First knowledge of C language -- structure, branch and loop statements

The pointer

#include <stdio.h>

int main()
{
   int a = 10;  // Applied to memory 4 Bytes of space 
   //  &a Take the address operator 
   // printf("%p\n",&a);
   int *p = &a; // *p Pointer to the variable ,
   //   printf("%p\n",p);
   *p = 15;   // * -- Indirect access operators 
   printf("a = %d\n",a);
   return 0;
}

1. Structure （ important ）

Structure is a type we created

#include <stdio.h>

//  Create a structure 
struct  Book
{
   char name[20]; //  Title 
   short price;   //  Price 
};    // ; essential 

int main()
{
   //  Use the structure type to create a structure variable of that type 
   struct  Book b1 = {"C Language ", 20};
   printf(" Title ：%s\n",b1.name);
   printf(" Price ：%d\n",b1.price);
   b1.price = 15;
   printf(" Revised price ：%d\n",b1.price);
   return 0;
}

1.1 Structure + Pointer application （ important ）

The operator ：

.    Structural variable . member

->    Structure pointer ->  member

Using this method, we can define a structure to store the data of the sensor

#include <stdio.h>

//  Create a structure 
struct Book
{
   char name[20]; //  Title 
   short price;   //  Price 
};                // ; essential 

int main()
{
   //  Use the structure type to create a structure variable of that type 
   struct Book b1 = {"C Language ", 20};
   struct Book *p = &b1;
   printf("%s\n", p->name);  // -> Is the operator whose pointer points to the object 
   printf("%d\n", p->price);  
   return 0;
}

#include <stdio.h>
#include <string.h>
//  Create a structure 
struct Book
{
   char name[20]; //  Title 
   short price;   //  Price 
};                // ; essential 

int main()
{
   //  Use the structure type to create a structure variable of that type 
   struct Book b1 = {"C Language ", 20};
   strcpy(b1.name, "C++"); // strcpy -- String copy , Library function --string.h
   //  All structure members are of string type , All need to be adopted strcpy To modify its content 

   struct Book *p = &b1;
   printf("%s\n", p->name); // -> Is the operator whose pointer points to the object 
   printf("%d\n", p->price);
   return 0;
}

2. Branches and loops

C Language is a structured programming language , Sequential structure 、 Selection structure 、 Loop structure

2.1 Branch statement （ Selection structure ）

C There is a semicolon in language ; Separated by a statement .

else Match the nearest if sentence , Unless added {} Segmentation

Code style optimization , When comparing and judging the values of constants and variables , The constant is on the left , Put the variable on the right

int num = 1;
if(5 == num){
   printf("%d\n",num);
}

practice ：

Output 1--100 Between the odd numbers

/*
 * Output 1~100 Between the odd numbers 
 * Ideas 1： Odd is divided by 2 An integer followed by a remainder 
 * Ideas 2： Odd number is 2 A multiple of 1
 */
#include <stdio.h>

// int main()
// {
//    int i;
//    for(i = 1; i <= 100; i++)
//    {
//       if (i % 2 != 0)
//          printf("%d\t", i);
//    }
//    return 0;
// }

//  Method 2
int main()
{
   int i;
   for (i = 1; i < 100; i += 2)
   {
      if (i % 2 != 0)
         printf("%d\t", i);
   }
   return 0;
}

Output results ：

 2.2 switch sentence

switch Nesting is allowed inside

2.3 Loop statement

while   for     do--while

continue stay while The role in the cycle ：

continue Is used to terminate this cycle , In this cycle continue The following code is no longer executed , It's a jump to while The judgment part of a sentence , Make the entry judgment of the next cycle .

for loop ：

1.for Initialization of the loop 、 adjustment 、 Judgment can be omitted , however for The judgment part of the loop is omitted , Then the judgment condition is always true

2. If not very skilled , Don't omit for Each part of the cycle

  practice 1： Calculation n The factorial

#include <stdio.h>

int main()
{
   int i = 0;
   int n = 0;
   int ret = 1;
   scanf("%d", &n);
   for (i = 1; i <= n; i++)
   {
      ret = ret * i;
   }
   printf("ret = %d\n", ret);
   return 0;
}

practice 2： Calculation 1！+2！+....+10！

#include <stdio.h>
int main()
{
   int i = 0;
   int n = 0;
   int ret = 1;
   int sum = 0;
   for (n = 1; n <= 10; n++)
   {
      ret = ret * n;
      sum = sum + ret;
   }
   printf("sum = %d\n", sum);
   return 0;
}

practice 3： Find a specific number in an ordered array n, To write intbinsearch(int x, int v[],int n); function ： stay v[0]<=v[1]<=v[2]<=.....<=v[n-1] Find in the array of x.

#include <stdio.h>
int main()
{
   int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
   int k = 7;
   //  Write a code , stay arr Found... In an ordered array 7
   int i = 0;
   int sz = sizeof(arr) / sizeof(int);
   for (i = 0; i < sz; i++)
   {
      if (k == arr[i])
      {
         printf(" Already found , The following table is ：%d\n", i);
         break;
      }
   }
   if (i == sz)
      printf(" Not found \n");
   return 0;
}

Binary search

/*
 Find by dichotomy , Ordered array 
*/
#include <stdio.h>
int main()
{
   int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
   int sz = sizeof(arr) / sizeof(int); //  Count the number of elements in an array 
   int left = 0;                       //  Left subscript 
   int right = sz - 1;                 //  Right subscript 
   int mid = (left + right) / 2;       //  The subscript of the intermediate element 

   int k = 7;
   while (left <= right)
   {
      if (arr[mid] > k)
         right = mid - 1;
      else if (arr[mid] < k)
         left = mid + 1;
      else
      {
         printf(" Already found , The following table is ：%d\n", mid);
         break;
      }
   }
   if (left > right)
      printf(" Can't find \n");
   return 0;
}