char arr[10]//定义一个字符数组，用来存放字符元素（未初始化）
double arr[]={0,1,2,3,4,5};//定义一个浮点数师数组，元素个数可以不写，会根据存放的数据个数自己生成
int a[10]={0};//初始化一般赋值0，若初始化的数据个数不够元素个数，则后面元素数据自动补0

The third is called incomplete initialization, such as:

int a[4]={1,2};//这个数组可以存放四个数据元素，但只给它赋值了1，2；则剩下的自动赋值为0

2. The subscript of the array

Each element of the array has a subscript, and the subscript starts from 0.

int a[4]={1，3，4，6};

int arr[4]	1	3	4	6
subscript	0	1	2	3

3. The use of arrays

For example, if we want to print the 3 elements in the array, we need to find the subscript 1

printf("%d",arr[1]);

If you need to print the entire array, you need to use a loop

int i=0;
while(i<4)
{
printf("%d ",arr[i]);
i++;

}

Two, the operator

1. Arithmetic operators

+(Add) -(Subtract) *(Multiply) /(Divide) %(Remainder)

printf("%d",3+5);//打印结果为8
printf("%d"，5-3);//打印结果为2
printf("%d"，3*5);//结果为15
printf("%d"，3/5);//打印结果为0
printf("%d"，5/3);//结果为1
printf("%lf",5.0/3);//结果为1.6666
printf("%d",5%3);//结果为2

Division sign /: If both sides are integers, the result is divisible. If at least one of the two sides is written as a decimal, the result will also be calculated to the decimal part.

Remainder (modulus)%: used to calculate the remainder.

2. Shift operator

>> <<

don't need to know

3. Bitwise operators

& ^ |

don't need to know

4. Assignment operator

= += -= *= /= &= ^= |= >>= <<=

5. Unary operator

! - + & sizeof ~ -- ++ * (type)

1) ! logical inverse operation

For example, a!=5 means that a is not equal to 5; it means the opposite

2) - Negative values

int a=-1;
printf("%d\n",-a);//结果为1

3) + Positive values generally have little effect, and cannot turn negative numbers into positive numbers

int a=-1;
printf("%d\n",a);//结果为-1
printf("%d\n",+a);//结果为-1

4) & get address

(described later)

5) The type length of the operand of sizeof (in bytes)

(later)

6) ~ Reverse the binary bitwise of a number

(later)

7) -- pre-position, post-position -- is to implement a subtraction operation on itself

int a=5;
//后置--
printf("%d\n",a--);//先打印a,所以打印结果为5，再让a-1，所以此时a变为4
//前置--
printf("%d\n",--a);//先a-1，所以此时a为4，再打印a，打印结果为4

8) ++ pre, post ++ ++ is to implement a subtraction operation on itself

int a=5;
//后置++
printf("%d\n",a++);//先打印a,所以打印结果为5，再对a+1，所以此时a变为6
//前置++
printf("%d\n",++a);//先a+1，所以此时a为6，再打印a，打印结果为6

9) * indirect access operator

(described later)

10) (type) mandatory type conversion

int a=(int)3.14//将3.14强制转换为整形3

date cannot be converted

6. Relational Operators

> >= < <= != ==1)>
1)> greater than sign

2)>= greater than or equal to

3) <= less than or equal to

4) != is used to test for inequality

5) == is used to test for equality

7. Logical operators

&& ||

1) && logical AND (and)

a&&b requires both a and b to be true for a&&b to be true

2) || logical or (or)

a||b only needs one of a or b to be true, then a||b is true

Purpose: used for conditional judgment, not like this a<=x<=b, only ||

8. Conditional operator (ternary operator)

exp1?exp2:exp3

Parsing: If exp1 is true, execute exp2; if it is false, execute exp3

int a=3;
int b=5;
int max=a>b？a:b;//如果a>b,则最大值为a,否则为b

9. Comma expressions

exp1,xep2,exp3,.....expn

Features: It is calculated sequentially from left to right, and the result is the value of the entire expression.

int a=3;
int b=4;
int c=1;
int d=(a=2+c,b=a-1,c=a+b);
//先计算a=2+c,此时a=3,再计算b=a-1,此时b=2,再计算c=a+b,所以此时c=5，所以d=5.

10. Subscript references, function calls, and structure members

[] () . ->

Initial C language (fourth class hour)

1. Array

1. Definition: a combination of a group of elements of the same type