1. Initialization

1.1 overall initialization

Try the following three pieces of code to analyze the output.

• Code one

   int arr[12];
    for(int i=0;i<12;++i){
        printf("%d ",arr[i]);
    }

• Code two

 int arr[12] = {0};
    for(int i=0;i<12;++i){
        printf("%d ",arr[i]);
    }

• Code three

 int arr[12] = {2};
    for(int i=0;i<12;++i){
        printf("%d ",arr[i]);
    }
    

Conclusion:

1. The array is uninitialized. The values in the array are all random values.
2. The array is initialized to {0}, and the values in the array are all 0.
3. The array is initialized to {nonzero value}. The first value in the array is a nonzero value, and the other values are all 0.

#include <stdio.h>

int main(){
	int n = 12;
	int arr[n];// = {0};
	// int arr[12] = {0};
	for(int i=0;i<12;++i){
	    printf("%d ",arr[i]);
	}

1.2 partial initialization

Try the code and analyze the output.

int arr[12] = {[2]=2,[5]=5};
for(int i=0;i<12;++i){
    printf("%d ",arr[i]);
}

The value of the specified subscript is assigned, and all other values are 0. This is the C99 syntax.

1.3 size

printf("carr = %d\n",sizeof(carr));//12
	printf("iarr = %d\n",sizeof(iarr));//48
	printf("farr = %d\n",sizeof(farr));//96

printf("sizeof(tests) = %d\n",sizeof(tests));
	printf("sizeof(tests[0]) = %d\n",sizeof(tests[0]));
	printf("num = %d\n",sizeof(tests)/sizeof(tests[0]));//(2) How to find the number of elements;
}

#include <stdio.h>

int main(){
	char carr[12];  //12
	int iarr[12];//48
	double farr[12];//96
	printf("carr = %d\n",sizeof(carr));//12
	printf("iarr = %d\n",sizeof(iarr));//48
	printf("farr = %d\n",sizeof(farr));//96

	int tests[] = {1,4,5,7,3,2,8,9,10,11,45,32,12,67,90,78,56,32,89,45,1,3,5,7,8,9,0};
	printf("sizeof(tests) = %d\n",sizeof(tests));
	printf("sizeof(tests[0]) = %d\n",sizeof(tests[0]));
	printf("num = %d\n",sizeof(tests)/sizeof(tests[0]));//(2) How to find the number of elements;
}

1.4 assignment

Try the following code

int days[]={31,28,31,30,31,30,31,31,30,31,30,31};
int arr = days;

2. Array and pointer

#include <stdio.h>

int main(){
	int days[]={31,28,31,30,31,30,31,31,30,31,30,31};
	//int arr = days;
	printf("days = %p\n",days);
	printf("days = %ld\n",days);
	printf("&days[0] = %ld\n",&days[0]);

}

#include <stdio.h>

int main(){
	int m=0;
	int n=0;
	printf("&m=%p\n",&m);
	printf("&n=%p\n",&n);

	int* p;
	scanf("%p",&p); //(1) Enter the address to be modified
	scanf("%d",p);//(2.  1) Enter the value to be modified
	scanf("%d",&m);//(2.  2) Enter the value to be modified
       
	printf("m=%d\n",m);
	printf("n=%d\n",n);

}

So, traversal arrays can be

#include <stdio.h>

int main(){
int arr[] = {1,2,3,4,5,6,7,8};
for(int i=0;i<8;++i){
    printf("%d\n",*(arr+i)); 
}
}

The array name is immutable.

3. Functions and arrays

3.1 passing arrays to functions

When an array is used as a function parameter, it is usually necessary to pass in an array size with another parameter.

  Return value type function name (type parameter name [], int size){
    
    }
    

perhaps

Return value type function name (type * parameter name, int size){

}

When an array is used as a parameter, the array degenerates into a pointer. You cannot use sizeof to get the size of the array or count the number of array elements.

• practice
  1. Print all elements of the array.
  2. lookup
  3. replace
  4. Any range + the given array randomly populates the data in the specified range.
     Use srand(time(NULL)) and rand() in stdlib.h.

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

int* add_num(int* arr,int size,int n){
	for(int i=0;i<size;++i){
		arr[i] += n;
	}
	return arr;
}

int* rand_arr(int* arr,int size,int start,int end){
	srand(time(NULL));
	for(int i=0;i<size;++i){
		arr[i] = rand()%(end-start)+start;
	}
	return arr;
}

(4) Find the index of the element;

int find_arr(int* arr,int size,int m){
	int index = -1;
	for(int i=0;i<size;++i){
		if(arr[i] == m){
			index = i;
			break;
		}
	}
	return index;
}

(see 006 for complete code_ array_ func.c)

#include <stdio.h>
#include <stdlib.h> // srand() rand()
#include <time.h> // time()
//void print_arr(int arr[],int size){
//(1) Print function;
void print_arr(int* arr,int size){
	for(int i=0;i<size;++i){
		printf("%d ",arr[i]);
	}
	printf("\n");
}
//(2) Add a number to each item;
int* add_num(int* arr,int size,int n){
	for(int i=0;i<size;++i){
		arr[i] += n;
	}
	return arr;
}
//(3) Generate any range of arrays;
int* rand_arr(int* arr,int size,int start,int end){
	srand(time(NULL));
	for(int i=0;i<size;++i){
		arr[i] = rand()%(end-start)+start;
	}
	return arr;
}
//(4) Find the index of the element;
int find_arr(int* arr,int size,int m){
	int index = -1;
	for(int i=0;i<size;++i){
		if(arr[i] == m){
			index = i;
			break;
		}
	}
	return index;
}


int main(){
	int n;
	scanf("%d",&n);
	int arr[n];
	//for(int i=0;i<n;++i){
	//	scanf("%d",&arr[i]);
	//	scanf("%d",arr+i);
	//}
	//for(int i=0;i<n;++i){
	//	printf("%d ",arr[i]);
	//	printf("%d ",*(arr+i));
	//}
	//printf("\n");
//(5) How to call
	rand_arr(arr,n,20,50);
	print_arr(arr,n);
	int m;
	scanf("%d",&m);
	int index = find_arr(arr,n,m);
	printf("%d The subscript of is%d\n",m,index);
	// int* t = add_num(arr,n,m);
	// print_arr(t,n);
}

int cmp(const void* a, const void* b){
    int l = *(int*)a;
    int r = *(int*)b;
    if(l==r) return 0;
    return (l>r) ?1:-1;
} 

 qsort(res,n+m,sizeof(int),cmp);   //① Object, ② size ③ unit ④ sorting function

for(int i=0;i<m;++i){
		scanf("%d",b+i);
		res[i+n] = b[i];
	}

#include <stdio.h>
#include <string.h>
int cmp(const void* a, const void* b){
    int l = *(int*)a;
    int r = *(int*)b;
    if(l==r) return 0;
    return (l>r) ?1:-1;
} 

int main(){
	int n,m;
	scanf("%d",&n);
	int a[n];
	
	int res[n+m];
	for(int i=0;i<n;++i){
		scanf("%d",a+i);
		res[i] = a[i];
	}

	int b[m];
	scanf("%d",&m);
	for(int i=0;i<m;++i){
		scanf("%d",b+i);
		res[i+n] = b[i];
	}

     qsort(res,n+m,sizeof(int),cmp);   //① Object, ② size ③ unit ④ sorting function
    for(int i=0;i<m+n;++i){
		printf("%d\n",*(res+i));
	}

    
    
    
}

void merge(int* nums1, int nums1Size, int m, int* nums2, int nums2Size, int n){
    int len = m + n;
    int i, a, b;

    a = m - 1;
    b = n - 1;
    for (i = len - 1; i >= 0; i--) {
        printf("i = %d, a = %d, b = %d\n", i, a, b);
        //compare
        if (a >= 0 && b >= 0) {
            nums1[i] = nums1[a] > nums2[b] ? nums1[a] : nums2[b];
            nums1[a] > nums2[b] ? a-- : b--;
            continue;
        }
        //Only b is left;
        if (a < 0) {
            nums1[i] = nums2[b];
            b--;
        } 
    }
}

3.2 return array from function

C does not allow returning a complete array as a function's argument. Returns a pointer to an array by specifying the name of the array without an index.

Type * function name (){
   return array name;
}

practice

**Key points**

  int arr[] = {1,2,3,4,5,6};
    // Sizeof (array name)
    printf("sizeof(arr) = %ld\n",sizeof(arr));
    // Value of array name
    printf("arr=%p\n&arr[0]=%p\n",arr,&arr[0]);

The value of array name is address

1. You can use pointer operations: dereference
2. When an array is used as a function parameter and return value, a pointer / address is passed.

4. Multidimensional array

4.1 statement

• grammar

  Type array name [number of elements 1] [number of elements 2]... [number of elements N];

The most common form of multidimensional array is 2D array. A two-dimensional array is a table of rows and columns.

Type 2D array name [number of rows] [number of columns];

For example, here is a table with four rows and three columns of element type int.

  int days[4][3];

4.2 initialize 2D array

Multidimensional arrays can be initialized by specifying values for each row in parentheses.

   int days[4][3]={
        {31,28,31},
        {30,31,30},
        {31,31,30},
        {31,30,31}
    };
    

4.3 accessing 2D array elements

The elements in a two-dimensional array are accessed by using subscripts (that is, the row index and column index of the array).

   // Get days of January
    int n = days[0][0];
    // Modify days in February
    days[0][1] = 29;
    // Print January days
    printf("%d",days[0][0]);

4.4 element traversal of two-dimensional array

Nested loops are often used to work with 2D arrays.

   for (int i = 0; i < 4; i++ ) {
        for (int j = 0; j < 3; j++ ) {
           printf("a[%d][%d] = %d\n",i,j,a[i][j]);
        }
    }
    

4.5 2D array input

   int n,m;
    scanf("%d%d",&n,&m);
    int arr[n][m];
    for (int i=0;i<n;++i){
      for (int j=0;j<m;++j){
          scanf("%d",&arr[i][j]);
      }
    }
    

practice

1. Input the scores of n courses of m students, and output the scores of each course as well as the total scores and average values.

for(int i=0;i<m;++i){
		for(int j=0;j<n;++j){
			scanf("%d",&arr[i][j]);
		}
	}

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

#include <stdio.h>

int main(){
	int m,n;
	scanf("%d%d",&m,&n);

	int arr[m][n];
	for(int i=0;i<m;++i){
		for(int j=0;j<n;++j){
			scanf("%d",&arr[i][j]);
		}
	}
	for(int i=0;i<m;++i){
		int sum = 0;
		for(int j=0;j<n;++j){
			sum += arr[i][j];
			printf("%d ",arr[i][j]);
		}
		printf("%d %f\n",sum,1.0*sum/n);
	}
}

1. Two dimensional arrays are often used to represent matrices in mathematics. Basic matrix operations (addition, subtraction, number multiplication, transposition)

4.6 simplification

Initialization of 2D arrays can be simplified as follows:

1. Omit inner nested parentheses.

 int days[4][3]={31,28,31,30,31,30,31,31,30,31,30,31};

1. The first dimension size is omitted, and the second dimension cannot be omitted.

int days[][3]={31,28,31,30,31,30,31,31,30,31,30,31};

problem
Can 2D arrays be initialized as a whole? sure;
practice

• LeetCode 36. Effective Sudoku

/* isValidSudoku: Sudoku is valid, return true, otherwise return false */
bool isValidSudoku(char** board, int boardSize, int* boardColSize)
{
    int row[9][9] = {{0}}, col[9][9] = {{0}}, rc[9][9] = {{0}};   /* Row, column, 9-house number occurrence flag, initial 0 */
    
    for (int i = 0; i < boardSize; ++i)
        for (int j = 0; j < boardSize; ++j)
            if (board[i][j] != '.') {   /* Fill in the number. Only 1-9 cells in each row, column and 9 cells can be used once */
                if (++row[i][board[i][j] - '1'] > 1 || 
                    ++col[j][board[i][j] - '1'] > 1 || 
                    ++rc[i/3*3+j/3][board[i][j] - '1'] > 1)
                    return false;
            }
    return true;
}

• LeetCode 892. Surface area of three-dimensional body
  

int surfaceArea(int** grid, int gridSize, int* gridColSize){
    int i, j, k, res = 0;
    for(i=0; i<gridSize; i++){
        for(j=0; j<gridColSize[i]; j++){
            for(k=0; k<grid[i][j]; k++){
                res += 6;
                // Adjacent below
                if(k>0){
                    res -= 2;
                }
                // Left adjacent
                if(i>0 && grid[i-1][j]>k){
                    res -= 2;
                }
                // Front adjacent
                if(j>0 && grid[i][j-1]>k){
                    res -= 2;
                }
            }
        }
    }
    return res;
}

4.7 multidimensional array

The use of arrays larger than two dimensions is the same as that of two dimensions, but less.

 int days[][4][3]={
        {31,28,31,30,31,30,31,31,30,31,30,31}, // Weekday
        {31,29,31,30,31,30,31,31,30,31,30,31}  // leap year
    };

    
    printf("The number of days in February is%d\n",days[0][0][1]);// The second month of the first quarter of the year
    printf("In leap year, February days are%d\n",days[1][0][1]);// The second month of the first quarter of leap year 

Multidimensional array initialization can only omit the first dimension.

5 const array

5.1 what is const array?

const int arr[]={1,2,3,4,5,};

Array variable is already const pointer, indicating that every element in the array is const int, that is, every element cannot be changed by arr.
For example:

const int arr[]={1,2,3,4,5,};
arr[0] = 0;

5.2 how to use const array?

Protect array values

Because arrays are passed by address as function parameters, the values of arrays can be modified inside a function.
To protect the array from being destroyed by the function, set the parameter const.
For example:

  int sum(const int arr[],int len);

perhaps

int sum(const int* arr,int len);

6 variable pointer vs array pointer

Variable pointer: pointer to a single variable.
Array pointer: pointer to array.

#include <stdio.h>

int main () {
    int n = 10;
    int *p;
    p = &n; // p pointer to variable
    printf("*p = %d\n",*p);
    
    int arr[] = {1,2,3,4,5,};
    p = arr;// p pointer to array
    printf("*p = %d\n",*p);
    printf("*(p+1) = %d\n",*(p+1));
    printf("*(p+2) = %d\n",*(p+2));
    printf("*(p+3) = %d\n",*(p+3));
    printf("*(p+4) = %d\n",*(p+4));
    
    return 0;
}

A pointer can point to both a basic type variable and an array. So we should pay attention to distinguish when using.

7 project

Enter the year to print the perpetual calendar.

Option 1: print 12 months based on the year of the year

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
int qq(int x, int t)
{
    int i = 0, p, s = 365, y, j, b, k = 1, h = 1, u = 0;
    int a[13];
    FILE *fp;
    fp = fopen("tai.txt", "a+");
    i = 0; k = 1; h = 1; u = 0;
    printf("\t^^^^^^^^^^^^^\t%d year%d month\t^^^^^^^^^^^^^^^^\n\n", x, t);
    fputs("\t^^^^^^^^^^^^^^^^", fp);
    fputc(x / 1000 + 48, fp);
    fputc(x / 100 % 10 + 48, fp);
    fputc(x / 10 % 10 + 48, fp);
    fputc(x % 10 + 48, fp);
    fputs("year", fp);
    fputc(t / 10 + 48, fp);
    fputc(t % 10 + 48, fp);
    fputs("month^^^^^^^^^^^^^^^^^^^^^^^", fp);
    fputs("\n\n", fp);
    {for (p = 1900; p <= x; p++)
    if (p % 4 == 0 && p % 100 != 0 || p % 400 == 0) i++;
    if (x % 4 == 0 && x % 100 != 0 || x % 400 == 0) s = i * 366 + (x - 1900 - i) * 365;
    else s = i * 366 + (x - 1900 - i) * 365 + 1;
    printf("\t day\t One\t Two\t Three\t Four\t Five\t Six\n");
    fputs("\t day\t One\t Two\t Three\t Four\t Five\t Six\n", fp);
    for (i = 1; i <= t; i++)
    {
        if (i == 2 || i == 4 || i == 6 || i == 8 || i == 9 || i == 11) u += 31;
        if (i == 5 || i == 7 || i == 10 || i == 12) u += 30;
        if (i == 3){
            if (x % 4 == 0 && x % 100 != 0 || x % 400 == 0) u += 29;
            else u += 28;
        };
    };
    for (i = 1; i <= 12; i++)
    {
        if (i == 1 || i == 3 || i == 5 || i == 7 || i == 8 || i == 10 || i == 12) a[i] = 31;
        if (i == 4 || i == 6 || i == 9 || i == 11) a[i] = 30;
        if (i == 2){
            if (x % 4 == 0 && x % 100 != 0 || x % 400 == 0) a[i] = 29;
            else a[i] = 28;
        };
    };
    b = a[t];
    y = (s % 7 + u) % 7;
    for (i = 1; i <= 6; i++)
    {
        for (j = 1; j <= 7; j++)
        {
            if (h <= y) { printf("\t"); fputc('\t', fp); };
            if (h>y)  {
                if (k <= b) { printf("\t%d", k); fputc('\t', fp); if (k>9) fputc(k / 10 + 48, fp), fputc(k % 10 + 48, fp); else fputc(k + 48, fp); };
                if (k == b + 1) { printf("\n"); fputc('\n', fp); };
                k++;
            };
            h++;
        };
        printf("\n"); fputs("\n", fp);
    };
    };
    printf("\t***************************************************\n\n");
    fputs("\t***************************************************", fp);
    fputs("\n\n", fp);
    fclose(fp);
    return (0);
}
void main()
{
    int x, t, q = 0, e, i, c, cc, xx, tt;
    float p, pp;
    FILE *fp;
    if ((fp = fopen("tai.txt", "w")) == NULL)
    {
        printf("error!");
        exit(0);
    }
    fclose(fp);
    do{
        printf("Please enter the year and month you want to query,\n");
        printf("The input format is:"year+Space+month+Enter". (particular year >=1900)");
        printf("(If you want to show the whole year,Lose "year"+Space+0+Enter"). For example, "2012+Space+0+Enter")\n");
        printf("(If you want to output XXXX reach xxxx Year round,Lose "year"+Space+year+Enter"). For example, "2010+Space+2012+Enter")\n");
        printf("(If you want to show xxxx Before or after x Year, lose year+Space+.x+Enter"). For example, in the first three years of 2010, enter "2010"+Space+"-.3"+Enter")\n");
        printf("(If you want to show xxxx Of m Before or after x Months, lose "years"+Space+m.x+Enter"). For example, three months after May 2010, enter "2010"+Space+"+5.3"+Enter")\n");
        scanf("%d%f", &x, &p);
        pp = p * 10;
        c = (int)pp;
        cc = c % 10;
        xx = x;
        t = abs((int)p);
        tt = t;
        if (x<1900) { printf("The input is not satisfactory, please re-enter!!"); q = 1; }

        else if (x >= 1900 && (t>0 && t <= 12) && cc == 0)//Output the specified month and year.
        {
            qq(x, t);
        }

        else if (x >= 1900 && t == 0 && cc == 0)//Outputs the specified full year.
        {
            for (t = 1; t <= 12; t++)
                qq(x, t);
        }
        else if (x >= 1900 && t >= 1900)//Output the whole year from xxxx to xxxx.
        {
            for (i = xx; i <= tt; i++, x++)
            for (t = 1; t <= 12; t++)
                qq(x, t);
        }
        else if (x >= 1900 && p<1.0&&tt == 0)//Output x years before or after xxxx years.
        {
            if (cc + x<1900 && tt == 0)
                printf(" Too many previous years entered.");
            else if (cc>0)
            {
                for (i = 0; i <= cc; i++, x++)
                for (t = 1; t <= 12; t++)
                    qq(x, t);
            }
            else {
                for (i = cc; i <= 0; i++, x--)
                for (t = 1; t <= 12; t++)
                    qq(x, t);
            }
        }
        else if (x >= 1900 && t <= 12 && t >= 1 && cc != 0)//Output x months before or after m months of xxxx. "Haha, imagine" let me ask any number before or after x months "
        {
            if (cc>0)
            {
                for (tt = 0; tt <= cc; tt++, t++)
                {
                    if (t>12) qq(x + 1, t - 12);
                    else qq(x, t);
                }
            }
            else{
                for (tt = 0; tt >= cc; tt--, t--)
                {
                    if (t<1) qq(x - 1, t + 12);
                    else qq(x, t);
                }
            }
        }

    } while (q == 1);
    scanf("%d", &e);
    if (e)exit(0);
}

Scheme 2: print the current month of the year according to the year and month of the year

The Date.h file is as follows:

//Print perpetual calendar from 1900
#ifndef DATE_H
#define DATE_H
#include <string.h>
#define OUT_OF_YEAR -1
//Leap year or not
bool isLeap(int year){
	if(year<1900) return OUT_OF_YEAR;
	return (year%4==0)&&(year%100!=0)||(year%400==0);
}
//Number of leap years from year to 1900
int  LeapYearNum(int year){
	if(year<1900) return OUT_OF_YEAR;
	int LeapYear=0;
	for (int i=1900;i<=year;i++)
	{
		if(isLeap(i)){
			++LeapYear;
		}	
	}
	return LeapYear;
}
//Days from year to 1900
int GetDayNum(int year){
	if(year<1900) return OUT_OF_YEAR;
	return LeapYearNum(year)+(year-1900)*365;
}
//year, number of days in month
int GetDayOfMouth(int year,int mounth){
	if (year<1900 || mounth<1||mounth>12) return OUT_OF_YEAR;
	int dayNum=0;
	switch (mounth)
	{
	case 1:
	case 3:
	case 5:
	case 7:
	case 8:
	case 10:
	case 12:
		dayNum=31;
		break;
	case 4:
	case 6:
	case 9:
	case 11:
		dayNum=30;
		break;
	case 2:
		if(isLeap(year)){
			dayNum=29;
		}else{
			dayNum=28;
		}
		break;
	default:
		dayNum=0;
	}
	return dayNum;
}
//Days to 1900
int getDay(int year,int mouth){
	int mouthDay=0;
	for (int i=1;i<mouth;i++)
	{
		mouthDay+=GetDayOfMouth(year,i);
	}
	return GetDayNum(year)+mouthDay;
}
int getWeek(int year,int mouth){
	return getDay(year,mouth)%7+1;
}
//Get week abbreviation
char* getWeekChar(int i){
	if (i<1||i>7) return "";
	char *WeekChar="   ";
	switch(i){
	case 1:
		WeekChar="Mon";
		break;
	case 2:
		WeekChar="Tue";
		break;
	case 3:
		WeekChar="Wed";
		break;
	case 4:
		WeekChar="Thu";
		break;
	case 5:
		WeekChar="Fri";
		break;
	case 6:
		WeekChar="Sat";
		break;
	case 7:
		WeekChar="Sun";
		break;
	default:
		WeekChar="";
	}
	return WeekChar;

}
//Print month
void printMouth(int year,int mouth){
	int startWeek=getWeek(year,mouth);
	int days=GetDayOfMouth(year,mouth);
	printf("*********************%d year%d month******************************\n",year,mouth);
	int dayIndex=1;
	printf("\n");
	for (int i=1;i<=7;i++)
	{
		printf(" %s\t",getWeekChar(i));
	}
	printf("\n");
	for (int i=1;i<=50;i++)
	{
		if (dayIndex>days) break;
		if (i<startWeek)
		{
			printf("   \t");
		}else{
			printf("%4d\t",dayIndex);
			dayIndex++;
		}
		if (i%7==0) printf("\n");
	}
	printf("\n");
}
#endif

Call as follows:

#include <stdio.h>
#include "Date.h"
int main( void )
{	

	printMouth(2013,10);
	
}