C語言經典習題100例（九）41-45

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習題41

學習static定義靜態變量的用法。

實現思路：
用static聲明和未用static聲明變量進行對比，即可得出static的作用。

代碼如下：

#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

習題42

學習使用定義自動變量的用法。

實現思路：
自動變量用auto關鍵字聲明，函數中的形參和在函數中定義的變量(包括在複合語句中定義的變量)都屬於自動變量，默認不用關鍵字修飾即爲自動變量，可進行對比分析。

代碼如下：

#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

習題43

學習使用static的另一用法。

實現思路：
static在單獨的代碼塊中也可以實現在一次函數執行結束後變量未被銷燬、保留當前的值，下次調用函數時即使用保留在內存中的變量值，直到運行期間結束才釋放該變量。

代碼如下：

#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

習題44

學習使用external的用法。

實現思路：
用extern來聲明外部變量，以擴展外部變量的作用城，從變量的定義處開始，到本程序文件的末尾。

代碼如下：

#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;

習題45

學習使用register定義變量的方法。

實現思路：
C語言允許將局部變量的值放在CPU中的寄存器中，需要用時直接從寄存器取出參加運算，不必再到內存中去存取，從而提高執行效率，這種變量叫做寄存器變量，用關鍵字register聲明。

代碼如下：

#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