[C question set] of VII

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Question 31 → Simulation Implementation strcat() function

Let's introduce what is strcat() function ,strcat()  The function is declared as follows  

char *strcat(char *dest, const char *src)

Append a copy of the source string to the destination string .

dest

 →  Point to target array , The array contains a  C  character string , And enough to hold the appended string .

src

 →  Point to the string to append , The string does not override the target string .

This function returns a string pointing to the final destination  

dest

  The pointer to .

hold  

src

  The string pointed to is appended to  

dest

  The end of the string that you are pointing to .

The idea of simulation implementation is very simple. In fact  

Ⅰ→dest  Find the target string '\0'

Ⅱ→ Then append the source data to the string '\0' Behind , Be careful ： Is included '\0' Of .

Question 32 → Enter ten numbers randomly , The numbers are arranged from large to small

Enter ten numbers randomly , establish ⑩ An integer array of elements , use  for  Loop through , Then enter the function .

The numbers are arranged from large to small , Put the first number and all the numbers (2~10) Compare , If the first number is greater than the second number, the values of both sides will not be exchanged ( By analogy ), Exchange if less than . If you know bubble sorting, you know what to do (╹ڡ╹ )

Finally, we begin to print out, and then we enter ⑩ The numbers are arranged from large to small

The idea of doing this topic is still relatively strong , Therefore, we must do more questions of this type in order to know which can make perfect .

Be careful ： Arrange from large to small ( Descending order ).

Question 33 → Use a function to create a variable inside the function to exchange variables with two values

Be careful → When you exchange values, you need to take out their addresses , Because it's equivalent to you and changing their memory number ！ We expect it to bring

a

and

b

The value of , In this process, we will

swap()

  Exchange the value of the function . therefore , Our external and internal functions must

Make connections

. Then we have to pass the address to , Only by the number of the memory unit of the address can we find it back .

By pointer transmission , A parameter is a pointer to the address of an argument , When pointing to a parameter , It is equivalent to the operation of the argument itself .

Pass by reference , A parameter is a pointer to the address of an argument , When pointing to a parameter , It is equivalent to the operation of the argument itself .

Passing a pointer allows multiple functions to access the object the pointer refers to , Instead of declaring objects globally accessible .

Question 34 → Receive an integer value ( Unsigned form ), Print out each bit in order . for example ：1234, Output  1 2 3 4( Recursive form )

The key point of this topic is actually recursion ！ So first we need to know what recursion is .

summary ： A function that calls itself within its body is called a recursive call , This function is called a recursive function . Executing a recursive function will call itself repeatedly , Each call enters a new layer , When the innermost function is executed , Exit layer by layer from the inside to the outside .

Then recursion also has restrictions , Otherwise it will cause death , This is equivalent to an endless cycle .

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

• After each recursive call, it gets closer and closer to this limit .

In fact, everyone has a different understanding of recursion , In the end, you need to practice the corresponding topics more .

Question 35 → Simulate the print length of string function , In the form of recursion , Cannot create temporary variable

Topic content ： Simulate the print length of string function , In the form of recursion , Cannot create temporary variable .

When doing this problem, let's explain what recursion is ↓

【C Language 】 Familiar with Chinese characters → function ＋ recursive _ Chen Ze's blog -CSDN Blog

I believe you will know what recursion is after reading this article (●'◡'●)

strlen()  The function is declared as follows ↓

size_t strlen ( const char * str );


Get string length .

return  C  character string  str  The length of .

str

 

→

  To calculate the length of a string .

A string of '\0' As an end sign ,strlen The return value of the function is the number of characters in the string ( It doesn't contain '\0')

Be careful ： The return value of the function is unsigned  unsigned  It's easy to make mistakes の.

Size_t

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

Question 31 → Code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<assert.h>
char *My_strcat(char *dest, const char *src)
{
 assert(dest && src != NULL);
 char *ret = dest;
 while (*dest != '\0')//'\0' Of ASCLL The code value is 0
 {
 dest++;
 }
 //dest Pointing to '\0'
 while (*dest++ = *src++)
 {
 ;
 }
 return ret;
}
int main(void)
{
 char arr1[20] = &quot;hello C&quot;;
 char arr2[20] = &quot;yuyan&quot;;
 printf(&quot;%s\n&quot;, My_strcat(arr1, arr2));
 return 0;
}

Running results

hello Cyuyan

Question 32 → Code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main(void)
{
 int arr[10] = { 0 };
 int i = 0;
 int j = 0;
 int k = 0;
 puts(&quot; Please enter a number :&quot;);
 for (i = 0; i < 10; i++)
 {
 scanf(&quot;%d&quot;, &arr[i]);
 }
 for (i = 0; i < 10; i++)
 {
 for (j = 0; j < 10 - i - 1; j++)
 {
 if (arr[j] > arr[j + 1])
 {
 int temp = arr[j];
 arr[j] = arr[j + 1];
 arr[j + 1] = temp;
 }
 }
 printf(&quot;program result:%d\n&quot;, arr[j]);
 }
 // Compare adjacent elements . If the first one is bigger than the second one , Just swap them .
 // Each trip starts with the first pair of adjacent elements , Do the same for each pair of adjacent elements , Until the last pair .
 // Repeat the above steps for all elements , Except for sorted elements ( The last element after each sort ), Until there's no pair of numbers to compare
 return 0;
}

Possible running results

Please enter a number ：1 2 3 4 5 6 7 8 9 10

program result：10 9 8 7 6 5 4 3 2 1

Question 33 → Code

#include<stdio.h>
void swap(int *x, int *y)
{
 int tep = *x;
 *x = *y;
 *y = tep;
}
int main(void)
{
 int a = 10;
 int b = 20;
 printf(&quot; Before the exchange :a=%d,b=%d\n&quot;, a, b);
 swap(&a, &b);
 printf(&quot;------------------\n&quot;);
 printf(&quot; After the exchange :a=%d,b=%d\n&quot;, a, b);
 return 0;
}

Running results

Before the exchange ：10,20

After the exchange ：20,10

Question 34 → Code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
void print(unsigned int number)
{
 if (number > 9) // Limiting conditions
 {
 print(number / 10); // Call this function , Until the expression is false, execute the following statement ,1234 123 12 1
 }
 printf(&quot;%d &quot;, number % 10);
}
int main(void)
{
 unsigned int number = 0;
 printf(&quot; Please enter a number :&quot;);
 scanf(&quot;%u&quot;, &number);

 print(number);

 return 0;
}

Possible running results

Please enter a number ：1234

1 2 3 4

Question 35 → Code

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int My_strlen(char *str)
{
 if (*str != '\0')
 return 1 + My_strlen(1 + str);
 else
 return 0;
}
int main(void)
{
 char arr[20] = {0};
 printf(&quot; Please enter the characters :&quot;);
 scanf(&quot;%s&quot;, &arr);

 printf(&quot;str = %d\n&quot;, My_strlen(arr));

 return 0;
}

Running results

It is possible to enter characters ：HelloC

str = 6

Be careful → It assumes that the character you enter is ：Hello C If so, include spaces , The program will stop on the space character . therefore , Here it is  str = 5. In fact, it should be 7, Then you can put scanf() The input function of is changed to gets(), such str = 7 了 .

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