[C language] pointer & array

array

• pass array to function

    double getAverage(int arr[], int size)
    {
        int    i;
        double avg;
        double sum=0;
        for (i = 0; i < size; ++i){
            sum += arr[i];
        }
        avg = sum / size;
        return avg;
    }
    int main ()
    {
        int balance[5] = {1000, 2, 3, 17, 50};
        double avg;
        avg = getAverage( balance, 5 ) ;
        printf( "平均值是： %f ", avg );
        return 0;
    }
  

• Function returns an array: returns a pointer to an array

  int * getRandom()
  {
      static int r[10];
      int i;
      srand((unsigned)time(NULL));
      for(i=0;i<10;++i){
          r[i] = rand();
      }
      return r;
  }
  int main()
  {
      int *p;
      int i;
      p=getRandom();
      for(i=0;i<10;i++){
          printf("*(p+%d):%d\n",i,*(p+i));
      }
      return 0;
  }
  

• pass pointer to function

  void getSeconds(undigned long *par)
  {
      *par = time(NULL);
      return;
  }
  getSeconds(&sec);
  

• function returns pointer

  int * getRandom()
  {
    srand((unsigned)time(NULL));
    r[3] = rand();
    return r;
  }
  p = getRandom();
  printf("%d",*p);
  

---

pointer

operator

• &: return variable address
• *: point to a variable
  precedence
• NULL: If there is no exact address when declaring a variable, a NULL value can be assigned

  int *ptr = NULL;
  

  • Address 0x0, most systems do not allow access to memory at address 0, indicating that it does not point to an accessible memory location.

operation

• increase or decrease
  • Increment ptr++: Point to the memory location of the next element
  • decrement ptr--: point to the storage unit of the previous element
  • The number of bytes jumped depends on the data type length of the variable pointed to by the pointer (int4 bytes)
• Compare
  • ==, <, >
  • ptr <= &var[MAX-1]

array of pointers

int *ptr[MAX];
ptr[i]=&var;
x=*ptr[i];

const char *names[]={   //  指向字符的指针数组来存储一个字符串列表
    "KCZX",
    "XHYZ",
    "TJDX",
    "GKGZ",
};

pointer pointer

int **ptr2;

ptr1 = &V;
ptr2 = &ptr1;

printf("%d",V);
printf("%d",*ptr1);
printf("%d",**ptr2);