c language bits and pieces

C language is a general computer programming language, which is widely used in Bottom development. The design goal of C language is to provide a programming language that can compile and process low-level memory in a simple way, generate a small amount of machine code and run without any running environment support. Although C language provides many low-level processing functions, it still maintains good cross platform characteristics. C language programs written in a standard specification can be compiled on many computer platforms, even including some embedded processors (single chip microcomputer or MCU) and supercomputers. In the 1980s, in order to avoid differences in C language syntax used by various developers, the American National Bureau of standards formulated a complete set of American national standard syntax for C language, called ANSI C, as the original standard of C language. [1] At present, the C11 standard issued by the international organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) on December 8, 2011 is the third official standard of C language and the latest standard of C language. The standard better supports Chinese function name and Chinese identifier, and realizes Chinese character programming to a certain extent. C language is a process oriented computer programming language, which is different from object-oriented programming languages such as C + +, Java and so on. Its compilers mainly include Clang, GCC, WIN-TC, SUBLIME, MSVC, Turbo C, etc.

2, Common keywords

auto break case char const continue default do

double else enum extern float for goto if

int long register return short signed sizeof static

struct switch typedef union unsigned void volatile while

1. Keyword typedef

typedef, as the name suggests, is a type definition, which should be understood here as type renaming.

//Rename unsigned int uint_32, so uint_32 is also a type name
typedef unsigned int uint_32;
int main()
{
    //Look at num1 and num2. These two variables are of the same type
    unsigned int num1 = 0;
    uint_32 num2 = 0;
    return 0;
}

2. Keyword register

Register serves as a recommendation. It is recommended to put the value of num in the register, which is finally determined by the compiler

int main()
{
	register int num = 10;
	return 0;
}

Simply record how the CPU processor works

3, Data type

char short int long

long long float double

When using char type, you need to use '' when the object is a single character and '' when the object is a string

Eg:1.'w' 2."linqinyan"

#include <stdio.h>
//sizeof is an operator that calculates the size of types and variables
int main()
{
    printf("%d\n", sizeof(char));
    printf("%d\n", sizeof(short));
    printf("%d\n", sizeof(int));
    printf("%d\n", sizeof(long));
    printf("%d\n", sizeof(long long));
    printf("%d\n", sizeof(float));
    printf("%d\n", sizeof(double));
    printf("%d\n", sizeof(long double));
    return 0;
}

IV. ASCII code table

5, Variable type

1. Global variables

If a global variable is not initialized, it will be initialized to 0 by default

2. Local variables

If a local variable is not initialized, it defaults to a random value

#include <stdio.h>
int global = 2019;//global variable
int main()
{
    int local = 2018;//local variable
    //Is there a problem with the global defined below?
    int global = 2020;//local variable
    printf("global = %d\n", global);
    return 0;
}

Ext can also introduce variables in header files or other cpp sub files

extern int g_val;   //Declare external symbols, such as those in the header file
int main()
{
	printf("%d\n", g_val);
	return 0;
}

Corresponding header file

#define _CRT_SECURE_NO_WARNINGS 1
int g_val = 2022;//global variable

6, Constants in C language

1. Literal constant

int main()
{
    3.14;//Literal constant
    1000;//Literal constant
    return 0;
}

2.const modified constant

const int n = 10;
printf("n=%d\n", n);//10
n = 20;
printf("n=%d\n", n);//20
const int n = 10;//Here n has a constant property, but it is essentially a variable

3.# Identifier constant defined by define

#define MAX 100
int main()
{
	//MAX = 101;//err error reporting
	int m = MAX;
	printf("%d\n", m);
	printf("%d\n", MAX);
	return 0;
}

4. Enumeration constants

Enumerating constants are enumerations one by one. They are a custom type method provided by C language
eg: Gender: male, female, three primary colors: red green blue

enum Sex
{
	MALE,//The contents are sorted from 0, 0 1 2 3 4 5 6
	FEMALE,
	SECRET
};
enum COLOR
{
	RED,
	GREEN,
	BLUE
};
int main()
{
	//MALE = 5;//err
	enum Sex s = MALE;
	printf("%d\n", MALE);    //0
	printf("%d\n", FEMALE);  //1
	printf("%d\n", SECRET);  //2
	return 0;
}

7, Escape character

Suppose we want to print a directory on the screen: C: \ code \ test c

How do we write code? Is it like this?

#include <stdio.h>
int main()
{
    printf("c:\code\test.c\n");
    return 0;
}

In fact, the result of the program is as follows:

It's different from what we expected.

I have to mention the escape character here.

Escape character table

Escape character	interpretation
\?	Used when writing consecutive question marks to 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 represent a backslash to prevent it from being interpreted as an escape sequence character.
\a	Warning character, beep
\b	Backspace character
\f	Feed character
\n	Line feed
\r	enter
\t	Horizontal tab
\v	vertical tab
\ddd	ddd represents 1 ~ 3 octal digits. For example: \ 130 X
\xdd	dd represents 2 hexadecimal digits. For example: \ x30

Program verification:

int main()
{
	printf("c:\\test\\test.c");
	printf("abcndef");
	printf("abc\ndef");
	//Three letter word:?) Will be transformed into]
	printf("(are you ok\?\?)");//(are you ok]
	printf("%c\n", 'a');
	printf("%c\n", '\'');
	printf("%s\n", "abc");
	printf("%s\n", "a");
	printf("%s\n", "\"");
	printf("\a\a\a\a\a");
	printf("%c\n", '\135');
	       //The number converted to decimal is 64 + 24 + 5 = 93 
	printf("%c\n", '\x42');
	return 0;
}

What will the following program output?

#include <stdio.h>
int main()
{
    printf("%d\n", strlen("abcdef"));                //6
    // \62 is parsed into an escape character
    printf("%d\n", strlen("c:\test\628\test.c"));    //14
    return 0;
}

VIII. Comma expression

int main()
{
	int a = 1;
	int b = 3;
	int c = 4;
	int d = (a = b - 3, b = a * 2, c = a - b);  //Comma expression d takes the last value
	printf("%d %d %d %d\n", a,b,c,d);
	return 0;
}

9, The essence of scanf

int main()
{
    int iq = 0;
    while (scanf("%d", &iq) != EOF)
    {
        if (iq >= 140)
            printf("Genius\n");
    }
    return 0;
}

int main()
{
    int iq = 0;
    while(scanf("%d", &iq) == 1)
    {
        if(iq>=140)
            printf("Genius\n");
    }
    return 0;
}

10, Static - static

1. static can modify local variables

#include <stdio.h>
void test()
{
	static int n = 1;
	n++;
	printf("%d ", n);
}
int main()
{
	int i = 0;
	while (i < 10)
	{
		test();
		i++;
	}

	return 0;
}

2. static can modify global variables

extern int g_val;
int main()
{
	printf("%d\n", g_val);
	return 0;
}

Header file

static int g_val = 2022;

3. static can modify functions

extern int Add(int, int);   //Declare functions from outside
int main()
{
	int a = 0;
	int b = 0;
	scanf("%d %d", &a, &b);
	int sum = Add(a, b);
	printf("%d\n", sum);

	return 0;
}