Exchange two values with a pointer , Temporary variables cannot be created for exchange replacement

The title says that we should use pointers to realize the exchange of two values , So obviously we need to put two variables into the pointer variable .

secondly , You cannot create temporary variables to exchange . In fact, this is very easy to do. Just use bitwise XOR , So we need to know what a bitwise XOR is. Next, let's introduce what a bitwise XOR is （＾∀＾●）ﾉｼ

Then we need to know the operation rules of bitwise XOR .

0^0=0; 0^1=1; 1^0=1; 1^1=0;

Question 23 → Write code , Demonstrate the movement of multiple characters from both ends , Converging in the middle .( One cycle delay 1s Then clear the screen , Finally print out the characters )

First, I'll show you how to move a string from both ends , Then we must first create two string arrays . hypothesis ：

char arr1[] = "Cyuyuanyyds";
char arr2[] = "###########";


Then converge to the middle , This cycle + The left and right subscripts cycle once each time --++ Exit if the straight loop condition expression is not satisfied .

Here we introduce the use of screen clearing function and delay function ： The header file is #include<Windows.h>

Sleep();// The time delay function , Be careful :S Use capital letters
system("cls");// Screen clearing function


Question 24 → use  switch  Branch statements enter Monday through Sunday , If you enter a range other than Monday to Sunday, print  eroor (switch Statements for )

This topic mainly depends on your right switch I understand , It's an easy topic .

C Language can be used &nbsp;

[switch]

  Statement directly handles the case of multi branch selection , Improve the readability of our program code ！

[switch]

  Statement is a multi branch select statement , In our program code , If we think we use more branch statements . At this time we can use  

[switch]

  Instead of &nbsp;

[if - else]

  Branch statement .

for example ： If you only need to check the possible value of a certain shaping variable , Then you can use a simpler  

[switch]

  sentence . 

[switch]

  The general form of the statement is as follows ：

switch( expression )
{ 
 case  Constant expression  1:
  sentence  1;
 case  Constant expression  2: 
  sentence  2;
 … 
 case  Constant expression  n: 
  sentence  n; 
 default : 
  By default, statement blocks ;
}


Evaluate the value in the expression , And compare them with constant expressions one by one . When the value of an expression is equal to a constant expression , That is, execute the following statements , And then stop judging . Continue with all the following  

[case]

  sentence , For example, the value of the expression does not have a sum  

[case]

  When the constant expressions in are equal , execute  

[default]

  By default, statement blocks .

Question 25 → Please input a password , Then input  Y  Yes, confirm the password ,N  Yes, confirmation failed

First we need to enter the password , Then we will create an array . Then use the input function scanf() Enter password

Enter the password and we can use getchar() Input function , The function is to input to the display device ① Unsigned characters .

But here we need to pay attention ： When we program scanf() The function has... When reading the buffer **\n( The reason is that we enter '\n'), Remember that the buffer will remain '\n' It will not be read .**

therefore , We should put '\n' This character of is read , So we should build a function while() Loop until the buffer '\n' Finished reading .

Program realization ：while ((ch = getchar()) != '\n');

After reading , We'll use getchar() Input function input ① A single character .

then , Input with judgment  Y  Is to confirm success ,N  Yes, confirmation failed . If you don't enter these two characters , Then it will print out ： The input format is incorrect .

Question 21 code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>

int face(int n)
{
 if (n <= 1)
 return 1;
 else
 return (n+face(n - 1));// Attention priority  
}
int main(void)
{
 int n;
 printf(&quot; Please enter your number :&quot;);
 scanf(&quot;%d&quot;, &n);
 int ret = face(n);
 printf(&quot;N = %d\n&quot;, ret);

 return 0;
}


Running results

Please enter a number ：5

ret = 15

1+recursion(0) = 1

2+recursion(1) = 3

3+recursion(3) = 6

4+recursion(6)=10

5+recursion(10)=15

Question 22 code

#include<stdio.h>

void swap(int *x, int *y) 
{
 // Bitwise XOR   hypothesis   yes  x = 1,y = 2
 *x ^= *y;//x = x^y // 0001 ^ 0010 = 0011 = 3  here x = 3
 *y ^= *x;//y = x^y // 0011 ^ 0010 = 0001 = 1  here y = 1 
 *x ^= *y;//x = x^y // 0011 ^ 0001 = 0010 = 2  here x = 2  result  x = 2,y = 1 
}
int main(void) 
{
 int a, b;
 printf(&quot; Please enter two numbers :&quot;);
 scanf(&quot;%d %d&quot;, &a, &b);
 printf(&quot;a,b Exchange before ： a = %d,b = %d\n&quot;, a, b);
 swap(&a, &b);
 printf(&quot;a,b After exchanging ： a = %d,b = %d\n&quot;, a, b);

 return 0;
}


Running results

Please enter two numbers ：10 20

a,b Exchange before ：a = 10,b = 20

a,b After exchanging ：a = 10,b = 20

Question 23 code

#include<stdio.h>
#include<string.h>//strlen Long header file
#include<windows.h>//system(&quot;&quot;); Execute command header file

int main(void)
{
 char arr1[] = &quot;nageshijieshimeng&quot;;
 char arr2[] = &quot;#################&quot;;
 int left = 0;// Subscript   Access subscript elements from 0 Start
 int right = strlen(arr1) - 1;// Length string   Subscript   Subscripts are less than the number of elements 1, So you have to subtract 1

 while (left <= right)// When subscript <= Subscript ( character ) Then stop the cycle
 {
 arr2[left] = arr1[left]; // hold [arr1]n＝[arr2]#  On the left left First character
 arr2[right] = arr1[right];// hold [arr2]g＝[arr2]#  On the right right First character

 printf(&quot;%s\n&quot;, arr2);
 Sleep(1000); // Delay 1000ms = 1s
 system(&quot;cls&quot;);// Clear the current screen
 left++;
 right--;
 }
 printf(&quot;%s\n&quot;, arr2);// Finally print out the result
 return 0;
}


Running results

C########s

Cy######ds

Cyu####yds

Cyuy##yyds

Cyuyanyyds

Cyuyanyyds

Question 24 code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main(void)
{
 int input = 0;
 printf(&quot;Please enter:&quot;);
 scanf(&quot;%d&quot;, &input);

 switch (input)
 {
 case 1:
 printf(&quot;Monday\n&quot;);
 break;
 case 2:
 printf(&quot;Tuesday\n&quot;);
 break;
 case 3:
 printf(&quot;Wednesday\n&quot;);
 break;
 case 4:
 printf(&quot;Thursday\n&quot;);
 break;
 case 5:
 printf(&quot;Friday\n&quot;);
 break;
 case 6:
 printf(&quot;Saturday\n&quot;);
 break;
 case 7:
 printf(&quot;Sunday\n&quot;);
 break;
 default:
 printf(&quot;eroor\n&quot;);
 }
}


Running results

Possible running results ：

Please enter：7

Sunday

Question 25 code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main(void)
{
 char password[20] = { 0 };
 int ret = 0;
 printf(&quot; Please input a password :&quot;);
 //password It's an array name , The array name itself is already addressed .
 scanf(&quot;%s&quot;, password);
 // When we program scanf The function has... When reading the buffer \n, So we should take \n This character of is read ！ Note that the buffer will remain \n
 int ch;
 while ((ch = getchar()) != '\n');// build while Loop until the buffer '\n' Finished reading
 printf(&quot; Please confirm the password (Y/N):&quot;);
 ret = getchar();// After reading , Proceed again getchar() An input to
 if (ret == 'Y')
 {
 printf(&quot; Confirm success !\n&quot;);
 }
 if (ret == 'N')
 {
 printf(&quot; Confirmation failed !\n&quot;);
 }
 if (ret != 'Y'&&'N')
 {
 printf(&quot; The input format is incorrect !\n&quot;);
 }
 return 0;
}


Running results

Please input a password ：123456

Please confirm the password (Y/N)：Y

Confirm success