C language circular statement

Catalog

One 、 Type of cycle

for Loop compiler error

do...while loop

Two 、 Loop control statement  

break sentence  

continue sentence

3、 ... and 、 Infinite loop

---

One 、 Type of cycle

C The language provides the following types of loops .

Type of cycle	describe
while loop	When a given condition is true , Repeat a statement or group of statements . It tests the condition before executing the loop body .
for loop	Execute a sequence of statements multiple times , Simplify the code for managing loop variables .
do...while loop	Except that it tests the condition at the end of the loop body , Others and while Statements like .
Nested loop	You can go to while、for or do..while Use one or more loops within the loop .

for Loop compiler error

Some compilers will report errors when running the above statements , as follows ：

[Error] 'for' loop initial declarations are only allowed in C99 or C11 mode

intend “ Do not allow the for Internally defined variables ”.

Change it to ：

int a=10;
for(; a<20; a=a+1){
}

Then it can run .

If you are using Dev C++ The above problems occur , You can find tools directly above the editor --> Compilation options --> The generated code / Optimize --> Code generation --> Language standard ISO C99 Then click below to save .

do...while loop

Unlike  for  and  while  loop , They test the loop conditions at the loop head . stay C In language ,do...while  A loop is to check its condition at the end of the loop .do...while  Circulation and while Circulation similar , however do...while The loop will ensure that At least once loop .

grammar

do
{
   statement(s);

}while( condition );

example  

// example 
#include <stdio.h>
 
int main ()
{
   /*  Definition of local variables  */
   int a = 10;

   /* do  Loop execution , Execute at least once before the condition is tested  */
   do
   {
       printf("a  Value ： %d\n", a);
       a = a + 1;
   }while( a < 20 );
 
   return 0;
}

When the above code is compiled and executed , It will produce the following results ：

a  Value ： 10
a  Value ： 11
a  Value ： 12
a  Value ： 13
a  Value ： 14
a  Value ： 15
a  Value ： 16
a  Value ： 17
a  Value ： 18
a  Value ： 19

C Nesting in languages do...while The syntax of circular statements ：

do
{
    statement(s);
    do
    {
        statement(s);
        ... ... ...
    }while (condition2);
    ... ... ...
}while (condition1

example  

//do-while  Nested instances 
#include <stdio.h>
int main()
{
    int i=1,j;
    do
    {
        j=1;
        do
        {
            printf("*");
            j++;
        }while(j <= i);
        i++;
        printf("\n");
    }while(i <= 5);
    return 0;
}

  When the above code is compiled and executed , It will produce the following results ：

*
**
***
****
*****

Two 、 Loop control statement  

Loop control statements change the execution order of the code . It can realize the jump of code .C The following loop control statements are provided .

Control statement	describe
break sentence	End loop or  switch  sentence , The program flow will continue to execute immediately following the loop or switch The next sentence of .
continue sentence	Tell a loop body to stop this iteration immediately , Restart the next iteration of the loop .
goto sentence	Transfer control to the marked statement . however It is not recommended to use goto sentence .

break sentence  

C In language  break  Statement has the following two uses ：

1. When  break  When a statement appears in a loop , The loop ends immediately , And the program flow continues to execute the next statement that follows the loop .
2. It can be used to terminate  switch  One of the sentences case.

If you are using nested loops （ That is, one loop embeds another ）,break Statement will stop executing the innermost loop , Then start executing the next line of code after the block .

grammar

break;

example  

// example 
#include <stdio.h>
 
int main ()
{
   /*  Definition of local variables  */
   int a = 10;

   /* while  Loop execution  */
   while( a < 20 )
   {
      printf("a  Value ： %d\n", a);
      a++;
      if( a > 15)
      {
         /*  Use  break  Statement to terminate the loop  */
          break;
      }
   }
 
   return 0;
}

When the above code is compiled and executed , It will produce the following results ：

a  Value ： 10
a  Value ： 11
a  Value ： 12
a  Value ： 13
a  Value ： 14
a  Value ： 15

continue sentence

C In language  continue  It's a little bit like that  break  sentence . But it's not a mandatory termination ,continue Code in the current loop will be skipped , Force to start the next cycle .

about  for  loop ,continue  After the statement is executed, the auto increment statement will still execute . about  while  and  do...while  loop ,continue  Statement to re execute the conditional judgment statement .

grammar

continue;

example

// example 
#include <stdio.h>
 
int main ()
{
   /*  Definition of local variables  */
   int a = 10;

   /* do  Loop execution  */
   do
   {
      if( a == 15)
      {
         /*  Skip iteration  */
         a = a + 1;
         continue;
      }
      printf("a  Value ： %d\n", a);
      a++;
     
   }while( a < 20 );
 
   return 0;
}

When the above code is compiled and executed , It will produce the following results ：

a  Value ： 10
a  Value ： 11
a  Value ： 12
a  Value ： 13
a  Value ： 14
a  Value ： 16
a  Value ： 17
a  Value ： 18
a  Value ： 19

3、 ... and 、 Infinite loop

If the conditions are never false , Then the loop will become an infinite loop .for  Loop can be used to realize infinite loop in traditional sense . Because none of the three expressions that make up the loop is necessary , You can leave some conditional expressions blank to form an infinite loop .

// example 
#include <stdio.h>
 
int main ()
{
   for( ; ; )
   {
      printf(" The cycle will go on forever ！\n");
   }
   return 0;
}

When the conditional expression does not exist , It is assumed to be true . You can also set an initial value and an incremental expression , But in general ,C Programmers tend to use for(;;) Structure to represent an infinite loop .

Be careful ： You can press  Ctrl + C  Key to terminate an infinite loop .