Elementary --4 C language extensions: Array

Elementary --4 C language extensions: Array

1. Return Value Meaning

The return value Sometimes there are two cases: the legal value and an illegal value.
If there are illegal values, the value of certain commonly used to refer to special circumstances. For example: array subscript 0 and can only be a positive number. We find the circumstances to achieve an array of time scale, there is an element not found in the next element, then, to use -1as a return value in this case.

Return value and meaning of the definition is artificial.

2. The use of a two-dimensional array pointer

• A one-dimensional array pointer usage

No.	operating	Subscript	pointer
1	The first ielement of value	arr[i]	*(arr+i)
2	The first ielement address	&arr[i]	arr+i

• Two-dimensional array pointer usage

No.	operating	Subscript	pointer
1	The first irow jcolumn element values	arr[i][j]	*(*(arr+i)+j)
2	The first irow of the jcolumn element address	&arr[i][j]	*(arr+i)+j

In the two-dimensional array a[i]is a one-dimensional array.

• problem

1. Two-dimensional array array name is the first address of the first element of it?

2. Try the following code

#include <stdio.h>

int main () {
 int days[4][3]={31,28,31,30,31,30,31,31,30,31,30,31};
 printf("days[0]\t\t = %p\n&days[0][0]\t = %p\n",days[0],&days[0][0]);
 printf("days[1]\t\t = %p\n&days[1][0]\t = %p\n",days[1],&days[1][0]);
 printf("days[2]\t\t = %p\n&days[2][0]\t = %p\n",days[2],&days[2][0]);
 printf("days[3]\t\t = %p\n&days[3][0]\t = %p\n",days[3],&days[3][0]);

 return 0;
}

One-dimensional elements in a two-dimensional array is an array of pointers.

int days[4][3]={31,28,31,30,31,30,31,31,30,31,30,31};
int* p = days[0];
for(int i=0;i<3;++i){
  printf("%d ",p[i]);
}

3. The one-dimensional array using multi-dimensional way

3.1 one-dimensional array to a two-dimensional array

int n = 24;
int arr[n];
for(int i=0;i<n;++i){
    arr[i] = i;
}
for(int i=0;i<n;++i){
    printf("%d ",arr[i]);
}
printf("\n");

3.2 one-dimensional array to a two-dimensional array

One-dimensional array using a two-dimensional array access method (double loop), can be seen as a box placed in accordance with the ranks of the way.

元素下标 = 当前行序号*列元素个数 + 当前列序号

int n = 24;
int arr[n];
for(int i=0;i<n;++i){
    arr[i] = i;
}
int rows = 4; 
int cols = 6;
for(int i=0;i<rows;++i){
   for(int j=0;j<cols;++j){
     printf("%d ",arr[i*cols+j]);
   }
   printf("\n");
}

A three-dimensional array to a 3.3-dimensional array

One-dimensional array using a three-dimensional array of access methods (triple loop), can be seen as a box placed in accordance with the ranks of high fashion.

元素下标 = 当前面序号 * 面元素个数 + 当前行序号 * 列元素个数 + 当前列序号

int n = 24;
int arr[n];
for(int i=0;i<n;++i){
    arr[i] = i;
}
int pages = 2;
int rows = 4; 
int cols = 3;
for(i=0;i<pages;++i){
   for(j=0;j<rows;++j){
     for(k=0;k<cols;++k){
       printf("%d ",arr[i*rows*cols+j*cols+k]);
     }
     printf("\n");
   }
   printf("\n");
}

4. few weeks calculation

Kim Larson formula

w=(d + 2*m + 3*(m+1）/5 + y + y/4 - y/100 + y/400) % 7

• Explanation

1. Formula should January and February 13, respectively, as the previous year and month 14 month treatment.
   For example: January 4, 2008 to be replaced by 2007 to March 4, into the formula.
2. Corresponding to the formula a little different from Zeller's formula: 0as week 1, ......, 6Sunday