按键K0～K3，用三种分支语句实现P0口的多值输出

程序1
1-1
#include <reg51.h> 
void main()//主函数
{
   P0=0x00; // P0口写0 
   while(1)
   {
     P1=0x0f; //P1口写1
     if((P1&0x0f)==0x0e)           P0=0x01;  //按下K0,D1点亮
     if((P1&0x0f)==0x0d)           P0=0x02;  //按下K1,D2点亮
     if((P1&0x0f)==0x0b)           P0=0x04;  //按下K2,D3点亮
     if((P1&0x0f)==0x07)           P0=0x08;  //按下K3,D4点亮
    }
}



1-2
#include <reg51.h> 
void main() //主函数
{
   P0=0x00;  // P0口写0 
   while(1)
   {
     P1=0x0f;  //P1口写1 
     if((P1&0x0f)==0x0e)  //按下K0 
 {
  P0=0x01; // D1点亮 
      }

    else if((P1&0x0f)==0x0d)   //按下K1
     {    
      P0=0x02;  //D2点亮
      }
     else if ((P1&0x0f)==0x0b)  //按下K2
     {
 P0=0x04;  //D3点亮
     }
 else if((P1&0x0f)==0x07)  //按下K3 
     {
 P0=0x08;  //D4点亮
     }
  }
}



1-3
#include <reg51.h> 
unsigned char a;
void main()  //主函数
{
   P0=0x00; // P0口写0 
   while(1)
   {
     P1=0x0f; //P1口写1
     a=P1; //读P1口的状态送到a
     a=a&0x0f;//屏蔽掉高4位无关位
     switch(a)
     {