[C language series] - branch and loop statements

Branch and loop statements

One ： What is a sentence ？

Two ： Branch statement

1.if sentence

2.swltch sentence

3、 ... and ： Loop statement

1.while loop

2.for loop

3.do... while loop

Four ：goto sentence

---

One ： What is a sentence ？

C Sentences can be divided into the following five categories ：

1. Expression statement

2. Function call statements

3. Control statement

4. Compound statement

5. Empty statement

Later this week, we will introduce the control statement .

Control statement Used to control the execution flow of the program , In order to realize the various structures of the program , They are made up of specific statement definers ,C language

There are nine control statements .

It can be divided into three categories ：

1. Conditional judgment statements are also called branch statements ：if sentence 、switch sentence ;

2. Loop statement ：do while sentence 、while sentence 、for sentence ;

3. Turn to statement ：break sentence 、goto sentence 、continue sentence 、return sentence .

Two ： Branch statement ：

 1.if sentence ：

  1.1： Grammatical structure ：

if sentence ：

 1: Single branch ：

    if（ expression ）

            sentence ;

 2. Double branch ：

    if（ expression 1）

            sentence 1;

    else if（ expression 2）

            sentence 2;

 3. Multiple branches ：

      if（ expression 1）

            sentence 1;

    else if（ expression 2）

            sentence 2;

    else 

            sentence 3;

  1.2： In the air else：

  notes ：else The matching of ：else It's the closest thing to it if Matching .

 2.swltch sentence ：

  2.1： Grammatical structure ：

switch sentence ：

 switch（ Integer expression ）

   {

          Statement item     case Integer constant expression :

                   sentence ;

   }

  Code demonstration ：

#include <stdio.h>
//switch Code demonstration 
int main()
{
    int day = 0;
    switch(day)
   {
        case 1：

        case 2:
        case 3:
        case 4:
        case 5:
            printf("weekday\n");
            break;
        case 6:
        case 7:
            printf("weekend\n");
            break;
   }
    return 0; 
}

  notes ：break sentence The practical effect is to divide the statement list into different branches .

  2.2：default Clause ：

If the expressed value is the same as all case What if the values of the tags don't match ？

It's nothing , Structure is that all statements are skipped .

The program does not terminate , No errors reported , Because this situation is C I don't think it's a mistake .

however , What if you don't want to ignore the values of expressions that don't match all tags ？

You can add an item to the statement list default Clause , Put the label below

default：

Write in any one case Where labels can appear .

When switch The value of the expression does not match all case The value of the tag , This default The statement following the clause will execute .

therefore , Every switch Only one... Can appear in the statement default Clause .

But it can appear anywhere in the statement list , And the statement flow will execute a case Execute as tag default Clause .

Programming good habits

At every switch Put one in every statement default Clause is a good habit , You can even add another one in the back break .

3、 ... and ： Loop statement ：

 1：while loop ：

  1.1： Grammatical structure

    while（ expression ）

            Loop statement ;

  1.2：while Statement execution process

  1,3：while Statement break and continue

  break:

//break  Code instance 
#include <stdio.h>
int main()
{
 int i = 1;
 while(i<=10)
 {
 if(i == 5)
 break;
 printf("%d ", i);
 i = i+1;
 }
 return 0; 
}

summary ：

      break stay while The role in the cycle ：

        In fact, in the loop, as long as you encounter break, Just stop all the later cycles , Terminate the cycle directly

        therefore ：while Medium break It's used to terminate the loop permanently

  continue：

//continue  Code instance 1
#include <stdio.h>
int main()
{
 int i = 1;
 while(i<=10)
 {
 if(i == 5)
 continue;
 printf("%d ", i);
 i = i+1;
 }
 return 0;
 }

//continue  Code instance 2
#include <stdio.h>
int main()
{
 int i = 1;
 while(i<=10)
 {
    i = i+1;
 if(i == 5)
 continue;
 printf("%d ", i);
 }
 return 0; 
}

summary ：

   continue Is used to terminate the cycle , In this cycle continue The following code is no longer executed

   It's a jump to while The judgment part of a sentence , Make the entry judgment of the next cycle .

 2：for loop ：

  2.1： Grammatical structure

for ( expression 1; expression 2; expression 3)

                  Loop statement ;

expression 1

expression 1 by Initialization part , Used to initialize loop variables .

expression 2

expression 2 by Condition judgment part , Used to determine when a loop ends .

expression 3

expression 3 by Adjustment part , For adjustment of cyclic conditions .

  2.2：for Execution flow chart of the loop

 2.3：for Statement loop control variable

Suggest ：

1. Don't be in for Modify the loop variable in the loop body , prevent for The cycle is out of control .

2. Suggest for The loop control variable value of the statement adopts “ Front closing back opening section ” How to write it .

int i = 0;
// The writing method of closing the front and opening the back 
for(i=0; i<10; i++)
{}
// Both sides are closed intervals 
for(i=0; i<=9; i++)
{}

 3.do... while loop

  3.1：do... while Cyclic syntax format ：

do

Loop statement ;

while( expression );

  3.2： do... while loop Execution flow chart of

  3.3：do Characteristics of sentences

Loop at least once , The scenarios used are limited , So I don't often use

#include <stdio.h>
int main()
{
 int i = 10;
 do
 {
 printf("%d\n", i);
 }while(i<10);
 return 0; }

  Four ：goto sentence ：

C The language provides a language that can be abused at will goto Statement and mark the label of jump .

In theory, goto Statements are not necessary , In practice, there is no goto Statements can also easily write code .

But on some occasions goto Statements are still useful , The most common use is to terminate the processing of structures nested in some depth

cheng .

for example ： Jump out of two or more loops at a time .

Multilayer loops are used in this case break You can't achieve your goal . It can only exit from the innermost loop to the upper loop .

goto The scenarios where the language is really suitable are as follows ：

for(...)
    for(...)
   {
        for(...)
       {
            if(disaster)
                goto error;
       }
   }
    …
error:
 if(disaster)
         //  Handling error situations 

  goto sentence An example of a sentence ： Shutdown Sequence

#include <stdio.h>
int main()
{
    char input[10] = {0};
    system("shutdown -s -t 60");
again:
    printf(" The computer will be in 1 Shut down in minutes , If input ： I am a pig , Just cancel the shutdown !\n Please enter :>");
    if(0 == strcmp(input, " I am a pig "))
   {
        system("shutdown -a");
   }
    else
   {
        goto again;
   }
    return 0; }