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Question 36 → Find the Fibonacci number ！ Enter a number to find the corresponding Fibonacci sequence

Here, let's explain what Fibonacci sequence is .

The Fibonacci sequence refers to a sequence that looks like this ：1,1,2,3,5,8,13,21,34,55,89...

Fibonacci sequence in nature .

This sequence starts at number one 3 A start , Each of these terms is equal to the sum of the first two terms .

In Mathematics , The Fibonacci sequence is defined recursively as follows

F(0)=0

F(1)=1

F(n)=**

F(n - 1)+F(n - 2)

however , Here we don't mind you using recursion , because ： recursive n Time , Time complexity O(2^n), This is very, that what , therefore , Must be optimized . So if you want to make your code more efficient, you have to use other methods , Using recursive time complexity ↑ 了 .

Question 37 → Calculation 1 To 100 Number of numbers , Find the number in one digit 9 And the ten digit number 9 Print it out , And use Count Calculate the sum of the printed numbers

First , Calculation 1~100 We can solve this problem with a loop , In fact, this kind of problem is solved by circulation at first sight . The cycle has a total of  for  and  while  We usually use both of them  for  Loop to solve this problem . because ,for  The cycle will actually be more efficient than  while  The efficiency is good . notes ： It refers to different occasions .

Find the number in one digit, that is, the single digit 9 The number of , The number on the ten digit, that is, the ten digit number, has 9 The number of .

Judgment statements can solve , Use the modulo operator in the expression (%) You can find the number in a bit , Use the division operator (/) You can find the number in ten digits , notes ： The number required for the title is ：9

then , Print the numbers above . Every time you print a number  Count++, Finally, put Count Also print it out , This problem is solved . This question is actually taking a model of you (%) And division (/) What's the understanding of becoming , This topic is not difficult and easy .

Question 38 → Separate calculation  1/1+1+2/1+3/1+4/1+5/+......+1/99+1/100  and  1/1-1/2-1/3-1/4-1-5-......-1/99-1/100  Value

Separate calculation  1/1+1+2/1+3/1+4/1+5/+......+1/99+1/100.

Separate calculation  1/1-1/2-1/3-1/4-1-5-......-1/99-1/100.

One is to divide and then add, and then divide and then add , The other is to divide, subtract, and then divide . From which we can find its law, which is actually the key to this question , Then the total value is 1~100, Then this problem can be solved well . Because this topic has division , Then the type we use here can be floating point type , Double precision type is recommended , notes ： The format character of double precision floating-point type is %lf、 The single precision formatter is %f.

Question 39 → Simulate the implementation of string functions , Any input ~ Find the length of string function

In fact, the above problem is nothing more than asking us to realize  strlen  The function of , Then we need to know  strlen()  Function declaration of .

size_t strlen ( const char * str );

Size_t

→ It's an unsigned thing (unisgned int) Integer type .

str

 

→

  To calculate the length of a string . Be careful ： It is a character pointer type , And it's  const  Defined by the , That is, a constant string .

Their thinking → It's nothing more than an array passing parameters to formal parameters , The pointer is used because , The array name parameter actually passes the address of the first element , The address should be stored with pointer variables .

Question 40 zero → Any input string to print in reverse order , Out of commission C Its own string library function

First , We create a function to print the string in reverse order , The title stipulates that library functions in strings cannot be used , Then this topic is likely to be used, but we are not allowed to use the library function string function , So we have to create our own functions to use .

Reverse order is the key to this question. Of course, this is particularly easy , It's an old routine. It's no different from the exchange of variables .

Input function is recommended here  gets()  function , Let's talk about  gets()  Input function   and  scanf()  The difference between input functions ↓

**

scanf()：

 ** Spaces are not acceptable 、 tabs Tab、 Return, etc ;  When there is a carriage return , Space and tab The key will automatically add '\0', But enter , Space and tab The key remains in the input buffer .  You can read all types of variables .

**

gets()：

 ** Able to accept spaces 、 tabs Tab And carriage return, etc ;  All characters entered before the Enter key are acceptable , And use '\n' replace  '\0'. Enter does not stay in the input buffer  , Use to read the string , End the input with enter .

Question 36 → Code

#include<stdio.h>
// Fibonacci sequence : The sum of the first two numbers is equal to the third number
/* 1 1 2 3 5 8 13 21 34 
 a b c 
 1 2 3 4 5 6 7 8 9*/
int Fib(int n)
{
 int a = 1;
 int b = 1;
 int c = 1;

 while (n > 2)// Only the third party is required 2 The Fibonacci number will be executed after three months while loop
 {
 c = a + b;
 a = b;
 b = c;
 n--;//n = n - 1; Each execution is reduced to n=2 until .
 }
 return c;
}
int main(void)
{
 int n = 0;
 int let = 0;
 printf(&quot; Please enter a number :&quot;);
 scanf_s(&quot;%d&quot;, &n);
 ret = Fib(n);
 printf(&quot;ret = %d\n&quot;, ret);
 return 0;
}

Running results

Please enter a number ：9

ret = 34

The recursive algorithm is as follows ↓

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
int fib(int n) {
if (n==1 || n==2) {
return 1;
}
return fib(n - 1) + fib(n - 2);
}

int main()
{
int n = 0;
printf(&quot; Please enter the required number ：&quot;);
scanf(&quot;%d&quot;, &n);
printf(&quot;%d\n&quot;, fib(n));
return 0;
}

 

Question 37 → Code

 

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main(void)
{
 int i = 0;
 int Count = 0;
 for (i = 0; i < 101; i++)
 {
 // bits
 if (i % 10 == 9)
 {
 printf(&quot;%d &quot;, i);
 Count++;
 }
 // ten
 if (i / 10 == 9)
 {
 printf(&quot;%d &quot;, i);
 Count++;
 }
 }
 printf(&quot;\nCount=%d\n&quot;, Count);
 return 0;
}

Running results

9 19 29 39 49 59 69 79 89 90 91 92 93 94 95 96 97 98 99 99 

Count=20

 

Question 38 → Code

 

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h> 
int main(void)
{
 int i = 1;
 double sum1 = 0;
 double sum2 = 0;
 for (i = 1; i < 100; i++)
 {
 // Be careful → If you divide, keep in mind 1 Modified into 1.0 Become a floating point type
 sum1 = 1.0 / i + sum1;//sum1 Save the last value and add
 sum2 = 1.0 / i - sum2;//sum2 Save the last value and add
 }
 printf(&quot;sum1 = %lf\n&quot;, sum1);
 printf(&quot;sum2 = %lf\n&quot;, sum2);
 return 0;
}

Running results

sum1 = 5.177378

sum2 = 0.698172

Question 39 → Code

​#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<string.h>
#include<assert.h>//assert The header file
int My_strlen(const char *arr)
{
 unsigned int Count = 0;// Statistical characters cannot be negative !
 assert(arr!=NULL);// By adding assertions here, we can ensure that when we enter a string, it will not be a null pointer
 while (*arr != '\0')
 {
 Count++;// Self increasing ++
 *arr++;//++, Until I met '\0' Quit
 }
 return Count;// Returns the length of the computer
}
int main(void)
{
 char enter[20] = { 0 };
 printf(&quot; Please enter the string :&quot;);
 scanf(&quot;%s&quot;, &enter);
 int ret = My_strlen(enter);
 printf(&quot;The total number of input strings:%d\n&quot;,ret);
 return 0;
}

The operation results are as follows

Please enter the string ：C Language is the best in the world

The total number of input strings：7

Question 40 → Code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<assert.h>
//2. simulation strlen() Use of functions
int my_strlen(char *str)
{
 int count = 0;
 while (*str != '\0')
 {
 *str++;
 count++;
 }
 return count;
}
//1. Realize reverse printing
void print(char *str)
{
 assert(str != NULL);
 int left = 0;
 int right = my_strlen(str) - 1;
 while (left <= right)
 {
 char tep = str[left];
 str[left] = str[right];
 str[right] = tep;
 left++;
 right--;
 }
}
int main(void)
{
 char arr[20] = { 0 };
 puts(&quot; Please enter the string ↓&quot;);
 gets(arr);
 print(arr);
 printf(&quot; The reverse の character string ↓\n%s\n&quot;, arr);
 return 0;
}

The operation results are as follows

Please enter the string ↓

8 Printf();

The reverse の character string ↓

;)(tnirP 8