Learning notes on February 8, 2022 (C language)

22/2/8 Learning notes （Part 2）

notes

Comments are used to explain complex code .

 //int a = 10;
 //C++ Annotation style 

 /* int b = 10; */ 
 //C Language annotation style - Nested comments are not supported 

Array

An array is a collection of elements of the same type .

• int arr —— Array of numbers

int arr[10] = {
    1,2,3,4,5,6,7,8,9,10};

• char ch—— A character array

 char ch[5] = {
    ‘a’,’b‘,’c‘}; // Incomplete initialization , The rest of the following defaults to 0

Escape character

Escape character （ Changed the original meaning ）

Escape character	paraphrase
\ ?	Use... When writing multiple consecutive question marks , Prevent them from being parsed into three letter words
\ ’	Used to represent character constants ‘
\ "	Double quotation marks used to represent the inside of a string
\ \	Used to indicate a backslash , Prevent it from being interpreted as an escape sequence character
\ a	Warning characters , Beep
\ b	Back space
\ f	Paper in
\ n	Line break
\ r	enter
\ t	Horizontal tabs
\ v	Vertical tabs
\ ddd	ddd Express 1~3 Eight octal numbers . Such as ：\130X
\ xdd	dd Express 2 Hexadecimal numbers . Such as ： \x30 0

Three letter words such as "??) It will be interpreted as ]

Simple operators

type	Specifically
Arithmetic operators	+ - * / %
Shift operator	>> <<
Bit operators	&( Bitwise AND ) 、 ^( Press bit or ) 、 l( Bitwise XOR )
Assignment operator	=、+=、-=、*=、/=、&=、^=、l=、>>=、<<=
Monocular operators	【 See the table below 】
Relational operator	>、>=、<、<=、!=( The tests are not equal )、==( Test equality )
Logical operators	&&( Logic and )、ll( Logic or )
Conditional operators	exp( expression )1？exp2：exp3
Comma expression	exp1,exp2,exp3,…,expN
Other operators	[] Subscript reference 、() Function call 、. Structural members 、-> Structural members

Monocular operators	paraphrase
!	Logical anti operation
-	negative
+	Comes at a time
&	Address fetch
sizeof	The type length of the operands （ In two bytes ）
~	To reverse the binary of a number
- -	In front of - -、 After - -
++	In front of ++、 After ++
*	Indirect access operators （ Dereference operator ）
( type )	Cast

1、 The arithmetic operation needs to be programmed according to the floating-point precision ：

float a = 9 / 2.0;  // First float The premise for decimal calculation , Secondly, when both sides of the division sign are integers, the result is an integer , When a floating-point precision bit appears , The result will float to this bit .
printf("%f\n",a);

2、% The meaning of the arithmetic operator of is “ modulus ”, namely “ Remainder ”.

int a = 9 % 2;
printf("%d\n",a);   // The result displayed on the screen is 1

3、+= This kind of meaning is ：

a＝10;
a = a + 1;
//  Can be written  a += 1
a = a % 3;
//  Can be written  a %= 3

4、a + b here + It's a binocular operator , Empathy , Unary operator means that there is only one operand .

5、 The size of an element is 4 byte .

int arr[10] = {
    0};
printf("%d\n",sizeof(arr)); // The total size of the array is calculated , Unit is byte , The result is 40 //10( Elements )*4( Unit byte )=40
printf("%d\n",sizeof(arr[0])) // The result is 4

6、~ Bitwise negation means that all the digits in the binary bits are 1 become 0,0 become 1.

int main()
{
    
int a = 0;
printf("%d\n",~a);
return 0;
}    // The output is -1
// 0  The binary of is ：00000000000000000000000000000000
//~0  The binary of is ：1111111111111111111111111111111111111

7、++ Usage of

int main()
{
    
int a = 10;
int b = ++a; // In front of ++, First ++, After use 
printf（“%d\n”,b);    //  The result is 11
printf("%d\n",a);   // The result is 11
return 0;
}

int main()
{
    
int a = 10;
int b = a++; // After ++, First use , after ++
printf（“%d\n”,b);    //  The result is 10
printf("%d\n",a);   // The result is 11
return 0;
}

8、 Cast

int main()
{
    
int a = (int) 3.14; // This is a different type , Use (int) Coercive transformation 
printf("%d\n",a);
return 0;
}

9、 The conditional operator is also called the trinocular operator
exp1？exp2：exp3
exp1 establish ,exp2 Calculation , The result of the entire expression is exp2 Result .
exp1 Don't set up ,exp3 Calculation , The result of the entire expression is exp3 Result .

10、 Comma expressions are evaluated from left to right , The result of the entire expression is the result of the last expression .

Little tips

• Why? VC6.0 When a written program cannot run , Show this file does not exist ？
  One 、 open VC6.0 after , When creating a new file , You need to pay attention to the files you create No “C Header File” Format , It is “C source File” Format .
  Two 、 Add a file name to your file and the location where the file is stored , At this point, you can see that it is “c or cpp” For the suffix The suffix file of , and Not in “txt or h” For the suffix The suffix file of .
  3、 ... and 、 Ensure that the code format is correct .
  Four 、 When you run the program , If prompted “ Unable to execute program ”, Press CTRL Add F7 Composite key , Build , Press the shortcut key again CTRL Add F5 Composite key , Finally according to the Y, That is, the program can be executed successfully .

• The binary representation of an integer has 3 Kind of ： Original code 、 Inverse code 、 Complement code .
  Integers are stored in memory as complements .
  The way negative numbers are calculated （ Original code of positive integer 、 Inverse code 、 The complement is the same ）：-1 For example ：
  10000000000000000000000000001（ Original code ）
  11111111111111111111111111110（ Inverse code ）
  11111111111111111111111111111（ Inverse code +1 For the complement ）

• printf When printing data , Format can be specified .

printf("%d",100);
printf("%c",'a');
printf("%s","abc");

• ASCII surface （ Character base conversion ）
• int Table ID ;sacnf Table input
  
  When the above figure is converted to function （ Function as called module , Very important in complex code ） When expressing ：

#include<stdio.h>;
Add(int x,int y)
{
    
	int z = 0;
	z = x + y;
	return z;
}                 // And the function 
int main()
{
    
	int num1 = 0; // Identification number 1
	int num2 = 0; // Identification number 2
	scanf("%d%d", &num1, &num2); // Input   Numbers 1  Numbers 2 
    int sum = Add(num1,num2); // Identify usage and functions 
	printf("%d\n",sum); // Print out and 
	return 0;
}

subject

What does the following program output ？

 #include<stdio.h>;
 int main()
 {
    
 printf("%d\n",strlen("abcdef"));
 // \32 Parsed into an escape character 
 printf（“%d\n”,strlen("c:\text\328\text.c"));
 return 0;
 }

answer

Output ：
6（“abcdef” It's six , All lengths are 6）
14（ Escape character , Having a length 14 The bit characters are ：‘c’、’:’、’\t’、‘e’、‘x’、‘t’、’\32’、‘8’、’\t’、‘e’、‘x’、‘t’、’.’、‘c’）