04-分支语句和逻辑运算符

news2024/10/7 10:25:22

分支语句和逻辑运算符

6.1、if语句

  • if

结构

image-20230815112909543

例子

统计输入的字符数、空格数

// if.cpp -- using the if statement
#include <iostream>
int main()
{
    using std::cin;     // using declarations
	using std::cout;
    char ch;
    int spaces = 0;
    int total = 0;
    cin.get(ch);
    while (ch != '.')   // quit at end of sentence
    {
        if (ch == ' ')  // check if ch is a space
            ++spaces;
        ++total;        // done every time
        cin.get(ch);
    }
    cout << spaces << " spaces, " << total;
    cout << " characters total in sentence\n";
    // cin.get();
    // cin.get();
    return 0;
}
  • if else
// ifelse.cpp -- using the if else statement
#include <iostream>

int main() {
	char ch;

	std::cout << "Type, and I shall repeat.\n";
	std::cin.get(ch);
	while (ch != '.') {
		if (ch == '\n')
			std::cout << ch;     // done if newline
		else
			std::cout << ++ch;   // done otherwise
		std::cin.get(ch);
	}
// try ch + 1 instead of ++ch for interesting effect
	std::cout << "\nPlease excuse the slight confusion.\n";
	// std::cin.get();
	// std::cin.get();
	return 0;
}
/*
Type, and I shall repeat.
An ineffable joy suffused me as I beheld
Bo!jofggbcmf!kpz!tvggvtfe!nf!bt!J!cfifme
the wonders of modern computing
uif!xpoefst!pg!npefso!dpnqvujoh
*/
  • if else if else结构
// ifelseif.cpp -- using if else if else
#include <iostream>
const int Fave = 27;

int main() {
	using namespace std;
	int n;

	cout << "Enter a number in the range 1-100 to find ";
	cout << "my favorite number: ";
	do {
		cin >> n;
		if (n < Fave)
			cout << "Too low -- guess again: ";
		else if (n > Fave)
			cout << "Too high -- guess again: ";
		else
			cout << Fave << " is right!\n";
	} while (n != Fave);
	// cin.get();
	// cin.get();
	return 0;
}

/*
Enter a number in the range 1-100 to find my favorite number: 50
Too high -- guess again: 25
Too low -- guess again: 31
Too high -- guess again: 28
Too high -- guess again: 27
27 is right!
*/

最好不要把variablevalue写成了valuevariable

6.2、逻辑表达式

  • or
// or.cpp -- using the logical OR operator
#include <iostream>

int main() {
	using namespace std;
	cout << "This program may reformat your hard disk\n"
	     "and destroy all your data.\n"
	     "Do you wish to continue? <y/n> ";
	char ch;
	cin >> ch;
	if (ch == 'y' || ch == 'Y')             // y or Y
		cout << "You were warned!\a\a\n";
	else if (ch == 'n' || ch == 'N')        // n or N
		cout << "A wise choice ... bye\n";
	else
		cout << "That wasn't a y or n! Apparently you "
		     "can't follow\ninstructions, so "
		     "I'll trash your disk anyway.\a\a\a\n";
	// cin.get();
	// cin.get();
	return 0;
}

  • &&
// and.cpp -- using the logical AND operator
#include <iostream>
const int ArSize = 6;

int main() {
	using namespace std;
	float naaq[ArSize];
	cout << "Enter the NAAQs (New Age Awareness Quotients) "
	     << "of\nyour neighbors. Program terminates "
	     << "when you make\n" << ArSize << " entries "
	     << "or enter a negative value.\n";

	int i = 0;
	float temp;
	cout << "First value: ";
	cin >> temp;
	while (i < ArSize && temp >= 0) { // 2 quitting criteria
		naaq[i] = temp;
		++i;
		if (i < ArSize) {           // room left in the array,
			cout << "Next value: ";
			cin >> temp;            // so get next value
		}
	}
	if (i == 0)
		cout << "No data--bye\n";
	else {
		cout << "Enter your NAAQ: ";
		float you;
		cin >> you;
		int count = 0;
		for (int j = 0; j < i; j++)
			if (naaq[j] > you)
				++count;
		cout << count;
		cout << " of your neighbors have greater awareness of\n"
		     << "the New Age than you do.\n";
	}
	// cin.get();
	// cin.get();
	return 0;
}
/*
Enter the NAAQs (New Age Awareness Quotients) of
your neighbors. Program terminates when you make
6 entries or enter a negative value.
First value: 28
Next value: 721
Next value: 15
Next value: 6
Next value: 130
Next value: 145
Enter your NAAQ: 50
3 of your neighbors have greater awareness of
the New Age than you do.
*/

首先将第一个输入值读入到临时变量(temp)中。然后,while测试条件查看数组中是否还有空间(i<ArSize)以及输入值是否为非负(temp>0)。

for more

// more_and.cpp -- using the logical AND operator
#include <iostream>

const char *qualify[4] = {      // an array of pointers*/
	// to strings
	"10,000-meter race.\n",
	"mud tug-of-war.\n",
	"masters canoe jousting.\n",
	"pie-throwing festival.\n"
};

int main() {
	using namespace std;
	int age;
	cout << "Enter your age in years: ";
	cin >> age;
	int index;

	if (age > 17 && age < 35)
		index = 0;
	else if (age >= 35 && age < 50)
		index = 1;
	else if (age >= 50 && age < 65)
		index = 2;
	else
		index = 3;

	cout << "You qualify for the " << qualify[index];
	// cin.get();
	// cin.get();
	return 0;
}

/*
Enter your age in years: 33
You qualify for the 10,000-meter race.
*/
  • not
if(!(x>5))		//if(x<=5)

例子

// not.cpp -- using the not operator
#include <iostream>
#include <climits>
bool is_int(double);

int main() {
	using namespace std;
	double num;

	cout << "Yo, dude! Enter an integer value: ";
	cin >> num;
	while (!is_int(num)) {  // continue while num is not int-able
		cout << "Out of range -- please try again: ";
		cin >> num;
	}
	int val = int (num);    // type cast
	cout << "You've entered the integer " << val << "\nBye\n";
	// cin.get();
	// cin.get();
	return 0;
}

bool is_int(double x) {
	if (x <= INT_MAX && x >= INT_MIN)   // use climits values
		return true;
	else
		return false;
}
/*
Yo, dude! Enter an integer value: 234567890345678
Out of range -- please try again: 1234567890
You've entered the integer 1234567890
Bye
*/

​ 如果给读取int值的程序输入一个过大的值,很多C++实现只是将这个值截断为合适的大小,不会通知丢失了数据。因此首先将可能得int值作为double值来读取。double类型的精度足以存储典型的int值,且取值范围更大。 另一种选择是,使用long long来存储输入的值,因为其取值范围比int大。

  • 逻辑运算符与优先级
x>5 && x<10
//被解释为
(x>5)&&(x<10)

另外一个例子

!(x>5)		//is it false that x is greater than 5

!x>5		//is !x greater than 5

6.3、字符函数库cctype

简化诸如确定字符是否是大写字母、数字、标点符号等工作。

例如,如果ch是一个字母,则isalpha(ch)函数返回一个非零值,否则返回0.

同样,如果ch是标点符号(逗号或句号),函数ispunct(ch)将返回true。(这些函数的返回类型为int,而不是bool,但通常bool转换能让我们将它视为bool类型。)

例子

使用isalpha()来检查字符是否为字母字符,

使用isdigit()来测试字符是否为数字字符。

使用isspace()来测试字符是否为空白,如换行符、空格和制表符。

使用ispunct()来测试字符是否为标点符号。

// cctypes.cpp -- using the ctype.h library
#include <iostream>
#include <cctype>              // prototypes for character functions

int main() {
	using namespace std;
	cout << "Enter text for analysis, and type @"
	     " to terminate input.\n";
	char ch;
	int whitespace = 0;
	int digits = 0;
	int chars = 0;
	int punct = 0;
	int others = 0;

	cin.get(ch);                // get first character
	while (ch != '@') {          // test for sentinel
		if (isalpha(ch))        // is it an alphabetic character?
			chars++;
		else if (isspace(ch))   // is it a whitespace character?
			whitespace++;
		else if (isdigit(ch))   // is it a digit?
			digits++;
		else if (ispunct(ch))   // is it punctuation?
			punct++;
		else
			others++;
		cin.get(ch);            // get next character
	}
	cout << chars << " letters, "
	     << whitespace << " whitespace, "
	     << digits << " digits, "
	     << punct << " punctuations, "
	     << others << " others.\n";
	// cin.get();
	// cin.get();
	return 0;
}

/*
Enter text for analysis, and type @ to terminate input.
wsteqdtfgyipo1245836782,..
125r6t38ehwcv fwefjhcdy 89u2n fcxt76 revf
ty8uijtyhjn   wegyh3782..,,yugh@
58 letters, 11 whitespace, 26 digits, 7 punctuations, 0 others.
*/

cctype字符函数

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6.4、? :运算符

例子

// condit.cpp -- using the conditional operator
#include <iostream>

int main() {
	using namespace std;
	int a, b;
	cout << "Enter two integers: ";
	cin >> a >> b;
	cout << "The larger of " << a << " and " << b;
	int c = a > b ? a : b;   // c = a if a > b, else c = b
	cout << " is " << c << endl;
	// cin.get();
	// cin.get();
	return 0;
}
/*
Enter two integers: 14 78
The larger of 14 and 78 is 78
*/

与if else序列相比,条件运算符更佳简洁。可以将条件表达式嵌套在另一个条件表达式。

const char x[2][20]={"Jason", "at your service\n"};
const char*y="Quillstone";

for(int i=0;i<3;i++){
	cout<<((i<2)? !x?x[i]:y :x[i]);
}

6.5、switch语句

switch语句的通用格式:

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// switch.cpp -- using the switch statement
#include <iostream>
using namespace std;
void showmenu();   // function prototypes
void report();
void comfort();
int main()
{
    showmenu();
    int choice;
    cin >> choice;
    while (choice != 5)
    {
        switch(choice)
        {
            case 1  :   cout << "\a\n";
                        break;
            case 2  :   report();
                        break;
            case 3  :   cout << "The boss was in all day.\n";
                        break;
            case 4  :   comfort();
                        break;
            default :   cout << "That's not a choice.\n";
        }
        showmenu();
        cin >> choice;
    }
    cout << "Bye!\n";
    // cin.get();
    // cin.get();
    return 0;
}

void showmenu()
{
    cout << "Please enter 1, 2, 3, 4, or 5:\n"
            "1) alarm           2) report\n"
            "3) alibi           4) comfort\n"
            "5) quit\n";
}
void report()
{
    cout << "It's been an excellent week for business.\n"
        "Sales are up 120%. Expenses are down 35%.\n";
}
void comfort()
{
    cout << "Your employees think you are the finest CEO\n"
        "in the industry. The board of directors think\n"
        "you are the finest CEO in the industry.\n";
}

也可以使用字符

image-20230815165933599
  • 将枚举量用作标签
// enum.cpp -- using enum
#include <iostream>

// create named constants for 0 - 6
enum {red, orange, yellow, green, blue, violet, indigo};

int main() {
	using namespace std;
	cout << "Enter color code (0-6): ";
	int code;
	cin >> code;
	while (code >= red && code <= indigo) {
		switch (code) {
			case red     :
				cout << "Her lips were red.\n";
				break;
			case orange  :
				cout << "Her hair was orange.\n";
				break;
			case yellow  :
				cout << "Her shoes were yellow.\n";
				break;
			case green   :
				cout << "Her nails were green.\n";
				break;
			case blue    :
				cout << "Her sweatsuit was blue.\n";
				break;
			case violet  :
				cout << "Her eyes were violet.\n";
				break;
			case indigo  :
				cout << "Her mood was indigo.\n";
				break;
		}
		cout << "Enter color code (0-6): ";
		cin >> code;
	}
	cout << "Bye\n";
	// cin.get();
	// cin.get();
	return 0;
}

/*
Enter color code (0-6): 1
Her hair was orange.
Enter color code (0-6): 2
Her shoes were yellow.
Enter color code (0-6): 3
Her nails were green.
Enter color code (0-6): 4
Her sweatsuit was blue.
Enter color code (0-6): 0
Her lips were red.
Enter color code (0-6): 6
Her mood was indigo.
Enter color code (0-6): 8
Bye
*/
  • switch和if else

switch语句和if else语句都允许程序从选项中选择。相比之下,if else更通用。例如,它可以处理取值范围。

然而,switch并不是为处理取值范围而设计的。switch语句中的每一个case标签都必须是一个单独的值。另外,这个值必须是整数(包括char),但是Switch无法处理浮点数。

6.6、break和continue

continue结构

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break结构

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例子

// jump.cpp -- using continue and break
#include <iostream>
const int ArSize = 80;

int main() {
	using namespace std;
	char line[ArSize];
	int spaces = 0;

	cout << "Enter a line of text:\n";
	cin.get(line, ArSize);
	cout << "Complete line:\n" << line << endl;
	cout << "Line through first period:\n";
	for (int i = 0; line[i] != '\0'; i++) {
		cout << line[i];    // display character
		if (line[i] == '.') // quit if it's a period
			break;
		if (line[i] != ' ') // skip rest of loop
			continue;
		spaces++;
	}
	cout << "\n" << spaces << " spaces\n";
	cout << "Done.\n";
	// cin.get();
	// cin.get();
	return 0;
}

/*
Enter a line of text:
Let's do lunch today.You can pay!
Complete line:
Let's do lunch today.You can pay!
Line through first period:
Let's do lunch today.
3 spaces
Done.
*/

在C++中,也有goto语句。下面的语句将跳转paris:作为标签的位置:

char ch;
cin>>ch;
if(ch=='P')
	goto paris;
cout<...
....
paris:cout<<"You arriced at here.\n";

在大多数情况下,使用goto语句不好。尽可能不用goto语句去控制程序的流程。

6.7、读取数字的循环

例子

// cinfish.cpp -- non-numeric input terminates loop
#include <iostream>
const int Max = 5;

int main() {
	using namespace std;
// get data
	double fish[Max];
	cout << "Please enter the weights of your fish.\n";
	cout << "You may enter up to " << Max
	     << " fish <q to terminate>.\n";
	cout << "fish #1: ";
	int i = 0;
	while (i < Max && cin >> fish[i]) {
		if (++i < Max)
			cout << "fish #" << i + 1 << ": ";
	}
// calculate average
	double total = 0.0;
	for (int j = 0; j < i; j++)
		total += fish[j];
// report results
	if (i == 0)
		cout << "No fish\n";
	else
		cout << total / i << " = average weight of "
		     << i << " fish\n";
	cout << "Done.\n";
// code to keep VC execution window open if q is entered
//	if (!cin)  // input terminated by non-numeric response
//	{
//	    cin.clear();  // reset input
//	    cin.get();    // read q
//	}
//	cin.get();    // read end of line after last input
//	cin.get();    // wait for user to press <Enter>
	return 0;
}

/*
Please enter the weights of your fish.
You may enter up to 5 fish <q to terminate>.
fish #1: 3
fish #2: 2
fish #3: 3
fish #4: 1
fish #5: 4
2.6 = average weight of 5 fish
Done.
*/

为了让窗口打开以便能够看到输出,需要添加额外的代码。

// code to keep VC execution window open if q is entered
	if (!cin)  // input terminated by non-numeric response
	{
	    cin.clear();  // reset input
	    cin.get();    // read q
	}
	cin.get();    // read end of line after last input
	cin.get();    // wait for user to press <Enter>

例子2

使用cin输入得分。如果检测输入的不是数字,则拒绝,并要求用户继续输入数字。可以看到,可以使用cin输入表达式来检测输入是不是数字。程序发现用户输入错误内容时,应该采取3个步骤:

  1. 重置cin以接受新的输入
  2. 删除错误输入
  3. 提示用户再输入
// cingolf.cpp -- non-numeric input skipped
#include <iostream>
const int Max = 5;

int main() {
	using namespace std;
// get data
	int golf[Max];
	cout << "Please enter your golf scores.\n";
	cout << "You must enter " << Max << " rounds.\n";
	int i;
	for (i = 0; i < Max; i++) {
		cout << "round #" << i + 1 << ": ";
		while (!(cin >> golf[i])) {
			cin.clear();     // reset input
			while (cin.get() != '\n')
				continue;    // get rid of bad input
			cout << "Please enter a number: ";
		}
	}
// calculate average
	double total = 0.0;
	for (i = 0; i < Max; i++)
		total += golf[i];
// report results
	cout << total / Max << " = average score "
	     << Max << " rounds\n";
	// cin.get();
	// cin.get();
	return 0;
}
/*
Please enter your golf scores.
You must enter 5 rounds.
round #1: 12
round #2: 23
round #3: 23
round #4: ysdiwvf
Please enter a number: 2341
round #5: 23
484.4 = average score 5 rounds
*/

在程序中,错误处理代码的关键部分如下:

while (!(cin >> golf[i])) {
    cin.clear();     // reset input
    while (cin.get() != '\n')
        continue;    // get rid of bad input
    cout << "Please enter a number: ";
}

​ 如果用户输入88,则cin表达式为true,因此将一个值放到数组中;而表达式!(cin>>golf[i])为false,因此结束内部循环。

​ 如果用户输入ysyv,则cin表达式为false,因此不会将任何值放到数组中,而表达式!(cin>>golf[i])为true,因此进入内部的while循环。

​ 循环的是先用clear()方法重置输入,如果省略该句,程序将拒绝继续读取输入。接下来,程序在while循环中使用cin.get()来读取行尾之前的所有输入,从而删除这一行的错误输入。另一种方法是读取到下一个空白字符,这样每次删除一个单词,而不是一整行。最后,程序高速用户,需要 输入一个数字。

6.8、简单文件输入/输出

  • 文本IO和文本文件
char ch;
cin>>ch;

//输入整数
int n;
cin>>n;

//输入double
double x;
cin>>x;

//char数组
char word[50];
cin>>word;

cin.getline(word,50);


  • 写入到文本文件中
ofstream outFile;
ofstream fout;

//打开文件的两种方式
outFile.open("fish.txt");
char filename[50];
cin>>filename;
fout.open(filename);

//写入数据
double wt=1255.8;
outFile<<wt;						//写入一个数据
char line[87]="Objects are closer than they appear.";
fout<<line<<endl;					//写入一行数据

使用文件输出的主要步骤。

  1. 包含一个头文件fstream
  2. 创建一个ofstream对象
  3. 将该ofstream对象同一个文件关联起来
  4. 像使用cout那样使用ofstream对象

例子

要求用户输入信息,然后将信息显示到屏幕上,再将这些信息写入到文件中。

// outfile.cpp -- writing to a file
#include <iostream>
#include <fstream>                  // for file I/O

int main() {
	using namespace std;

	char automobile[50];
	int year;
	double a_price;
	double d_price;

	ofstream outFile;               // create object for output
	outFile.open("carinfo.txt");    // associate with a file

	cout << "Enter the make and model of automobile: ";
	cin.getline(automobile, 50);
	cout << "Enter the model year: ";
	cin >> year;
	cout << "Enter the original asking price: ";
	cin >> a_price;
	d_price = 0.913 * a_price;

// display information on screen with cout

	cout << fixed;
	cout.precision(2);
	cout.setf(ios_base::showpoint);
	cout << "Make and model: " << automobile << endl;
	cout << "Year: " << year << endl;
	cout << "Was asking $" << a_price << endl;
	cout << "Now asking $" << d_price << endl;

// now do exact same things using outFile instead of cout

	outFile << fixed;
	outFile.precision(2);
	outFile.setf(ios_base::showpoint);
	outFile << "Make and model: " << automobile << endl;
	outFile << "Year: " << year << endl;
	outFile << "Was asking $" << a_price << endl;
	outFile << "Now asking $" << d_price << endl;

	outFile.close();                // done with file
	// cin.get();
	// cin.get();
	return 0;
}
/*
Enter the make and model of automobile: Filly Perky
Enter the model year: 2008
Enter the original asking price: 13500
Make and model: Filly Perky
Year: 2008
Was asking $13500.00
Now asking $12325.50
*/

打开文件和关闭文件

ofstream outFile;               // create object for output
//文件不存在将创建文件;文件存在则清空文件内容
outFile.open("carinfo.txt");    // associate with a file   
.....
outFile.close();                // done with file
  • 读取文本文件
//1、创建对象
ifstream inFile;
ifstream fin;

//2、对象和文件关联
inFile.open("bowling.txt");		//way1
char filename[50];				//way2
cin>>filename;
fin.open(filename);


//3、使用对象
double wt;
inFile>>wt;				//read a number from txt
char line[81];
fin.getline(line,81);	//read a line of text

检查文件是否被成功打开

inFile.open("bowling.txt");
if(!inFile.is_open()){
	exit(EXIT_FAILURE);
}

例子

打开用户指定的文件,读取其中的数字,然后指出文件中包含多少个值以及它们的和与平均值

// sumafile.cpp -- functions with an array argument
#include <iostream>
#include <fstream>          // file I/O support
#include <cstdlib>          // support for exit()
const int SIZE = 60;

int main() {
	using namespace std;
	char filename[SIZE];
	ifstream inFile;        // object for handling file input

	cout << "Enter name of data file: ";
	cin.getline(filename, SIZE);
	inFile.open(filename);  // associate inFile with a file
	if (!inFile.is_open()) { // failed to open file
		cout << "Could not open the file " << filename << endl;
		cout << "Program terminating.\n";
		// cin.get();    // keep window open
		exit(EXIT_FAILURE);
	}
	double value;
	double sum = 0.0;
	int count = 0;          // number of items read

	inFile >> value;        // get first value
	while (inFile.good()) { // while input good and not at EOF
		++count;            // one more item read
		sum += value;       // calculate running total
		inFile >> value;    // get next value
	}
	if (inFile.eof())
		cout << "End of file reached.\n";
	else if (inFile.fail())
		cout << "Input terminated by data mismatch.\n";
	else
		cout << "Input terminated for unknown reason.\n";
	if (count == 0)
		cout << "No data processed.\n";
	else {
		cout << "Items read: " << count << endl;
		cout << "Sum: " << sum << endl;
		cout << "Average: " << sum / count << endl;
	}
	inFile.close();         // finished with the file
	// cin.get();
	return 0;
}

ed to open file
cout << "Could not open the file " << filename << endl;
cout << “Program terminating.\n”;
// cin.get(); // keep window open
exit(EXIT_FAILURE);
}
double value;
double sum = 0.0;
int count = 0; // number of items read

inFile >> value;        // get first value
while (inFile.good()) { // while input good and not at EOF
	++count;            // one more item read
	sum += value;       // calculate running total
	inFile >> value;    // get next value
}
if (inFile.eof())
	cout << "End of file reached.\n";
else if (inFile.fail())
	cout << "Input terminated by data mismatch.\n";
else
	cout << "Input terminated for unknown reason.\n";
if (count == 0)
	cout << "No data processed.\n";
else {
	cout << "Items read: " << count << endl;
	cout << "Sum: " << sum << endl;
	cout << "Average: " << sum / count << endl;
}
inFile.close();         // finished with the file
// cin.get();
return 0;

}


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