C language learning notes

data

Constant

Big classification

1. Literal constants : Intuitively written values
2. const Modifier hair constant
3. #define Defined constant
4. Enumeration constants

Constant ,const Constant variable

int main()
{
    const int num = 1;\\const  Indicates the creation of constant attribute variables 
    num = 8;
    return 0;
}
​

Can't run

int main()
{
    const int n = 4;
    printf("%d\n", n);
    int arr[n] = { 0 };
    return 0;
}
​
int main()
{
    const const int n = 4;
    printf("%d\n", n);
    int arr[n] = { 0 };
    return 0;
}

It doesn't work , const int n Express n Is a variable with constant properties , Is not a constant .

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#define MAX 10
int main()
{
    int arr[MAX] = { 0 };
    return 0;
}

This can run , because #define Defines a standard constant

Enumeration constants

enum Sex\\ Enumeration constants 
{
    MALE,
    FEMALE,
    SECRET
};// Possible values of enumeration constants ,  The enumeration constants given in braces cannot be changed ,  Enumerating variables can be changed 
int main()
{
    printf("%d",MALE);//0
    printf("%d",FEMALE);//1
    printf("%d",SECRET);//2
}

character string + Escape character + notes

character string

Multiple characters in double quotation marks are strings  "hello bit\n"

int main()
{
"abcdef";// character string 
"";// An empty string 
return 0;
}

---

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
    char arr1[] = "abc";
    //"abc"--'a','b','c','\0' =>'\0' Indicates the end of a string ,'\0' The value of is 0
    //char arr2[] = { 'a','b','c' };'c' Should be followed by a 0
    char arr2[]={'a','b','c',0};
    printf("%s\n", arr1);
    printf("%s\n", arr2);
    printf("%d\n", strlen(arr1));
    printf("%d\n", strlen(arr2));// Print string length ,3
    return 0;
}

The data stored on the computer is binary , The characters are written with ASCII Code storage .

Escape character \

 \n Change the original meaning , Make a character no longer the original meaning , Means line break

#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
int main()
{
    printf("abc\n");
    return 0;
}

\t It's called horizontal tab , similar Tab key

notes

c Notes /* */ c++ Notes //`  Both can be used.   Comments are used where the code is difficult to understand

Input and output

Format characters

Format symbol	effect
i, d	Decimal integer
x, X	Hexadecimal unsigned integer
o	Octal unsigned integer
u	An unsigned decimal integer
c	Single character
s	character string
e	Floating point decimal in exponential form
f	Decimal form floating point decimal

#include<stdio.h>
int main()
{
    float f=3.1415926;
    printf("%2.3f\n",f);
    printf("%f\n",f);
    return 0;
}

among %2.3f2 Representation format ( If it is exceeded, it will be ignored ), 3 It means to reserve the number of decimal places ( rounding )

Modifier	function
m	The output data field is wide , Data length <m, Left padding ; Otherwise, press the actual output
.n	For real numbers , Specify the number of digits after the decimal point （ rounding ）
The string , Specify the actual output bits
-	The output data is left aligned in the field （ Default right alignment )
+	Specifies that a positive sign is displayed before the positive number of signed numbers (+)
0	When the output value is specified, the empty position not used on the left side will be filled in automatically 0
#	Show leading before octal and hexadecimal numbers 0,0x
l	stay d, o, x, u front , Specify the output precision as long type
l	stay e, f, g front , Specify the output precision as double type

Character input function

putchar()

scanf()  If you need a prompt statement , Write a separate line printf, Can not be written in scanf in

Garbage left by input function

That is, enter an integer value and then press enter to enter another character , It will turn carriage return into input . The methods of clearing are

1.  use getchar() eat 
1. %[ Space ]c or %*c

String input function

1. gets(s)
2. scanf() Spaces will be used as the end flag of string input ( When dealing with continuous character input, you can deal with spaces )

String output function

puts

It will wrap itself

Operator

Arithmetic operator

Add reduce ride except (/) Remainder (%) The remainder cannot be given to floating-point numbers .

Relational operator

>``>=``<=``<

==``!=

Logical operators

1. ! Take the opposite : 1 change 0,0 change 1
2. || or

An operator

For binary bits (0/1)

Bit operations can be performed with any data type

Operator	Functional specifications
~	Bit logic inverse	Convert to binary , then 0 1 swap
&	Bit logic and
|	Bit logic or
^	Bit logic or	Difference is true , Same as false
>>	Shift right
<<	Shift left	Multiply by two

Special operators

Assignment operator

 

Conditional operator ?

The comma operator

z=(x+=5,y=x+0.2)

z The value of is determined by the last expression

Operator precedence

It is suggested to add more brackets to solve the priority problem

#include<stdio.h>
int main()
{
	int x = 1, y = 0, z = 0;
	x += y == z,y = x + 2, z = x + y + x > 0;
	printf("%d,%d,%d\n", x, y, z);
	return 0;
}

Add

Dereference operator * The priority of is less than the subscript operator [], Therefore, the array pointer needs to be bracketed, such as int (*p)[10]

Array

One dimensional array

C The language will not judge the array beyond the bounds , You need to limit yourself

1. If the array is not initialized , Values are random .
2. static Array elements are not assigned , The default is 0
3. Assign only partial values , Default after 0
4. If there is too much initialization, an error will be reported directly

Example : Bubble sort

#include<stdio.h>
int main(int argc, char const *argv[])
{
	int a[7]={49,38,97,76,13,27,30};
		int i = 0,j=6,temp;
	for (;j>=0;j--)
	{
		for(i=0;i<=j;i++)
			{
				if(a[i]>a[i+1])
				{
					temp = a[i];
					a[i] = a[i+1];
					a[i+1] = temp;
				}
			}
			
	}
	for(i=0;i<=6;i++)
		{
			printf("%d\n",a[i]);
		}

	return 0;
}

Two dimensional array

P[ That's ok ][ Column ]

Define the way : The number of rows can be omitted , The number of columns cannot

Two dimensional array storage

Verification of two-dimensional array storage

#include<stdio.h>
int main(int argc, char const *argv[])
{
	int a[4][5]={
   {1,2,3,4,5},{6,7,8,9,10},{11,12,13,14,15},{16,17,18,19,20}};
	for(int x = 0; x < 4; x++)
	{
		for(int y = 0; y < 5; y++)
		{
			printf("The id of a[%d][%d] is %p\n",x,y,&a[x][y] );
		}
	}
	return 0;
}

Multidimensional arrays

Example : Print the first ten lines of Yang Hui triangle

#include<stdio.h>

int main(int argc, char const *argv[])
{
	int a[10][10];
	for(int x = 0; x < 10; x++)
		{
			for(int y = 0; y < 10; y++)
			{
				if(y == 0 ||x == y)
					a[x][y]=1;

			}
		}
	for(int x = 2; x < 10; x++)
	{
		for(int y = 1; y < x; y++)
		{
			a[x][y]=a[x-1][y-1]+a[x-1][y];
		}

	}
	for(int x = 0; x < 10; x++)
	{
		for(int y = 0; y <= x; y++)
		{
			printf("%d ",a[x][y]);
		}
		printf("\n");
	}
	return 0;
}

Character arrays and strings

C In language , The string is attached to the character array

initialization

1. Assign values one by one
2. String assignment  "abc" ->a, b, c, \0
   0 \0 NULL It's the same thing , ASCII The code value is 0

, '0' It's another thing , ASCII The code value is 48

String array under two-dimensional array

#include<stdio.h>
int main()
{
	char fruit[][20] = { "bannana","apple","strawmerry","watermelen" };
	int i, j, n, m;
	n = sizeof(fruit) / sizeof(fruit[0]);
	m = sizeof(fruit[0]) / sizeof(fruit[0][0]);

	for (i = 0; i < n; i++)
	{
		for (j = 0; j < m; j++)
		{
			putchar(fruit[i][j]);
		}
		putchar('\n');
	}
	return 0;
}

sizeof() What you get is total space , Include unassigned parts , strlen What you get is the actual string length ( need string.h>)

String function

Common string functions  <string.h>

strlen Find the length

Calculate the actual length of a string ( It doesn't contain \0), The number of bytes returned , Often a char It's just a byte , Therefore, it can be regarded as length

char s[10] = { 'A', 0, 'b', '\0', '0' };
printf("%zd", strlen(s));// This returns 1,  Because I met 0,  It's the end of the string 
printf("%d", sizeof(s)/sizeof(char));// This returns 10,  Represents the entire array 

When special characters are encountered :

char s[20] = "\t\t\nasdczx0\0qwe";
printf("%zd\n", strlen(s));// return 10  \t,\n Each is treated as a character 
printf("%d", sizeof(s)/sizeof(char));// return 20
char s[20] = "\x69\141\n"; // \xhh Represents the symbol represented by hexadecimal numbers , \ddd Represents the symbol represented by octal numbers   It is generally not recommended to write 
printf("%zd\n", strlen(s));

strcpy String copy

1. strcpy( A character array 1, A character array 2)
2. The string 2 Copy to string 1 In the middle , Returns an array of characters 1 The first address .( Must be a string )
3. A character array 1 It has to be big enough

strcat String concatenation function

1. strcpy( A character array 1, A character array 2)
2. The string 2 Connect to string 1 Back , Returns an array of characters 1 The first address .( Must be a string )
3. A character array 1 It has to be big enough
4. Before connection , Both are based on '\0' The end of the , After connection strand 1 Of '\0 Cancel , Add... At the end of the new string '\0'

strcmp String comparison function

strcmp( character string 1, character string 2)

1. Compare the rules : The two strings are compared from left to right ASCII Code value , if 1<2 Returns a negative integer , if 1==2 return 0, if 1>2 Returns a positive integer .
2. If the lengths are not equal , Empty positions are equivalent to and \0 Than ,

Be careful : The premise of using string function must be string or with '\0' Array of characters , Otherwise, unpredictable problems will occur .

Extended usage of string functions

strncpy(p,p1,n) Copy a string of specified length

strncat(p,p1,n) Connect a string of specified length

strcasecmp Ignore case comparison strings

strncmp(p, p1, n) Compare a specified length string

strchr(p, c) Find the specified character in a string

#include<stdio.h>

#include<string.h>
int main(int argc, char const* argv[])
{
	char s1[]="123aeqw$asdsa";
	int ch;

	ch='$';

	printf("%p,%p,\n\n",s1,strchr(s1,ch));	
	return 0;
}

strstr(p, p1)  Find string

#include<stdio.h>
#include<string.h>
int main(int argc, char const* argv[])
{
	char s1[]="how are you";
	char s2[]="are";
	printf("%ld\n",strstr(s1,s2)-s1);
	return 0;
}

Functions about characters , need #include<ctype.h>

isalpha()  Check for alphabetic characters

isupper() Check whether it is an uppercase character

islower()  Check for lowercase characters

isdigit()  Check if it's a number

The pointer

stay C In language , The address of a memory unit is called a pointer . Without affecting understanding , Address , The pointer , Pointer variables are not distinguished .

Pointer arithmetic

1. Only pointer to the same type
2. The number of separated data is obtained by subtracting the pointer , Not the address difference .

assignment

You can assign an address to a pointer .

Quoting

* Operator gives a pointer to the stored value on the address

address-of

Like all variables , Pointer variables also have their own addresses and values . & Operator gives the address of the pointer itself .

Add a pointer to an integer

Integers are multiplied by the size of the type pointed to by the pointer , Then add the result to the initial address .

Increment pointer

Incrementing the pointer to an array element moves the pointer to the next element of the array . therefore ptr1 Point to urn[1].

Pointer minus an integer

The pointer must be a minuend . The rest is similar to addition .

Decrement pointer

Similar increment , For arrays

Pointer difference

Pointers and arrays

The array name is a pointer to the first element of the array .

A pointer in a two-dimensional array

int a[3][2]={
   {1,6},{9,12},{61,12}};
int *p,i,n;

n=sizeof(a)/sizeof(int);
p=a;//warning

It's not right to write like this , a And p Types are different , One p Point to four bytes , But one a Point to 8 Bytes , It should be :

p=a[0]

in other words   If you need to traverse a two-dimensional array with a first-order pointer , need for loop

A two-dimensional array consists of multiple arrays ,

In the above code a Is the a[0] a[1] a[2]  Three arrays form .

A dereference of the two-dimensional array name will get a pointer to the number of rows .

Character pointer and string

If you directly point the pointer to the string

char *p="hello world";

The pointer stores the starting address of the string constant , Cannot modify with pointer Point to the value of the object . You cannot do this for arrays , This is also the difference between string and character array .

Pointer array

An array of pointer variables with the same storage type .

double *pa[2], a[2][3]

* Just one

Pointer arrays can be used for two-dimensional arrays .

 

Multi level pointer

void The pointer

 void * [ Pointer name ] 

It can be changed to other types of pointers by forcing type conversion

Display conversion

Implicit conversion

An explicit call refers to calling Implicit calls are compiler background calls

function

Definition and Usage

l A function is a code module that performs a specific function , Its program code is independent , Usually a return value is required , It can also be null . It is called function body in braces

l The general form is as follows :

< data type > < The name of the function >( < Formal parameter description > ) {

​       Statement sequence ;

​      return[(< expression >)];

} 

for example

double p(int x);

call

Function name ( The actual parameter )

Arguments are when using functions , The data passed by the calling function to the called function . Exact data is needed

A function call can appear in an expression as an operand , It can also form a single statement . For functions that have no return value , Only one function call statement can be formed .

Parameter passing (C The language does not quote )

Passing array parameters ( The string ) You need to pass the size of the array at the same time .

Copy the delivery method

The value of the argument is assigned to the formal parameter

1. The calling function passes the arguments to the called function , The called function will create formal parameters of the same type and initialize with arguments
2. Formal parameters are newly developed storage space , therefore , Change the value of formal parameter in function , It doesn't affect the arguments

Pointer function

Usage of pointer function

A pointer function is a function whose return value is a pointer .

The space in the function will be released at the end , This makes the pointer inaccessible , You can apply the variable in the function as static Variable extended probation .

Or use String constant :

char *str = "hello"

But it can't be modified *str

in general , The return of the pointer function can be :

1. The address of the global variable
2. static The address of the variable
3. The address of the string constant
4. Dynamic memory

Recursive functions and function pointers

Recursion is recursion plus regression

Example 1: Find the factorial

#include<stdio.h>
int calcu(int x);
int main()
{
	printf("%d\n",calcu(10));
	return 0;
}
int calcu(int x)
{
	if (x==0)
	{
		return 1;
	}
	double factorial = x;
	factorial*=calcu(x-1);
	return factorial;
}

Example 2: Fiboracci series

int main()
{
	for(int num = 1; num <= 10; num++)
	{
		printf("%d\n",fib(num));
	}
	return 0;
}
int fib(int x)
{
	int fibx;
	if (x == 1 || x == 2)
	{
		return 1;
	}
	fibx = fib(x-1)+fib(x-2);
	return fibx;
}

A function pointer

The core of a function pointer is a pointer , Store the address of the function

The function address represents the entry address of the function ( Function name )

#include<stdio.h>
int add(int, int);

int main()
{
	int m = 10,n = 20;
	int (*p)(int ,int );
	p=add;
	printf("%d\n",p(m,n));
	return 0;
}
int add(int x,int y)
{
	return x+y;
}

Function pointers should be related to functions , Therefore, the return value type and parameter list should be consistent with the prototype of the function , Note the de quotation here  * The priority of the , Need to add ()

Function pointer array

The return type of the function pointer pointing to the function in the array is consistent with the parameter list .

Example : call qsort Sort

#include<stdio.h>
#include<stdlib.h>
int stol(const void *x, const void *y);

int main()
{
	int a[10] = {5.7,3,6,4,2,8,9,1};
	qsort(a,sizeof(a)/sizeof(int),sizeof(int),stol);
	for(int k = 0; k<= 9; k++)
	{
		printf("%d\n",a[k] );
	}
	return 0;
}
int stol(const void *x, const void *y)
{
	return *(int *)x - *(int *)y;
}

resources & More in-depth readings ：

Open the mysterious channel to get information, click the link to join the group chat 【C Language /C++ Programming learning base 】：828339809