C++ Primer Plus笔记: 2023.06.20

1.C++ 11将使用大括号的初始化称为列表初始化，因为这种初始化常用于给复杂的数据类型提供值列表。列表初始化不允许缩窄，即变量的类型可能无法表示赋给他们的值。例如，不允许将浮点型转化为整型。在不同的整型之间转换或将整型转换为浮点型可能被允许，条件是编译器知道目标变量能够正确存储赋给它的值。

比如说下面的这个程序：

#include 
using namespace std ;

int main()
{
	const int code = 66 ;
	int x = 66 ;
	char c1 {31325} ;
	char c2 = {66}  ;
	char c3 {code} ;
	char c4 = {x} ;
	x = 31325 ;
	char c5 = x ;
	
	cout << "code = " << code << endl ;
	cout << "x = " << x << endl ;
	cout << "c1 = " << c1 << endl ;
    cout << "c2 = " << c2 << endl ;
	cout << "c3 = " << c3 << endl ;
    cout << "c4 = " << c4 << endl ;
	cout << "c5 = " << c5 << endl ;

	return 0 ;
}

编译结果：

narrow.cpp: In function ‘int main()’:
narrow.cpp:8:16: error: narrowing conversion of ‘31325’ from ‘int’ to ‘char’ [-Wnarrowing]
    8 |  char c1 {31325} ;
      |                ^
narrow.cpp:11:13: warning: narrowing conversion of ‘x’ from ‘int’ to ‘char’ [-Wnarrowing]
   11 |  char c4 = {x} ;
      |             ^

注意: 初始化c4时，您知道x的值为66，但在编译器看来，x是一个变量，其值可能很大。编译器不会跟踪下述阶段可能发生的情况: 从x被初始化到它被用来初始化c4。

2.自动转换: 在计算表达式时，C++将bool、char、unsigned char、signed char和short值转化为int。具体来说，true被转化为1，false被转化为0。这些转换被称为整体提升。

3.通常将int类型选择为计算机最自然的类型，这意味着计算机使用int类型时，运算速度可能最快。

4.使用下面的方式进行强制类型转换时:

typeName (value) ;

value必须是一个值，而不是变量，否则就会导致类似于下面的后果:

change.cpp: In function ‘int main()’:
change.cpp:9:7: error: conflicting declaration ‘long int thorn’
    9 |  long(thorn) ;
      |       ^~~~~
change.cpp:6:6: note: previous declaration as ‘int thorn’
    6 |  int thorn = 987 ;
      |      ^~~~~

第三章编程题：

（1）

#include 
using namespace std ;

int main()
{
	double height ;
	cout << "Please report your height in inchs: " << "________\b\b\b\b\b\b\b\b";
	cin >> height ;
	
	const double change = 12 ;
	cout << "Your height is " << height << " in. " << endl;
	cout << "Your height is " << height/change << " ft. " << endl;
	
	return 0 ;
}

(2)

#include 
#include 
using namespace std ;

int main()
{
	double feet, inch, pounds ;
	cout << "Report your height like ...ft...in: " << endl;
	cin >> feet ;
	cin >> inch ;
	
	cout << "Report your weight in pounds: " ;
	cin >> pounds ;
	
	const double FT_IN = 12 ;
	const double IN_METER = 0.0254 ;
	const double KG_LBS = 2.2 ;
	
	cout << "Your height = " << feet*FT_IN+inch << " in. " << endl ;
	cout << "Your BMI = " << pounds/KG_LBS/pow((feet*FT_IN+inch)*IN_METER, 2) << endl ;
}

(3)

#include 
using namespace std ;

int main()
{
	double degrees, minutes, seconds ;
	
	cout << "Enter a latitude in degrees, minutes, and seconds:" << endl;
	cout << "First, enter the degrees: " ;
	cin >> degrees ;
	cout << "Next, enter the minutes of arc: " ;
    cin >> minutes ;
	cout << "Finally, enter the seconds of arc: " ;
    cin >> seconds ;

	const double change = 60 ;
	
	cout << degrees << " degrees " << minutes << " minutes " << seconds << " seconds " ;
	cout << " = " << ( seconds/change + minutes ) / change + degrees << " degrees " << endl ;
	
	return 0 ;
} 

(4)

#include 
using namespace std ;

int main()
{
	long seconds ;
	cout << "Enter the number of seconds: " ;
	cin >> seconds ;
	
	const int DAY_HOUR = 24 ;
	const int HOUR_MINUTE = 60 ;
	const int MINUTE_SECOND = 60 ;
	
	cout << seconds << " seconds = " << seconds/MINUTE_SECOND/HOUR_MINUTE/DAY_HOUR << " days, " ;
	cout << seconds/MINUTE_SECOND/HOUR_MINUTE%DAY_HOUR<< " hours, " ;
	cout << seconds/MINUTE_SECOND%HOUR_MINUTE << " minutes, " ;
	cout << seconds%MINUTE_SECOND << " seconds" ;
	cout << endl ;

	return 0 ;
}

可能在取余运算方面，C++不会自动进行类型转换，所以出现了这样的错误：

error: invalid operands of types ‘long int’ and ‘const double’ to binary ‘operator%’
   17 |  cout << seconds%MINUTE_SECOND << " seconds" ;
      |          ~~~~~~~^~~~~~~~~~~~~~
      |          |       |
      |          |       const double
      |          long int

（5）

#include 
using namespace std ;

int main()
{
	long long world, us ;
	
	cout << "Enter the world's population: " ;
	cin >> world ;
	cout << "Enter the population of the US: " ;
	cin >> us ;
	
	cout << "The population of the US is " ;
	cout << ((double)us)/((double)world)*(double(100)) ;
	cout << "% of the world population." << endl ;
	return 0 ;
}

(6)

#include 
using namespace std ;

int main()
{
	double length, gasoline ;
	
	cout << "Enter the journey your car have gone in miles: " ;
	cin >> length ;
	cout << "Enter the gasoline your car have spent in gallons: " ;
    cin >> gasoline ;
	
	cout << "1 Gal can drive your car to go " ;
	cout << length/gasoline << " m." << endl ;

	cout << "Enter the journey your car have gone in kilometers: " ;
    cin >> length ;
    cout << "Enter the gasoline your car have spent in litres: " ;
    cin >> gasoline ;

	cout << "100 kilometers can force your car to spend " ;
	cout << gasoline/(length/double(100)) << " litres gasoline" << endl ;

	return 0 ;
}

(7)

#include 
using namespace std ;

int main()
{
	double europ ;
	
	cout << "100 kilometers spent gasoline: " ;
	cin >> europ ;
	
	cout << "This is equal to " << 62.14/(europ/3.875) << " mpg." << endl ;
	return 0 ;
}