C语言家教记录（三）

导语

本次授课内容如下：选择语句、循环

辅助教材为 《C语言程序设计现代方法（第2版）》

选择语句

基本运算符

< > <= >=
== !=
! && ||

简单介绍各自含义

示例：

a<b&&a<c;
a==b||a==c;
a!=b
!(i<j)

if

if(a==b)
	a++;
if(i<0&&i>-2);
if(i<0||i>3);
/************************/
if(a==4)
{
	a=b;
	b=3;
}
/************************/
if(a==b)
{
	
}
else//a!=b
{

}
/************************/
if()
{
	if()
	{
	}
	else
	{
	}
}
else
{
	if()
	{
	}
	else
	{
	}
}
/************************/
if(a==1)
{}
else if(a==2)
{}
else if(a==3)
{}
else
{}
/************************/
//悬空else
if (y != 0)
 if (x != 0)
 result = x / y;
else
 printf("Error: y is equal to 0\n");
 

示例程序

#include 
int main(void)
{
 float commission, value;
 printf("Enter value of trade: ");
 scanf("%f", &value);
 if (value < 2500.00f)
 commission = 30.00f + .017f * value;
 else if (value < 6250.00f)
 commission = 56.00f + .0066f * value;
 else if (value < 20000.00f)
 commission = 76.00f + .0034f * value;
 else if (value < 50000.00f)
 commission = 100.00f + .0022f * value;
 else if (value < 500000.00f)
 commission = 155.00f + .0011f * value;
 else
 commission = 255.00f + .0009f * value;
 if (commission < 39.00f)
 commission = 39.00f;
 printf("Commission: $%.2f\n", commission);
 return 0;
}

条件表达式

int i,j,k;
i=1,j=2;
k=i>j?i:j;
k=(i>=0?i:0)+j;
/************************/
if(i>j)return i;
else return j;

return i>j?i:j;

switch

if (grade == 4)
 printf("Excellent");
else if (grade == 3)
 printf("Good");
else if (grade == 2)
 printf("Average");
else if (grade == 1)
 printf("Poor");
else if (grade == 0)
 printf("Failing");
else
 printf("Illegal grade");


switch (grade) {
 case 4: printf("Excellent");
 break;
 case 3: printf("Good");
 break;
 case 2: printf("Average");
 break;
 case 1: printf("Poor");
 break;
 case 0: printf("Failing");
 break;
 default: printf("Illegal grade");
 break;
}

switch (表达式) {
 case 常量表达式 : 语句
 ...
 case 常量表达式 : 语句
 default : 语句
 }

break

switch (grade) {
 case 4: printf("Excellent");
 case 3: printf("Good");
 case 2: printf("Average");
 case 1: printf("Poor");
 case 0: printf("Fai1ing");
 default: printf("Illegal grade");
}

循环

while

while(i<2)
	i*=2;

while(k>0)
{
	printf("%d\n",k);
	k--;
}

while(1)
{
	k++;
	printf("%d\n",k);
}

i = 10;
while (i > 0) {
 printf("T minus %d and counting\n", i);
 i--;
}

示例程序

#include 
int main(void)
{
 int i, n;
 printf("This program prints a table of squares.\n");
 printf("Enter number of entries in table: ");
 scanf("%d", &n);
 i = 1;
 while (i <= n) {
 printf("%10d%10d\n", i, i * i);
 i++;
 }
 return 0;
}



#include 
int main(void)
{
 int n, sum = 0;
 printf("This program sums a series of integers.\n");
 printf("Enter integers (0 to terminate): ");
 scanf("%d", &n);
 while (n != 0) {
 sum += n;
 scanf("%d", &n);
 }
 printf("The sum is: %d\n", sum);
 return 0;
}

do

先判断后执行，先执行后判断，有时候会有一次执行的区别

i = 10;
do {
 printf("T minus %d and counting\n", i);
 --i;
} while (i > 0);

int i=1;
while(i!=1)
{
	printf("%d\n",i);
}
printf("******************************\n");

do{
	printf("%d\n",i);
}while(i!=1);//不加分号可能会被别的识别
	i++;

示例程序

#include 
int main(void)
{
 int digits = 0, n;
 printf("Enter a nonnegative integer: ");
 scanf("%d", &n);
 do {
 n /= 10;
 digits++;
 } while (n > 0);
 printf("The number has %d digit(s).\n", digits);
 return 0;
}

for

同等目标下，用for实现循环更可控

for (i = 10; i > 0; i--)
 printf("T minus %d and counting\n",i);
 /**************************等价于*/
 i = 10;
while (i > 0) {
 printf("T minus %d and counting\n", i);
 i--;
}
 /**************************等价于*/
 for (i = 10; i > 0;)
 printf("T minus %d and counting\n", i--);
 /**************************等价于*/
 i=10;
 for (; i > 0;)
	 printf("T minus %d and counting\n", i--); 

for (i = 10; i > 0; --i)
 printf("T minus %d and counting\n",i);


for(i=0;i<n;i++);
for(i=1;i<=n;i++);
for(i=n-1;i>=0;i--);
for(i=n;i>0;i--);

while(1);
for(;;;);

int sum=0,i=0;
for(sum=0,i=1;i<=n;i++)sum+=i;//可以叠加无数变量

退出循环

continue

只能用于循环

for(i=0;i<n;i++)
{
	if(i%2==0)continue;
	printf("%d",i);
}

break

for(int i=2;i<n;i++)
	if(n%i==0)break;

for(int i=1;i<4;i++)
{
		printf("i\n");
		for(int j=1;j<4;j++)
		{
			printf("before break\n");
			if(j==2)break;
			printf("after break\n");
		}
}

while()
{
	switch(){
	case ():break;
	default:break;
}
}

空循环

for(d=2;d<n&&n%d!=0;d++);
	if(d<n)printf("%d可以被%d整除",n,d);

for(i=0;i<1000;i++);

总结和复习

本次授课讲述第五章和第六章内容，关键点：选择和循环