C language -- while statement, dowhile statement, for loop and loop structure, break statement and continue statement

Catalog

One 、while sentence

getchar function

Two 、dowhile sentence  

while And dowhile The difference between sentences ：

  3、 ... and 、for Statements and loop structures

1. form

2. Print 10 Time LoveC

3. Write a program ： Judge whether a number is prime

4.for Flexible use of sentences

  Add ：C99 The new standard

5. A nested loop

6. Print the multiplication table

  Four 、break Statement and continue sentence

1.break sentence

2.continue sentence

  Be careful

---

One 、while sentence

while( expression )
     The loop body 

1. Code practice one ： use C Language computing 1+2+3+……+100 Result

The code is as follows ：

#include <stdio.h>

int main()
{
	int i = 1,sum = 0;
	
	while(i <= 100)
	{
		sum = sum + i;
		i = i + 1;
	}
	
	printf(" The result is ：%d\n",sum);
	
	return 0;
	
}

Running results ：

getchar function

（1） Function summary ：getchar Function from standard input stream (stdin) Get the next character in . Equivalent to calling getc(stdin) function .

（2） The function prototype ：

#include <stdio.h>
…
int getchar()(void);

（3） Return value ：

If the function call succeeds , Return the obtained characters （ Use integer to express its ASCII code ）

Return value if yes EOF, Indicates that the function call failed ;

• If the standard input stream is at the end , This function returns EOF, And set the end flag of the standard input stream .
• If there are other mistakes , This function also returns EOF, And set the error flag instead .

2. Code Practice II ： Count the number of characters in a line of English sentences input from the keyboard

The code is as follows ：

#include <stdio.h>

int main()
{
	int count = 0;
	
	printf(" Please enter a line of English characters ：\n");
	
	while(getchar() != '\n')
	{
		count = count + 1;
	}
	
	printf(" You have entered a total of %d Characters \n",count);
	
	return 0;
	
}

Running results ：

Two 、dowhile sentence  

do
     The loop body 
while( expression );

  Practical application , Such as ： Verify that the user password is correct

• If use while sentence , The flow chart is as follows ：

  If use dowhile sentence , The flow chart is as follows ：

while And dowhile The difference between sentences ：

  3、 ... and 、for Statements and loop structures

1. form

for( expression 1; expression 2; expression 3)
     The loop body 

The three expressions are separated by semicolons , among ：

• expression 1 Is a loop initialization expression
• expression 2 It's a cyclic conditional expression
• expression 3 Is a loop adjustment expression

2. Print 10 Time LoveC

The code is as follows ：

#include <stdio.h>

int main()
{
	int count;
	
	for(count = 0;count < 10;count++)
	{
		printf("LoveC!\n");
	}
	
	return 0;
	
}

Running results ：

3. Write a program ： Judge whether a number is prime

Tips ：

prime number ： In more than 1 Of the natural number , except 1 And the number itself , A number that cannot be divided by other natural numbers .

One of the ways to find prime numbers ： Iterative test from 2 To num/2 Whether all integers can be divisible （num Is the integer to be tested ）, If there is no integer that can be divisible , So it's prime .

The code is as follows ：

#include <stdio.h>

int main()
{
	int i,num;
	_Bool flag = 1;
	
	printf(" Please enter an integer ：\n");
	scanf("%d",&num);
	
	for(i = 2;i < num / 2;i++)
	{
		if(num % i == 0)
		{
			flag = 0;
		}
	}
	
	if(flag)
	{
		printf("%d It's a prime number ！\n",num);
	}
	else
	{
		printf("%d It's not a prime number ！\n",num);
	}
	
	return 0;
	
}

Running results ：

 

4.for Flexible use of sentences

（1） expression 1, expression 2 And expressions 3 It can be omitted as needed （ But the semicolon cannot be omitted ）：

• for(   ; expression 2 ; expression 3)
• for( expression 1 ; expression 2 ; )
• for( expression 1 ; ; )
• for(    ; ; )
• ……

（2） expression 1 And expressions 3 It can be a simple expression , It can also be a comma expression （ That is, separate multiple expressions with commas ）

The code for ：

#include <stdio.h>

int main()
{
	int i,j;
	
	for(i = 0,j = 10;i < j; i++,j--)
	{
		printf("%d\n",i);
	}
	
	return 0;
	
}

Running results ：

  Add ：C99 The new standard

C99 Allow in for Expression of statement 1 Define variables in , Such as ：

for (int i = 0; i < 100; i++)

That is, the code of the above example can be written as ：

#include <stdio.h>

int main()
{
	
	for(int i = 0,j = 10;i < j; i++,j--)
	{
		printf("%d\n",i);
	}
	
	return 0;
	
}

5. A nested loop

The code for ：

#include <stdio.h>

int main()
{
	int i,j;
	for(i = 0;i < 3;i++)
	{
		for(j = 0;j < 3;j++)
		{
			printf("i = %d,j = %d\n",i,j);
		}
	}
	
	return 0;
	
}

Running results ：

6. Print the multiplication table

The code is as follows ：

#include <stdio.h>

int main()
{
	int i,j;
	
	for(i = 1;i <= 9;i++)
	{
		for(j = 1;j <= i;j++)
		{
			printf("%d*%d=%-2d  ",i,j,i*j);
		}
		putchar('\n');
	}
	
	return 0;
	
}

Running results ：

  Four 、break Statement and continue sentence

1.break sentence

Be responsible for jumping out of the loop of the statement at its level , If you want to jump out of the outer cycle, you need to arrange another break sentence .

It is also the example of the prime number mentioned above , If the number entered is 100000000, I added break After the statement , The efficiency of implementation is greatly increased , The code is as follows ：

#include <stdio.h>

int main()
{
	long long int i,num;
	_Bool flag = 1;
	
	printf(" Please enter an integer ：\n");
	scanf("%lld",&num);
	
	for(i = 2;i < num / 2;i++)
	{
		if(num % i == 0)
		{
			flag = 0;
			break; 
		}
	}
	
	if(flag)
	{
		printf("%d It's a prime number ！\n",num);
	}
	else
	{
		printf("%d It's not a prime number ！\n",num);
	}
	
	printf("i = %lld\n",i);
	return 0;
	
}

Running results ：

so , Just execute 2 You can get the result once , But if you will break Comment out the statement , The execution efficiency is greatly reduced , The operation results are as follows ：

 

2.continue sentence

When executed continue When the sentence is , The rest of the loop body will be ignored , Go straight to the next cycle .

The code for ：

#include <stdio.h>

int main()
{
	int ch;
	
	while((ch = getchar()) != '\n')
	{
		if(ch == 'C')
		{
			continue; 
		}
		
		putchar(ch);
	}
	
	putchar('\n');
	
	return 0;
	
}

Running results ：

  Be careful

• For nested statements ,break Statement and continue Statements can only act on one layer of loops ！
• for Statement and while Statements are not completely equivalent , They are different in execution ： appear continue When the sentence is ,while Is to skip the statement after the loop body , Go straight to the next cycle .