一、选择结构和条件判断（双分支结构、多分支结构）

二、用if语句实现选择结构 

 1.第一种基本形式if

#include <stdio.h>
int main(){
	int a,b,max;
	printf("input two number: ");
	scanf("%d%d",&a,&b);
	max=a;
	if(max<b) max=b;
	printf("max=%d",max);
	return 0;
} 

 2.第二种形式if...else

#include <stdio.h>
int main(){
	int a,b;
	printf("input two number:");
	scanf("%d%d",&a,&b);
	if(a>b)
	  printf("max=%d",a);
	else
	  printf("max=%d",b);
	return 0;
} 

 3.第三种形式if...else if..else

#include <stdio.h>
int main(){
	char c;
	printf("input a character:");
	c=getchar();
	if(c<32)
	   printf("this is a control character\n");
	else if(c>='0'&&c<='9')
	   printf("this is a digit\n");
	else if(c>='A'&&c<='Z')
	   printf("this is a capital letter\n");
	else if(c>='a'&&c<='z')
	   printf("this is a small letter\n");
	else
	   printf("this is an character\n"); 
	return 0;
} 

三、关系运算符和关系表...