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：
1#include <stdio.h> 2 3int main(){ 4 void func(); 5 int i; 6 for(i = 0;i < 10; i++){ 7 func(); 8 } 9 10 return 0; 11} 12 13void func(){ 14 int i = 0; 15 static int num = 0; 16 printf("In func:i = %d, num = %d\n", i, num); 17 i++; 18 num++; 19}
打印：
1In func:i = 0, num = 0 2In func:i = 0, num = 1 3In func:i = 0, num = 2 4In func:i = 0, num = 3 5In func:i = 0, num = 4 6In func:i = 0, num = 5 7In func:i = 0, num = 6 8In func:i = 0, num = 7 9In func:i = 0, num = 8 10In func:i = 0, num = 9 11
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：
1#include <stdio.h> 2 3int main(){ 4 int i, m = 0, n = 0; 5 for(i = 0;i < 10; i++){ 6 printf("In main: m = %d, n = %d. ", m, n); 7 { 8 auto int n = 10; 9 printf("In block: m = %d, n = %d\n", m, n); 10 } 11 m++; 12 n++; 13 } 14 15 return 0; 16}
打印：
1In main: m = 0, n = 0. In block: m = 0, n = 10 2In main: m = 1, n = 1. In block: m = 1, n = 10 3In main: m = 2, n = 2. In block: m = 2, n = 10 4In main: m = 3, n = 3. In block: m = 3, n = 10 5In main: m = 4, n = 4. In block: m = 4, n = 10 6In main: m = 5, n = 5. In block: m = 5, n = 10 7In main: m = 6, n = 6. In block: m = 6, n = 10 8In main: m = 7, n = 7. In block: m = 7, n = 10 9In main: m = 8, n = 8. In block: m = 8, n = 10 10In main: m = 9, n = 9. In block: m = 9, n = 10 11
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：
1#include <stdio.h> 2 3int main(){ 4 int i, m = 0, n = 0; 5 for(i = 0;i < 10; i++){ 6 printf("In main: m = %d, n = %d. ", m, n); 7 { 8 static int n = 10; 9 printf("In block: m = %d, n = %d\n", m, n); 10 n--; 11 } 12 m++; 13 n++; 14 } 15 16 return 0; 17}
打印：
1In main: m = 0, n = 0. In block: m = 0, n = 10 2In main: m = 1, n = 1. In block: m = 1, n = 9 3In main: m = 2, n = 2. In block: m = 2, n = 8 4In main: m = 3, n = 3. In block: m = 3, n = 7 5In main: m = 4, n = 4. In block: m = 4, n = 6 6In main: m = 5, n = 5. In block: m = 5, n = 5 7In main: m = 6, n = 6. In block: m = 6, n = 4 8In main: m = 7, n = 7. In block: m = 7, n = 3 9In main: m = 8, n = 8. In block: m = 8, n = 2 10In main: m = 9, n = 9. In block: m = 9, n = 1 11
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：
1#include <stdio.h> 2 3int main(){ 4 int max(int a, int b); 5 extern A, B; 6 printf("Max = %d\n", max(A, B)); 7 8 return 0; 9} 10 11int max(int a, int b){ 12 return a > b ? a : b; 13} 14 15int 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：
1#include <stdio.h> 2 3int main(){ 4 long fact(int n); 5 int i; 6 for(i = 1; i <= 10; i++){ 7 printf("%2d! = %d\n", i, fact(i)); 8 } 9 10 return 0; 11} 12 13long fact(int n){ 14 register long f = 1; 15 int i; 16 for(i = 1; i <= n; i++){ 17 f *= i; 18 } 19 20 return f; 21} 22
打印：
1 1! = 1 2 2! = 2 3 3! = 6 4 4! = 24 5 5! = 120 6 6! = 720 7 7! = 5040 8 8! = 40320 9 9! = 362880 1010! = 3628800 11
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