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习题41

学习static定义静态变量的用法。

实现思路：
用static声明和未用static声明变量进行对比，即可得出static的作用。

代码如下：

#include <stdio.h>int main(){void func();int i;for(i = 0;i < 10; i++){func();}	return 0;
}void func(){int i = 0;static int num = 0;printf("In func:i = %d, num = %d\n", i, num);i++;num++;
}

打印：

In func:i = 0, num = 0
In func:i = 0, num = 1
In func:i = 0, num = 2
In func:i = 0, num = 3
In func:i = 0, num = 4
In func:i = 0, num = 5
In func:i = 0, num = 6
In func:i = 0, num = 7
In func:i = 0, num = 8
In func:i = 0, num = 9

习题42

学习使用定义自动变量的用法。

实现思路：
自动变量用auto关键字声明，函数中的形参和在函数中定义的变量(包括在复合语句中定义的变量)都属于自动变量，默认不用关键字修饰即为自动变量，可进行对比分析。

代码如下：

#include <stdio.h>int main(){int i, m = 0, n = 0;for(i = 0;i < 10; i++){printf("In main: m = %d, n = %d. ", m, n);{auto int n = 10;printf("In block: m = %d, n = %d\n", m, n);}m++;n++;}	return 0;
}

打印：

In main: m = 0, n = 0. In block: m = 0, n = 10
In main: m = 1, n = 1. In block: m = 1, n = 10
In main: m = 2, n = 2. In block: m = 2, n = 10
In main: m = 3, n = 3. In block: m = 3, n = 10
In main: m = 4, n = 4. In block: m = 4, n = 10
In main: m = 5, n = 5. In block: m = 5, n = 10
In main: m = 6, n = 6. In block: m = 6, n = 10
In main: m = 7, n = 7. In block: m = 7, n = 10
In main: m = 8, n = 8. In block: m = 8, n = 10
In main: m = 9, n = 9. In block: m = 9, n = 10

习题43

学习使用static的另一用法。

实现思路：
static在单独的代码块中也可以实现在一次函数执行结束后变量未被销毁、保留当前的值，下次调用函数时即使用保留在内存中的变量值，直到运行期间结束才释放该变量。

代码如下：

#include <stdio.h>int main(){int i, m = 0, n = 0;for(i = 0;i < 10; i++){printf("In main: m = %d, n = %d. ", m, n);{static int n = 10;printf("In block: m = %d, n = %d\n", m, n);n--;}m++;n++;}	return 0;
}

打印：

In main: m = 0, n = 0. In block: m = 0, n = 10
In main: m = 1, n = 1. In block: m = 1, n = 9
In main: m = 2, n = 2. In block: m = 2, n = 8
In main: m = 3, n = 3. In block: m = 3, n = 7
In main: m = 4, n = 4. In block: m = 4, n = 6
In main: m = 5, n = 5. In block: m = 5, n = 5
In main: m = 6, n = 6. In block: m = 6, n = 4
In main: m = 7, n = 7. In block: m = 7, n = 3
In main: m = 8, n = 8. In block: m = 8, n = 2
In main: m = 9, n = 9. In block: m = 9, n = 1

习题44

学习使用external的用法。

实现思路：
用extern来声明外部变量，以扩展外部变量的作用城，从变量的定义处开始，到本程序文件的末尾。

代码如下：

#include <stdio.h>int main(){int max(int a, int b);extern A, B;printf("Max = %d\n", max(A, B));return 0;
}int max(int a, int b){return a > b ? a : b;
}int A = 12, B = 20;

习题45

学习使用register定义变量的方法。

实现思路：
C语言允许将局部变量的值放在CPU中的寄存器中，需要用时直接从寄存器取出参加运算，不必再到内存中去存取，从而提高执行效率，这种变量叫做寄存器变量，用关键字register声明。

代码如下：

#include <stdio.h>int main(){long fact(int n);int i;for(i = 1; i <= 10; i++){printf("%2d! = %d\n", i, fact(i));}return 0;
}long fact(int n){register long f = 1;int i;for(i = 1; i <= n; i++){f *= i;}return f;
}

打印：

 1! = 12! = 23! = 64! = 245! = 1206! = 7207! = 50408! = 403209! = 362880
10! = 3628800