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! = 1 2! = 2 3! = 6 4! = 24 5! = 120 6! = 720 7! = 5040 8! = 40320 9! = 362880 10! = 3628800
