题目42：学习使用static的另一用法。
#include “stdio.h”
main()
{
int i,num;
num=2;
for(i=0;i<3;i++)
{
printf(“\40: The num equal %d \n”,num);
num++;
}
static int num=1;
printf(“\40:The internal block num equal %d\n”,num);
num++;
}

题目43：学习使用external的用法。
#include “stdio.h”
int a,b,c;
void add()
{ int a;
a=3;
c=a+b;
}
void main()
{ a=b=4;
add();
printf(“The value of c is equal to %d\n”,c);
}

题目44：学习使用register定义变量的方法。
void main()
{ register int i;
int tmp=0;
for(i=1;i<=100;i++)
tmp+=i;
printf(“The sum is %d\n”,tmp);
}

题目45：宏#define命令练习(1)
#include “stdio.h”
#define TRUE 1
#define FALSE 0
#define SQ(x) (x)*(x)
void main()
{ int num;
int again=1;
printf(“\40: Program will stop if input value less than 50.\n”);
while(again)
{
printf(“\40:Please input number==>”);
scanf(“%d”,&num);
printf(“\40:The square for this number is %d \n”,SQ(num));
if(num>=50)
again=TRUE;
else
again=FALSE;
}
}

题目46：宏#define命令练习(2)
#include “stdio.h”
#define exchange(a,b){ \ /*宏定义中允许包含两道衣裳命令的情形，此时必须在最右边加上”\”*/
int t;
t=a;\
a=b;
b=t;\　　}
void main(void)
{ int x=10;
int y=20;
printf(“x=%d; y=%d\n”,x,y);
exchange(x,y);
printf(“x=%d; y=%d\n”,x,y);
}

题目47：宏#define命令练习(3)
#define LAG >
#define SMA <
#define EQ ==
#include “stdio.h”
void main()
{ int i=10;
int j=20;
if(i LAG j)
printf(“\40: %d larger than %d \n”,i,j);
else if(i EQ j)
printf(“\40: %d equal to %d \n”,i,j);
else if(i SMA j)
printf(“\40:%d smaller than %d \n”,i,j);
else
printf(“\40: No such value.\n”);
}

题目48：#if #ifdef和#ifndef的综合应用。
#include “stdio.h”
#define MAX
#define MAXIMUM(x,y) (x>y) x:y
#define MINIMUM(x,y) (x>y) y:x
void main()
{ int a=10,b=20;
#ifdef MAX
printf(“\40: The larger one is %d\n”,MAXIMUM(a,b));
#else
printf(“\40: The lower one is %d\n”,MINIMUM(a,b));
#endif
#ifndef MIN
printf(“\40: The lower one is %d\n”,MINIMUM(a,b));
#else
printf(“\40: The larger one is %d\n”,MAXIMUM(a,b));
#endif
#undef MAX
#ifdef MAX
printf(“\40: The larger one is %d\n”,MAXIMUM(a,b));
#else
printf(“\40: The lower one is %d\n”,MINIMUM(a,b));
#endif
#define MIN
#ifndef MIN
printf(“\40: The lower one is %d\n”,MINIMUM(a,b));
#else
printf(“\40: The larger one is %d\n”,MAXIMUM(a,b));
#endif
}

题目49：#include 的应用练习
test.h 文件如下：
#define LAG >
#define SMA <
#define EQ ==
#include “test.h” /*一个新文件50.c，包含test.h*/
#include “stdio.h”
void main()
{ int i=10;
int j=20;
if(i LAG j)
printf(“\40: %d larger than %d \n”,i,j);
else if(i EQ j)
printf(“\40: %d equal to %d \n”,i,j);
else if(i SMA j)
printf(“\40:%d smaller than %d \n”,i,j);
else
printf(“\40: No such value.\n”);
}

题目50：学习使用按位与 & 。1.程序分析：0&0=0; 0&1=0; 1&0=0; 1&1=1
#include “stdio.h”
main()
{ int a,b;
a=077;
b=a&3;
printf(“\40: The a & b(decimal) is %d \n”,b);
b&=7;
printf(“\40: The a & b(decimal) is %d \n”,b);