2022_ 2_ 16 the second day of learning C language_ Constant, variable

c Language standards

sizeof(long) >= sizeof(int)

Constant 、 Variable

Some values in life are constant （ such as ： PI , Gender , Id card number , Blood type, etc ） Constant

Some values are variable （ such as ： Age , weight , Salary, etc ） Variable

Constant value ,C The concept of constant is used to express , Variable value C In languages, variables are used to represent

How to define variables ：

int age=150;

float weight=45.5f;

char ch=“w”;

Classification of variables

local variable

Global variables

int main()

{

// It means age

//20

short age = 20;// Request two bytes from memory =16 individual bit position , For storage 20

float weighe = 95.6f;// Apply to memory 4 Bytes =32bit position , Store decimals

return 0;

}

int num1 = 20;// Global variables - Defined in the code block （{}） Other variables

int main()

{

int num2 = 10;// local variable - Defined in the code block （{}） Internal variables

return 0;

}

int a = 100;

int main()

{

int a = 10;

// It is recommended that the names of local variables and global variables should not be the same - Easy to misunderstand -bug

// When local and global variables have the same name , Local variables take precedence

printf("%d\n", a);

return 0;

}

Use of variables

// Use of variables

int main()

{

// Calculate the sum of two numbers

int num1 = 0;

int num2 = 2;

int sum = 0;

// input data - Using input functions scanf

scanf("%d%d", &num1, &num2);// Take the address symbol &

//C Phonetic grammar regulations , The variable should be defined at the front of the current code block

sum=num1 + num2;

printf("sum=%d\n", sum);

return 0;

}

The scope and life cycle of variables

Scope ：

Scope （scope） Programming concept , Generally speaking , The names used to summarize a piece of program code are not always valid / Usable

The scope of the code that defines the usability of the name is the scope of the name .

1、 The action of a local variable on the local range of the variable .

2、 The scope of the global variable is the scope of the whole project .

// The scope of a local variable

int main()

{

int num = 0;

{printf("num=%d\n",num); }

return 0;

}

// Scope of global variables

int global = 2020; 

void test()

{

printf("test()--%d\n", global);

}

int main()

{

test();

printf("%d\n", global); 

return 0;

}

int main()

{

// undeclared identifier

// Make a statement extern External symbol

extern int g_val;

printf("g_val = %d\n", g_val);

return 0; 

}

Life cycle ：

The life cycle of a variable is the period between the creation of a variable and its destruction

1、 The life cycle of a local variable is ： Enter the scope lifecycle begins , Out of scope life cycle ends .

2、 The life cycle of global variables is ： The whole life cycle of the program

// Life cycle ：

int main()// Local life cycle

{

{

int a = 10;

printf("a=%d\n", a);//OK

}

printf("a=%d\n", a);//error

return 0;

}

// The life cycle of global variables =main The life cycle of a function = The whole life cycle of the program

Finally, I finished the first video of brother Peng ~ It doesn't feel particularly difficult to understand in the early stage , Each line is typed by itself , Keep going ！

Recent summary ：

C Speech is developed from binary language to assembly language to mnemonic B Language , Then derived C Language . Make the language easier to use , Produced C The standard of language （ANSI C）

main function -printf Need a lead file #include<stdio.h>- data type （7 Kind of ）- Variable （ Types are used to create variables ）- local variable / Global variables - The life cycle and scope of variables

Looking forward to the next issue ......