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C++ primer plus 第七章 讀書筆記

       C++的指針屬于比較難了解的内容，本章節中有相當一部分在講述指針、數組的關系與運用。

7.1 函數基本知識

  1. 提供函數定義。分為兩類函數有傳回值的函數和無傳回值的函數，無傳回值的函數為void函數。 
    void cheers(int n)
{
    for (int i = 0; i < n; i++)
        cout<<"cheers=:";
    cout<<endl;
}
    上述函數中int n意味調用函數cheers（）時，需要将一個int類型參數傳遞給函數。
  2. 提供函數原型。調用函數之前需要在main函數之前聲明函數。 
    void fufu(float a, b)//此為不正确聲明變量方式。
void fufu(float a, float b)//正确聲明方式。
  3. 調用函數。函數中定義的局部變量如x, n, i等，在另外一個函數中也定義這樣的同名變量，不會帶來沖突。

下屬代碼示範了一個接受兩個參數的函數

#include <iostream>
using namespace std;

void n_chars(char, int);

int main()
{
	int times;
	char ch;
	cout << "enter a character:";
	cin >> ch;
	cout << "\n";
	while (ch!='q')
	{
		cout << "enter a integer:";
		cout << "enter a character:";
		cout << "\n";
		cin >> times;
		if (!cin)
		{
			cout << "alpha only!!!! please enter again." << endl;
			cin.clear();
			cin.ignore();//清空讀取緩存
			continue;
		}
		n_chars(ch, times);
		cout << "enter another character or press q to quit." << endl;
		cin >> ch;
	}
	cout << "the value of times is " << times << endl;

	return 0;
}

void n_chars(char c, int n)//不停輸出c的值，直到n==0
{
	while (n-- > 0)
	{
		cout << c;
	}
	cout << endl;
}

形參跟局部變量的差別。

彩票中獎機率計算程式。

#include <iostream>
using namespace std;

long double probability(unsigned numbers, unsigned picks);

int main()
{
	double total, choices;
	long double P;
	cout << "enter the total number of choices on the game card and\n"
		<< "the number of picks allowed." << endl;
	while ((cin>>total)&&(cin>>choices)&&(choices<=total))
	{
		P = probability(total, choices);
		cout << "you have a chance in " << P << "of winning." << endl;
		cout << "please enter anohter two numbers, or press non-digit to quit." << endl;
	
	}

	return 0;
}

long double probability(unsigned numbers, unsigned picks)//不停輸出c的值，直到n==0
{
	long double R = 1.0;
	unsigned n;
	unsigned p;
	for ( n = numbers, p =picks ; p > 0; n--,p--)
	{
		R = R * (n / p);
	}
	return 1/R;
}

7.3.1 C++函數如何使用指針來處理數組

數組名即為指針，指向數組中第一個元素的位址

#include <iostream>
using namespace std;

const int s = 8;
int cookies[s];
int parameter;
int sum_arr(int arr[], int n);

int main()
{
	int cookies[s] = { 1,2,4,8,16,32,64,128 };
	cout << cookies << " = arry address." << sizeof(cookies) << " = sizeof cookies." << endl;
	int sum = sum_arr(cookies, s);
	cout << sum << "cookies eaten." << endl;
	sum = sum_arr(cookies, 3);
	cout << sum << "first three ate." << endl;
	sum = sum_arr(cookies+4, 4);
	cout << sum << "last four ate." << endl;
	while (1)
	{

	}
	return 0;
}

int sum_arr(int arr[], int n)//arr為指向數組的指針，sizeof（arr）輸出指針的大小，此處為4。
{
	int total = 0;
	cout << arr << " = arr, " << sizeof(arr) << "= sizeof arr." << endl;
	for (int i = 0; i < n; i++)
	{
		total = total + arr[i];
	}
	return total;
}

上述代碼中 main函數中cookies就是指向cookies數組的指針。

#include <iostream>
int c_in_str(const char * str, char ch);

int main()
{
	using namespace std;
	char mmm[15] = "minimum";
	const char *wail = "ululate";
	int ms = c_in_str(mmm, 'm');
	int us = c_in_str(wail, 'u');
	cout << ms << " m characters in " << mmm << endl;
	cout << us << " u characters in " << wail << endl;
	getchar();
	getchar();
	return 0;
}

int c_in_str(const char * str, char ch)
{
	int count = 0;
	while (*str)
	{
		if (*str==ch)
			count++;
		str++;
	}
	return count;
}

上述代碼中，const char *wail 如果不加const就無法編譯通過；

#include <iostream>
char * buildstr(char c, int n);

int main()
{
	using namespace std;
	char ch;
	int times;
	cout << "enter an character:";
	cin >> ch;
	cout << "enter an integer:";
	cin >> times;
	char *ps = buildstr(ch, times);
	cout << ps << endl;
	delete[] ps;
	ps = buildstr('+', 20);
	cout << ps << "-done-" << ps << endl;
	delete[] ps;
	getchar();
	getchar();
	return 0;
}

char * buildstr(char c, int n)
{
	char *pstr = new char[n + 1];
	pstr[n] = '\0'; //将字元串最後一個字元改成空值
	while (n-- > 0)
	{
		pstr[n] = c;
	}
	return pstr;
}

傳回字元串的函數就必須在函數之前加上 “*” ，不加的話程式就不能編譯完成，因為函數return的是一個指針

#include <iostream>
struct travel_time
{
	int hours;
	int mins;
};
const int mins_per_hr = 60;
travel_time sum(travel_time t1, travel_time t2);
void show_time(travel_time t);

int main()
{
	using namespace std;
	travel_time day1 = { 5, 45 };
	travel_time day2 = { 4, 55 };
	travel_time trip = sum(day1, day2);
	travel_time day3 = { 4, 32 };
	cout << "three day total: ";
	show_time(sum(trip, day3));
	getchar();
	return 0;

}
//将兩次時間加到一起：A:1小時20份 B：1小時15分鐘 則A+B為2小時35分鐘
travel_time sum(travel_time t1, travel_time t2)
{
	travel_time total;
	total.mins = (t1.mins + t2.mins) % mins_per_hr;
	total.hours = t1.hours + t2.hours + (t1.mins + t2.mins) / mins_per_hr;
	return total;
}

void show_time(travel_time t)
{
	using namespace std;
	cout << t.hours << "hours, " << t.mins << "minutes\n";
}

struct的應用 struct名稱 變量名 就是命名方式 。travel_time sum(travel_time t1, travel_time t2)表示sum函數接受兩個travel_time參數，并且傳回的是travel_time數值。

#include <iostream>
//聲明polar rect 結構
struct polar
{
	double distance;
	double angle;
};
struct rect
{
	double x;
	double y;
};
void rect_to_polar(const rect *pxy, polar *pda);
void show_polar(const polar *pda);


int main()
{
	using namespace std;
	rect rplace;
	polar pplace;
	cout << "enter the x and y:";
	while (cin>> rplace.x,cin >> rplace.y)
	{
		rect_to_polar(&rplace, &pplace);
		show_polar(&pplace);
		cout << "next x and y(no_digits to quit)";
	}
	cout << "done." << endl;
	return 0;
}
//直角坐标系轉換為極坐标系 void函數無傳回值的需求 無return
//從rect結構中用指針讀取數值并進行運算，用polar指針讀取運算結果并賦予distance和angle
void rect_to_polar(const rect *pxy, polar *pda)
{
	using namespace std;
	pda->distance = sqrt(pxy->x*pxy->x + pxy->y*pxy->y);
	pda->angle = atan2(pxy->y, pxy->x);
}

void show_polar(const polar *pda)
{
	using namespace std;
	const double Rad_to_deg = 57.29577951;
	cout << "distance: " << pda->distance << endl;
	cout << "angle: " << pda->angle*Rad_to_deg << endl;
}

用指針讀取struct類型資料，很精巧。

#include <iostream>
#include <string>
using namespace std;
const int Size = 5;
void display(const string sa[], int n);


int main()
{
	using namespace std;
	string list[Size];
	cout << "enter you string list, " << Size << " favorite astronomical sights." << endl;
	for (int i = 0; i < Size; i++)
	{
		cout << i + 1 << ":";
		getline(cin, list[i]);
	}
	cout << "display your list:"<<endl;
	display(list, Size);
	getchar();
	return 0;
}

void display(const string sa[], int n)
{
	for (int i = 0; i < n; i++)
	{
		cout << i + 1 << ":" << sa[i] << endl;
	}
}

在上述代碼中list[size]的實際結構為list[fdaf,gdfdg,llkj,jiefjd,jifafe]。

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