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Question 26 → Realization N The class of ( respectively while、for) 

First N The class of is actually a hypothesis 5 Your class is 1x2x3x4x5 This is called 5 The class of , This question requires while Circulation and for The topic of circular hierarchy is actually to examine your understanding of circular , This topic should pay special attention to the expression in the next loop , And create a sum The total value is used for each cycle i Multiply sum, Print until the end of the cycle sum.

Question 27 → Find a specific number in an ordered array k( Two points search )

 

Binary search is also called binary search （Binary Search）, It is an efficient search method . however , Half search requires that linear table must adopt sequential storage structure , And the elements in the table are arranged in order by keywords .

The basic idea of binary search is to n The elements are divided into roughly equal parts , take a[n/2] And x compare , If x=a[n/2], Then we find x, Algorithm abort ; If x<a[n/2], Then just in the array a The left half of the search continues x, If x>a[n/2], Then just in the array a Search the right half of x. Last , When we find the element, print it and then jump out . If you don't find it, jump out of the loop .

Question 28 → Use for Circular statements show 10 A random number ！ Printing produces random numbers 1~100~

 

stay C In language , We usually use  <stdlib.h>  In the header file  rand()  Function to generate a random number , Its usage is ：

int rand (void); Then the random number we need to generate in the above program is 1~100 How to generate this , It's actually easy . We don't know rand() The maximum value generated is 0~32768, So in this topic, we need to generate 1~100 In other words, we need a method to generate a certain random value , The modulo operator can be used at this time . The example is shown in the following code ：

int ret = rand() % 10; // produce 0~9 The random number  -  Be careful 10 It's divisible

Of course, if you take a mold here 100 It just produces 0~99 The number of , You have to +1 That's it 1~100 The numbers between the ranges just match the requirements of the topic . therefore , In this topic, we actually use modulo operator to solve this problem .

Use  <time.h>  In the header file  time()  Function to get the current time （ Accurate to seconds ）, It looks like this ：

srand((unsigned)time(NULL)); Usually you only need to quote once ！

Question 29

→

Print out the pyramid

Printing pyramids is nothing more than using for Loop nesting , When we enter numbers 5 When , Let's assume a running result of it to see if it is helpful for us to solve the problem ↓

 *
 ***
 ***** 
 *******
*********

The above is the input number 5, The printed pyramid . From this running result, it will be much easier for us to do the problem again .  If you don't understand its operation steps, you can debug more , Debugging can really help you solve many problems , Especially if you just learned C Language people or beginners , Even Daniel , All need debugging . It can be said that a person's time to write code may be only... Percent 30~40, While most people spend percent of their time writing and debugging code 60~70 了 .

Question 30

→

Enter two numbers , Find their greatest common divisor

The greatest common factor , Also known as the greatest common divisor 、 The greatest common factor , The largest of the common divisors of two or more integers .

a,b The greatest common divisor of is （a,b）, alike ,a,b,c The greatest common divisor of is （a,b,c）, The greatest common divisor of multiple integers has the same notation . There are many ways to find the greatest common divisor , There is a common factorization method 、 Short division 、 division 、 More derogation . The concept corresponding to the greatest common divisor is the least common multiple ,a,b The least common multiple of is [a,b].

This is also in mathematics. We often use short division .

Question 26 code

for The use of recycling ↓

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

 for (i = 1; i <= j; i++)
 {
 sum = sum * i;//sum  Realization N The sum of the factorials of
 }
 printf(&quot;sum = %d\n&quot;, sum);
 return 0;
}

while The use of recycling ↓

Running results

Possible running results  

Please enter a number ：5

sum = 120

Question 27 code

#include<stdio.h>
int main(void)
{
 int k = 7;
 int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

 int sz = sizeof(arr) / sizeof(arr[0]); 
 int left = 0; 
 int right = sz - 1; 

 while (left <= right) 
 {
 int mid = (left + right) / 2; 
 if (arr[mid] < k)
 {
 left = mid + 1;
 }
 else if (arr[mid] > k)
 {
 right = mid - 1;
 }
 else
 {
 printf(&quot; eureka , The array subscript :%d, Elements %d\n&quot;, mid,arr[mid]);
 break;
 }
 }
 if (left > right)
 {
 printf(&quot; Can't find !\n&quot;);
 }
 return 0;
}

Running results

eureka , The array subscript 6, Elements 7

Question 28 code

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#define NUM 10
int main(void)
{
 int i = 1;
 srand((unsigned)time(NULL));
 for (i = 1; i <= NUM; i++)
 {
 int ret = rand() % 100 + 1;
 printf(&quot; The first %-2d Time : Numbers -->%d\n&quot;, i, ret);
 }
 return 0;
}

Running results

The first 1  Time  :  Numbers -->13

The first 2  Time  :  Numbers -->74

The first 3  Time  :  Numbers -->71

The first 4  Time  :  Numbers -->42

The first 5  Time  :  Numbers -->23

The first 6  Time  :  Numbers -->10

The first 7  Time  :  Numbers -->10

The first 8  Time  :  Numbers -->47

The first 9  Time  :  Numbers -->11

The first 10 Time :  Numbers -->46

Question 29 code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main(void)
{
 int i, j, k;
 int input;
 printf(&quot; Please enter a number :-->&quot;);
 scanf(&quot;%d&quot;, &input);
 for (i = 1; i <= input; i++)
 {
 // Ideas : Space  &  Print  *
 for (j = 1; j <= input - i; j++)
 {
 printf(&quot; &quot;);
 }
 for (k = 1; k <= 2 * i - 1; k++)
 {
 printf(&quot;*&quot;);
 }
 printf(&quot;\n&quot;);
 }
 return 0;
}

Running results

Please enter a number ：-->5

 *
 ***
 ***** 
 *******
*********

Question 30 code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<stdlib.h>
int main(void)
{
 int a = 0, b = 0, t = 0;
 printf(&quot; Please enter two numbers :&quot;);
 scanf(&quot;%d %d&quot;, &a, &b);
 while (t = a%b)
 {
 a = b;
 b = t;
 }
 // hypothesis :a = 10、b = 20
 //t = 10 
 printf(&quot;|--------------------|\n&quot;);
 printf(&quot;| The greatest common divisor of two numbers :%d|\n&quot;,b);
 printf(&quot;|--------------------|\n&quot;);
 return 0;
}

Running results

Please enter two numbers ：10 20

The greatest common divisor of two numbers ：10

This series has not been updated. It has been more than three months since the last article , In fact, I have been writing this series. After all, I can brush it myself C Some topics of language, and then summarize these brushed topics. Write a blog with five topics , But there are many things left behind , Now I remember ( •̀ .̫ •́ )*

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