Embedded-c Language-3

One 、 Conversion of data types

1. Implicit conversion

characteristic ： If different numbers in the expression have different data types ,gcc The compiler first converts different data types , Convert to the same data type before operation

Three cases of implicit conversion ：

<1>. During implicit conversion, you must Convert the type with small memory into the type with large memory

<2>. If there are both integer data types and floating-point data types ,gcc The compiler automatically converts integer data types to floating-point types

<3>. If there are both unsigned and signed data types ,gcc The compiler automatically converts signed to unsigned data types

Be careful ： The fatal flaw of implicit conversion is that the code is not readable

2. Coercive transformation

Cast syntax format ： Target type variable = ( The target type ) Source type variable ;   

    for example ：

   char a = 90;

   int b = (int)a; //gcc Force will a convert to int Types are assigned to variables b

Be careful ： Forcing data type conversion may cause data loss

     for example ：  

  char a = (char)300; // Data lost 
    printf("a = %d\n", a); //a=44 = 300( It's too big ) - 128 = 172( It's too big )-128=44   ( Roll back )

     therefore ： Force conversion is from small to large , Or equal turns

Bear in mind ： Either type conversion does not modify the value of the variable itself

       for example ：     

 int a = 555;
      char b = (char)a;
      printf("b = %d, a = %d\n"); //b = ? a = 555

Two 、C Process control of procedure

2.1 Branching structure ： Conditional branch and switch branch

Conditional branch ： Grammar format ：

form 1： semantics ： If the expression 1 It's true , Then execute the statement 1

   if ( expression 1) {

             sentence 1;

   }

form 2：  semantics ： If the expression 1 It's true , Then run the statement 1, Otherwise, run the statement 2

  if( expression 1) {
       sentence 1;
  }
  else {
      sentence 2;
  }  

form 3： semantics ： If the expression 1 It's true , Then execute the statement 1, If the expression 1 Not true , So execute the expression 2, If the expression 2 It's true , Then execute the statement 2, If the expression 2 Not true , Judge down in turn

if( expression 1) {
   sentence 1;
}   
else if( expression 2) {
   sentence 2;

}

...

else if( expression N) {
    sentence N;
}

form 4: semantics ： If the expression 1 It's true , Then execute the statement 1, If the expression 1 Not true , So execute the expression 2, If the expression 2 It's true , Then execute the statement 2, If the expression 2 Not true , Judge down in turn , If none of the above holds true , Last run statement M

if( expression 1) {
   sentence 1;
}   
else if( expression 2) {
   sentence 2;
}

...

else if( expression N) {
    sentence N;
}
else {
    sentence M;
}

2.2 Precautions for using conditional branches

   1.if and else Matching principle of ：else perhaps else if And recent if pairing

   2. If there is only one statement ,{} It can be omitted ( Sincerely suggest adding )

      if(1 == a) {                                 if (1 == a)

printf("a be equal to 1.\n");      Equivalent to ->         printf("a be equal to 1.\n");

      }

   3.{} Whether to monopolize the same problem ： According to the coding specification of the company

       principle ： As long as it is linux System software development ,{} Don't monopolize a line    

      //linux The programmer 
      if(1 == a) {
printf("a be equal to 1.\n");
      }
      // Company code specification 
      if(1 == a)
      {
printf("a be equal to 1.\n");
      }

Switch branch ： Grammar format ：

   switch( Control expression ) {
         case  Constant expression 1:
  sentence 1;
 break;
         case  Constant expression 2:
  sentence 2;
 break;

         ...

         case  Constant expression N:
  sentence N;
 break;
         default:
  sentence M;
 break;
   }  

    Execution process ： Control the value of the expression and the following case The value of the corresponding constant expression is the same ( equal )

    Then the corresponding statement will be executed , Once encountered break,switch...case The switch branch ends immediately

    If case It's not equal , Finally, only default The corresponding statement

Precautions for use of switch branch structure

The control expression is treated as an integer (int), Can be character , however Cannot be floating point numbers or strings ( for example ："abc")

    for example ：

   switch(5) // Sure

   switch(5.5) // Can not be

   switch("abc") // Can not be

   switch('a') // Sure

Constant expression must be constant , for example ：5,'A',2+5 etc. , Duplicate branches are not allowed case

  int cmd = 250;
    switch(cmd) {
case 250:
break;
case 250: //gcc Compiler error 
break;

    }

If one Some case Corresponding break Does not add , When this case When established , And after running its statement Will continue with the next case So it is recommended that if the code needs break, finish writing sth. case After the list , Write a first break come out , Prevent forgetting ！

 int cmd = 250;
   switch(cmd) {
case 250:
printf("1.\n");
case 251:
printf("2\n");
break;

...

default:

break;
    }

    result ： Print 1 and 2

default Keywords can be written in any case In front of , But you need to add break, If default Key words are written at the end ,break It can be omitted

Branch case perhaps default If you define variables in , Remember to add {}, In the future, this variable can only be given to the corresponding case Use ！

     for example ：

 int cmd = 250;
   switch(cmd) {
case 250:
printf("1.\n");
case 251:  {
char c = '2'; // Defining character variables 
printf("%c\n", c);
break;
}

...

default:
break;
    }

summary ：if...else and switch..case contrast ：

1.switch...case What can be done ,if...else Can do it

2.if...else What can be done ,switch...case Not necessarily able to do , For example, dealing with floating-point numbers or strings

3.switch...case Some occasions are extremely tedious

    for example ： Judge 0~1000 The scope of the , If you use switch...case, You must add 1000 strip case

    But if you use if sentence , Take one ：if(xxx > 0 && xxx < 1000)

4.gcc The compiler to switch...case The amount of code generated is greater than if...else Less , therefore switch Efficient code execution

3、 ... and 、 The loop structure of the three structures of a program

3.1. Loop structure function ： Repeat a set of statements multiple times

       There are three types of loops ：for loop ,while loop ,do...while loop

3.2.for loop

a) Grammar format ：

   for( expression 1;  expression 2;  expression 3) {
 Loop statement ;
   }

   semantics ( Execute the process )：

   The first step ： First calculate , Run the expression 1( Only once )

   The second step ： And then calculate , Run the expression 2 If the expression 2 The result is true , Then execute the run loop statement if the expression 2 The result is false , that for Loop end exit

  The third step ： If if the expression 2 The result is true , Then execute the run loop statement , And when the loop statement is executed , Then calculate , Run the expression 3, expression 3 Operation completed , Repeat the second step

b)for The use of recycling ：

form 1:( Be careful ： here i There are corresponding technical terms ： Loop variable )

int i;
for(i = 0; i < 5; i++) {
printf("i = %d\n", i);
}

form 2：

int i = 0;
for(; i < 5; i++) {
printf("i = %d\n", i);
}

form 3：

int i = 0;
for(; i < 5; ) {
printf("i = %d\n", i);
i++; // Loop variables are placed in loop statements 
}

form 4： Dead cycle ( Be careful ： Press ctrl+c Key exit program )

for(;;) {
      printf("hello,world\n");
}
int i = 0;
for(;;) {
printf("i = %d\n", i);
i++; // Loop variables are placed in loop statements 
if(i >= 5) {
     break; // interrupt , end for loop , If for Loops are nested , Just interrupt the innermost layer for loop 
}
}

form 5：

int i, j;
for(i = 0, j = 0;  i < 5 && j < 5; i++, j++) {
printf("i = %d, j = %d\n", i, j);
}

form 6：

for(int i = 0; i < 5; i++) {
printf("i = %d\n", i);
}
printf("i = %d\n", i); //gcc Report errors 

Be careful ： If the loop variable is defined in the expression 1 Finish in , that Loop variables can only be used for for loop for Not available outside the loop

form 7：

int i;
for(i = 0; i < 5; i++) {
if(3 == i) {
continue; 
// interrupt , End this cycle , So let's go to the next loop 
// When i=3 when , perform continue keyword ,
// The following loop statements are no longer executed , Execute the expression directly 3,i++
// Then repeat the next cycle 
}
printf("i = %d\n", i);
}

c)for If there is only one statement in a loop statement ,{} It can be omitted ( I don't recommend it )

   for(int i = 0; i < 5; i++)
printf("i = %d\n", i);

3.3.while loop

a) Grammar format

   while( Loop control expression ) {
 Loop statement ;
   }

    semantics , Execute the process ：

    The first step ： First calculate , Operation loop control expression if the result of calculation is true , Then execute the loop statement if the result of the calculation is false ,while The cycle ends directly

   The second step ： When the loop statement is completed , Continue to run the first step

   for example ： 

 int i = 0;
  while(i < 5) {
printf("i = %d\n", i);
i++;
  }

  

 while Dead cycle ：

 while(1) {
printf("hello,world\n");
 }

b) Be careful ： If while There is only one loop statement in ,{} Can not add ( It's not recommended to do this )

  int i = 0;
  while(i++ < 5)
        printf("i = %d\n", i);

3.4.do-while loop

a) Grammar format    

do {
        Loop statement ;
   }while( Loop control expression ); // Remember that this semicolon cannot be omitted 

    semantics ： Execute the process ：

    The first step ： Execute the loop statement first ; // The first time I didn't make a judgment

    The second step ： And then calculate , Run the loop expression if true , Continue to execute the loop statement , Repeat in sequence , If it is false , The loop ends

    characteristic ：do-while At least once ！

    for example ：

   int i = 0;
   do {
      printf("i = %d\n", i);
   }while(i++ < 5);