C language pointer understanding

basic understanding

#include <stdio.h>
#include <windows.h>
int main(){
    SetConsoleOutputCP(65001);
    int var=20;
    int p=var;
    printf("%d\n",var);
    printf("%d\n",p);
    printf("%p\n",&var);
    printf("%p\n",&p);
    return 0;
}

%p means the value of the pointer

  As a result, although the values ​​of var and p are the same, the two variables point to two memory addresses.

Then if you change the value of the var variable

#include <stdio.h>
#include <windows.h>
int main(){
    SetConsoleOutputCP(65001);
    int var=20;
    int p=var;
    printf("%d\n",var);
    printf("%d\n",p);
    printf("%p\n",&var);
    printf("%p\n",&p);
    printf("-------\n");
    var=11;
    printf("%d\n",var);
    printf("%d\n",p);
    printf("%p\n",&var);
    printf("%p\n",&p);
    return 0;
}

 It can be seen that the value of var has changed, the value of p has not changed and the memory addresses of the two variables have not changed, which means that this is just a simple process of changing the value. If you want the variable p to change according to the value of var, Then p will point to the memory address of var

As follows, look at the code below to understand

#include <stdio.h>
#include <windows.h>
int main(){
    SetConsoleOutputCP(65001);
    int var=20;
    int *p;
    p=&var;
    printf("%d\n",var);
    printf("%d\n",*p);
    printf("%p\n",&var);
    printf("%p\n",p);
    printf("-------\n");
    var=11;
    printf("%d\n",var);
    printf("%d\n",*p);
    printf("%p\n",&var);
    printf("%p\n",p);
    return 0;
}

var is (value | address) is (20 | &var)

*p is a pointer variable, pointing to the address of var, relative to var expressed by p is (*p | p) is (value | address)

pointer arithmetic

#include <stdio.h>
#include <windows.h>
int main(){
    SetConsoleOutputCP(65001);
    int var[]={10,100,200};
    int *ptr;
    ptr=&var[0];// 也可以是ptr=var
    for(int i=0;i<3;i++){
        printf("var[%d]值：%d\n",i,*ptr);
        printf("var[%d]地址：%p\n",i,ptr);
        ptr++;
    }
    return 0;
}

array of pointers

#include <stdio.h>
#include <windows.h>
#define MAX 3
int main(){
    SetConsoleOutputCP(65001);
    int var[MAX]={10,100,200};
    int *ptr[MAX];
    for(int i=MAX-1;i>=0;i--){
        ptr[i]=&var[i];
        printf("%d\n",*ptr[i]);
    }
    return 0;
}

 string array pointer

#include <stdio.h>
#include <windows.h>
#define MAX 3
int main(){
    SetConsoleOutputCP(65001);
    char *name[]={
        "小白",
        "小红",
        "小明"
    };
    for(int i=MAX-1;i>=0;i--){
        // printf("%s\n",name[i]);
        puts(name[i]);
    }
}

pointer to pointer

#include <stdio.h>
#include <windows.h>
#define MAX 3
int main(){
    SetConsoleOutputCP(65001);
    int a=1;
    int *b,**c;
    b=&a;
    c=&b;
    printf("%d\n",a);
    printf("%d\n",*b);
    printf("%d\n",**c);
    printf("%p\n",&a);
    printf("%p\n",b);
    printf("%p\n",c);
}

pass pointer to function

functions with pointers as arguments

#include <stdio.h>
#include <windows.h>
void countFun(int *s){
    *s=1;
}
int main(){
    SetConsoleOutputCP(65001);
    int size=0;
    countFun(&size);
    printf("%d",size);
}

Functions that take pointers as parameters can also accept arrays as parameters

#include <stdio.h>
#include <windows.h>

double avg(int *arr,int size){
    int count=0;
    for(int i=0;i<size;i++){
        count+=arr[i];
    };
    double avgVal=(double)count/size;
    return avgVal;
}
int main(){
    SetConsoleOutputCP(65001);
    int arr[]={1,2,3,4,5,6};
    int size=sizeof(arr)/sizeof(arr[0]);
    double avgVal=avg(arr,size);
    printf("%d",avgVal);
}

 function return pointer

If you want to return in the function, you must use statci to define static variables. The usage of static in C/C++ is global variables and local variables | rookie tutorial

#include <stdio.h>
#include <time.h>
#include <stdlib.h> 
 
/* 要生成和返回随机数的函数 */
int * getRandom( )
{
   static int r[10];
   srand((unsigned) time(NULL));
   for(int i=0;i<10;i++){
    r[i]=rand()%10;
    printf("%d\n",r[i]);
   }
   return r;
}
 
/* 要调用上面定义函数的主函数 */
int main ()
{
    int *p;
    p=getRandom();
    printf("\n-------\n");
    for(int i=0;i<10;i++){
        printf("%d\n",*(p+i));
    }
    return 0;
}

function pointer

For example, the following is a normal function

#include <windows.h>
#include <stdio.h>
int cyc1(int x,int y){
   return x>y?x:y;
}
int main(void){
    SetConsoleOutputCP(65001);
    int a,b,c;
    printf("请输入3个整数：");
    scanf("%d %d %d",&a,&b,&c);
    int val=cyc1(cyc1(a,b),c);
    printf("%d",val);
    return 0;
}

then use the function pointer

#include <windows.h>
#include <stdio.h>
int cyc1(int x,int y){
   return x>y?x:y;
}
int main(void){
    SetConsoleOutputCP(65001);
    int a,b,c;
    int (*p)(int,int)=&cyc1;
    printf("请输入3个整数：");
    scanf("%d %d %d",&a,&b,&c);
    int val=p(p(a,b),c);
    printf("%d",val);
    return 0;
}