Embedded-c Language-2

One 、 about C Operators and expressions of language

1. Operator

1.1 Arithmetic operator : Add , reduce , ride , except , Remainder , Corresponding symbol :+,-,*,/,%

   <1>. If the two numbers involved in the division calculation are integer numbers, the calculation result is only Keep the integer part

       for example ：5/2=2  

   <2>./ and % Can't be used on integers 0 To operate , Otherwise, the program crashes

       for example ：5/0, 5%0

   <3>.% Floating point numbers cannot be used

       for example ：5%1.1, Otherwise, the program crashes

   <4>.% Result It is consistent with the number symbol on the left

       for example ：-7%2=-1 7%-2=1

   <5>./ If The divisor is a floating point number , Finally get inf infinity

       for example ：5/0.0

1.2 Assignment operator ：=, Is to give the value on the right to the variable on the left , That is to change the memory number corresponding to the variable

     form 1:

    int a;

    a = 10

     form 2：

    int a, b, c;

    a = b = c = 10;

    printf("%d %d %\n", a, b, c); //10 10 10

     form 3： Composite operators : Assignment operators are used in conjunction with other operators

     for example ：

    a += b;  // Equivalent to a = a + b;

    a -= b; // Equivalent to a = a - b;

    a *= b; // Equivalent to a = a * b;

    a /= b; // Equivalent to a = a/b;

    a %=b; // Equivalent to a = a%b;

   

     Pay attention to the realization of ： Cannot give constant ( Unchangeable numbers ) And expression assignment ：

    100 = 200; //gcc Report errors

    100 = a; //gcc Report errors

    a+b = c; //gcc Report errors

1.3 Self - increment operator (++) And the autodecrement operator (--)

<1>. Definition

    The auto increment operator is to add the memory value corresponding to the variable 1

    The self subtraction operator is to subtract the memory value corresponding to the variable 1

<2>. Four forms ：

    front ++: First, add... To the value of the variable 1, Calculate the value of the expression after

    for example ：

   int a = 1;

   int b = 0;

   b = ++a;

   printf("a = %d, b = %d\n", a, b); //a = 2, b = 2

    after ++: First calculate the value of the expression , After that, add... To the value of the variable 1

    for example ：

   int a = 1;

   int b = 0;

   b = a++;

   printf("a = %d, b = %d\n", a, b); //a = 2, b = 1

    front --: First, subtract the value of the variable 1, Calculate the value of the expression after

    for example ：

   int a = 2;

   int b = 0;

   b = --a;

   printf("a = %d, b = %d\n", a, b); //a = 1, b = 1

    after --: First calculate the value of the expression , Then subtract the value of the variable 1

    for example ：

   int a = 2;

   int b = 0;

   b = a--;

   printf("a = %d, b = %d\n", a, b); //a = 1, b = 2

<3>. Do not add or subtract constants

    100++; // Report errors

    ++100; // Report errors

1.4 Relational operator ：

    == be equal to /!= It's not equal to /> Greater than /< Less than />= Greater than or equal to /<= Less than or equal to

    Be careful ：

   <1>. The result of a relational operator is an integer ：1( really ) perhaps 0( false )

   <2>. Do not make the following logical judgments ( Continuous logical judgment )

        for example ：8<7<5, The result is always 1( really )

        Because first 8<7, The result is 0, and 0 There is less than 5, The result is 1

1.5 Logical operators

 <1>. clear ： What is true in a computer is false 0( Include 1),0 It's fake

    Logical operator action ： Realize logical judgment in computer program

              for example ： When the user name and password are correct , To log in

       The login method can be wechat or mobile phone number, and the latter can be Tiktok number

 <2>. Logical operator types ： Three types of

   1. Logic and : &&( also , At the same time )

   2. Logic or : ||( Or it means )

   3. Logic is not : !( The meaning of doing opposite )

 <3>. Logic and && Operator characteristics

    Use the syntax ：C= expression A && expression B

    semantics ： Only when A and B It's all true ( Not 0), Entire expression C The value of is true

              As long as one is false , Entire expression C All values are false

    for example ：1 && 1; // really

             0 && 1; // false

             1 && 0; // false

             0 && 0; // false

 <4>. Logic or || Operator characteristics

     Use the syntax ：C= expression A || expression B

     semantics ： as long as A and B One of them is true , Entire expression C It's all true

               Only when A and B It's all fake , The result can be false

     for example ：0 || 0; // false

              1 || 0; // really

              0 || 1; // really

              1 || 1; // really

 <5>. Characteristics of logical non operators

    Use the syntax ：! expression A

    semantics ：A False result is true ,A True results are false

    for example ：

           ！1;// The result is false

            !0;// The result is true

 <6>. Bear in mind ： Short-circuit operation ( The written examination questions are required )        

    form 1.A && B: If A The value of is false , be B Don't deal with ,B The code does not execute

    for example ：

   int a = 1;

   0 && ++a;

   printf("a = %d\n", a); //a=1

   

   int b = 1;

   250 && ++b;

   printf("b = %d\n", b); //b=2

    form 2.A || B:   If A The value of is true , be B Don't deal with ,B The code does not execute

    for example ：

   int a = 1;

   0 || ++a;

   printf("a = %d\n", a); //a=2

  

   int b = 1;

   250 || ++b;

   printf("b = %d\n", b); //b=1

1.6 position (bit) Operator

<1>. All the data in the computer ( Numbers ) All in memory , And they are stored in binary form (1 perhaps 0,bit position ) and 8,10,16 Base is just an artificial friendly representation of binary numbers ( To show )

<2>. Bitwise operators work ： Bit operations are operations on binary numbers in memory , also C Language specifically provides corresponding operators , Abbreviated as bitwise operator

<3>. There are four forms of bitwise operators ：

    Bit and ：&( clear 0)

    Bit or : |( Set up 1)

    Bit exclusive or : ^( reverse )

    It's the opposite : ~( Take the opposite )

<4>. Bit and operator characteristic ：

grammar ：C = A & B;

     for example ：  

    A:    01011010    0x5a

    B:    11100011    0xe3

    &-------------------

    C:    01000010    0x42

law ： Any number of heels 0 Do position and , The result is 0, Any number of heels 1 Do position and , Keep the original value

Remember the application ： It is generally used to assign a certain number to bit Positional clarity 0, And keep the other bits unchanged

     for example ： take 0x5a The second of this number 3 Positional clarity 0: 0x5a & 0xf7

       76543210 Binary number

    A：0x5a  01011010

    B :  0xf7  11110111

    C:   0x52 01010010 

    

     for example ： take 0x5a The second of this number 3 Position and number 4 Positional clarity 0: 0x5a & 0xE7

       76543210 Binary number

    A：0x5a  01011010

    B :  0xE7  11100111

    C:   0x42  01000010

Bit or operator characteristic

Grammar format ：C = A | B;

     for example ：

    A:    01011010    0x5a

    B:    11100011    0xe3

    |-------------------

    C:    11111011   0xfb

law ： Any number of heels 1 Do a bit or , The result is 1, Any number of heels 0 Do a bit or , Keep the original value

Remember the application ： It is generally used to assign a certain number to bit Location 1, And keep the other bits unchanged

    for example ： take 0x5a The second of this number 2 Location 1: 0x5a | 0x04

        76543210 Binary number

    A：0x5a   01011010

    B :  0x04  00000100

    C:   0x5e  01011110   

    for example ： take 0x5a The second of this number 1,2 Location 1: 0x5a | 0x06

        76543210 Binary number

    A：0x5a   01011010

    B :  0x06  00000110

    C:   0x5e  01011110

Bitwise exclusive or operation characteristic :^

1. grammar ：C = A ^ B;

    for example :

    A:    01011010    0x5a

    B:    11100011    0xe3

    ^-------------------

    C:    10111001   0xb9

2. law ： Same as 0, Different for 1, It is used in the case of inversion

Bit inverse operator characteristic :~

1. grammar ：C=~A;

    for example ：

    A:    01011010    0x5a

   ~A：10100101    0xa5

2. law ：0 change 1,1 change 0

3. Be careful ： It's the opposite (~) The operators are general and bitwise AND (&) Combined to achieve a certain number of bit clear 0 operation

for example ：

1.7 Shift Operators

<1>. function ： Is to move the binary numbers left or right n A place ( Shift left for short , Move right )

<2>. Shift operator types ： Two kinds of

    Move left ：A<<B

    semantics ： take A Move left B A place

    Move right :  A>>B

    semantics ： take A Move right B A place

    for example ：

   1 << 3: take 1 Move left 3 position

   3 >> 1: take 3 Move right 1 position

<3>. Shift operator features ：

   1. After moving to the left, the empty data on the right is used 0 Fill in

       for example ：A The data premise is char type

      A=0x5A = 01011010

      A << 2  =  01011010  << 2 The result is ： 01101000 = 0x68

   2. When an unsigned number is shifted to the right, the data left blank is used 0 Fill in

       for example ：01011010 >> 2 The result is ：00010110 = 0x16

   3. When a signed number is shifted to the right, the data left blank is filled with sign bits

       for example :10100101( Premise is char type ) >> 2 The result is ： 11101001  = 0xE9

   4. Move left n Bits are equal to times 2 Of n Power

   5. Move right n Bits are equal to dividing by 2 Of n Power

   6. If the program involves multiplication or division in the future 2 Of n Secondary operation , Be sure to move left or right * perhaps / Of The execution efficiency is quite low ！

       for example ：

      int a = 1;

      int b = a * 4; // Junk code

      int b = a << 2; // High salary code

   3. Whether it's a bitwise operator (&,|,^,~) Or move left , Moving them to the right does not change the value of the variable itself

      for example ：

     int a = 3;

     int b = a << 1;

     printf("a = %d, b = %d\n", a, b); //a = 3, b = 6

1.8 Fetch address operator & Dereference operator *

<1>. Specify address characteristics ： The address in the computer is determined by 32 Bit binary number composition , It's an address 32 position ,4 byte

<2>. Fetch address operator & effect ： Get the first address of a variable in memory , Place holder %p Used to display address information

<3>. Fetch address operator & Use grammar format ：& Variable name ;

    for example ：

   int a  = 250; // Distribute 4 Byte memory space and put a 250 Numbers

   printf(" Variable a The first address of is %p\n", &a);

<4>. Dereference operator * effect ： Get the data in memory according to the first address of the variable

   Or write new data to the memory according to the first address of the variable

<5>. Dereference operator * Grammar is ：* Address     As a result, the memory can be manipulated

    for example ：

   int a = 250;

   printf("a The first address %p\n", &a);

   printf("a The value of is %d\n", *&a); // Get the value of a variable by address

   *&a = 520; // According to the variable a The address of a Of memory , Then inward   Save and write a new number 520

1.9 Conditional operator ( Also known as the trinocular operator )

a) Grammar format ： The whole expression result D = Conditional expression A ? Expression B : expression C

    semantics ： If A It's true ,D= expression B The result of the operation , otherwise D= expression C The result of the operation

    for example ：

   int a = 1 ? 2 : 3; //a=2

1.10 Pay attention to the precedence of the operator

         for example ：

        printf("%d\n", 2+3*2); //8

        printf("%d\n", (2+3)*2); //10

         Conclusion ：() It has the highest priority , Not too much

         for example ：

        int a;

        a =  1 > 2  && 2 < 3;

         Equivalent to ：

        a = (1 > 2) && (2 << 3);