C++ Primer 第五版答案（全书各章节代码）

Chapter 2. Variables and Basic Types ==================================== Exercise 2.1 ------------ > What are the differences between `int`, `long`, `long long`, and `short`? Between an `unsigned` and a `signed` type? Between a `float` and a `double`? C++ guarantees `short` and `int` is **at least** 16 bits, `long` **at least** 32 bits, `long long` **at least** 64 bits. The `signed` can represent positive numbers, negative numbers and zero, while `unsigned` can only represent numbers no less than zero. The C and C++ standards do not specify the representation of float, double and long double. It is possible that all three implemented as IEEE double-precision. Nevertheless, for most architectures (gcc, MSVC; x86, x64, ARM) float is indeed a IEEE **single-precision** floating point number (binary32), and double is a IEEE **double-precision** floating point number (binary64). **Usage**: - Use `int` for integer arithmetic. `short` is usually too small and, in practice,`long` often has the same size as `int`. If your data values are larger than the minimum guaranteed size of an `int`, then use `long long`. (In a word: short < **int** < long < long long) - Use an unsigned type when you know that the values cannot be negative. (In a word: no negative, unsigned.) - Use double for floating-point computations; float usually does not have enough precision, and the cost of double-precision calculations versus single-precision is negligible. In fact, on some machines, double-precision operations are faster than single. The precision offered by long double usually is unnecessary and often entails considerable run-time cost. (In a word: float < **double** < long double) Reference: - [What are the criteria for choosing between short / int / long data types?](http://www.parashift.com/c++-faq/choosing-int-size.html) - [Difference between float and double](http://stackoverflow.com/questions/2386772/difference-between-float-and-double) - Advice: Deciding which Type to Use(This book.) Exercise 2.2 ------------ > To calculate a mortgage payment, what types would you use for the rate, principal, and payment? Explain why you selected each type. use `double`, or also `float`. - The rate most like that: `4.50 % per year`. - The principal most like that: `$854.36` - The payment most like that: `$1,142.36` Reference: - [mortgage-calculator](http://www.bankrate.com/calculators/mortgages/mortgage-calculator.aspx) - [What's in a Mortgage Payment?](http://www.homeloanlearningcenter.com/mortgagebasics/whatsinamortgagepayment.htm) Exercise 2.3 ------------ > What output will the following code produce? > > ```cpp > unsigned u = 10, u2 = 42; > std::cout << u2 - u << std::endl; > std::cout << u - u2 << std::endl; > int i = 10, i2 = 42; > std::cout << i2 - i << std::endl; > std::cout << i - i2 << std::endl; > std::cout << i - u << std::endl; > std::cout << u - i << std::endl; > ``` Output(g++ 4.8): ``` 32 4294967264 32 -32 0 0 ``` Exercise 2.4 ------------ > Write a program to check whether your predictions were correct. If not, study this section until you understand what the problem is. [Here](ex2_04.cpp) is the code, please test it in your computer. Exercise 2.5 ------------ > Determine the type of each of the following literals. Explain the differences among the literals in each of the four examples: > > - (a) 'a', L'a', "a", L"a" > - (b) 10, 10u, 10L, 10uL, 012, 0xC > - (c) 3.14, 3.14f, 3.14L > - (d) 10, 10u, 10., 10e-2 (a): character literal, wide character literal, string literal, string wide character literal. (b): decimal, unsigned decimal, long decimal, unsigned long decimal, octal, hexadecimal. (c): double, float, long double. (d): decimal, unsigned decimal, double, double. Exercise 2.6 ------------ > What, if any, are the differences between the following definitions: > > ```cpp > int month = 9, day = 7; > int month = 09, day = 07; > ``` The first line's integer is decimal. The second line: 1. `int month = 09` is invalid, cause octal don't have digit `9`. 2. `day` is octal. Exercise 2.7 ------------ > What values do these literals represent? What type does each have? > > - (a) "Who goes with F\145rgus?\012" > - (b) 3.14e1L > - (c) 1024f > - (d) 3.14L (a): Who goes with Fergus?(new line) "string" (b): 31.4 "long double" (c): 1024 "float" (d): 3.14 "long double" Reference: - [ASCII Table](http://www.asciitable.com/) Exercise 2.8 ------------ > Using escape sequences, write a program to print 2M followed by a newline. Modify the program to print 2, then a tab, then an M, followed by a newline. ```cpp #include <iostream> int main() { std::cout << "\062\115\012"; std::cout << "\062\t\115\012"; } ``` Exercise 2.9 ------------ > Explain the following definitions. For those that are illegal, explain what’s wrong and how to correct it. > > - (a) `std::cin >> int input_value;` > - (b) `int i = { 3.14 };` > - (c) `double salary = wage = 9999.99;` > - (d) `int i = 3.14;` (a): error: expected '(' for function-style cast or type construction. ```cpp int input_value = 0; std::cin >> input_value; ``` (b): error: type 'double' cannot be narrowed to 'int' in initializer list. ```cpp double i = { 3.14 }; ``` (c): if declared 'wage' before, it's right. Otherwise, error would be happened: `error: use of undeclared identifier 'wage'` ```cpp double wage; double salary = wage = 9999.99; ``` (d): Ok: but value will be truncated. ```cpp double i = 3.14; ``` Exercise 2.10 ------------- > What are the initial values, if any, of each of the following variables? > > ```cpp > std::string global_str; > int global_int; > int main() > { > int local_int; > std::string local_str; > } > ``` - `global_str` is global variable, so the value is empty string. - `global_int` is global variable, so the value is zero. - `local_int` is a local variable which is not uninitialized, so it has a undefined value. - `local_str` is also a local variable which is not uninitialized, but it has a value that is defined by the class. So it is empty string. PS: please read P44 in the English version, P40 in Chinese version to get more information. > Uninitialized objects of built-in type defined inside a function body have a undefined value. Objects of class type that we do not explicitly initialize have a value that is defined by class. Exercise 2.11 ------------- > Explain whether each of the following is a declaration or a definition: > > - (a) `extern int ix = 1024;` > - (b) `int iy;` > - (c) `extern int iz;` - (a): definition. - (b): definition. - (c): declaration. Exercise 2.12 ------------- > Which, if any, of the following names are invalid? > > - (a) `int double = 3.14;` > - (b) `int _;` > - (c) `int catch-22;` > - (d) `int 1_or_2 = 1;` > - (e) `double Double = 3.14;` `a`, `c`, `d` are invalid. Exercise 2.13 ------------- > What is the value of j in the following program? > > ```cpp > int i = 42; > int main() > { > int i = 100; > int j = i; > } > ``` `100`. cause the global `i` was hidden by the local `i`. Exercise 2.14 ------------- > Is the following program legal? If so, what values are printed? ```cpp int i = 100, sum = 0; for (int i = 0; i != 10; ++i) sum += i; std::cout << i << " " << sum << std::endl; ``` Yes. It is legal.Printed:`100, 45.` Exercise 2.15 ------------- > Which of the following definitions, if any, are invalid? Why? > > - (a) `int ival = 1.01;` > - (b) `int &rval1 = 1.01;` > - (c) `int &rval2 = ival;` > - (d) `int &rval3;` - (a): valid. - (b): invalid. initializer must be an object. - (c): valid. - (d): invalid. a reference must be initialized. Exercise 2.16 ------------- > Which, if any, of the following assignments are invalid? If they are valid, explain what they do. > > ```cpp > int i = 0, &r1 = i; > double d = 0, &r2 = d; > ``` > > - (a) r2 = 3.14159; > - (b) r2 = r1; > - (c) i = r2; > - (d) r1 = d; - (a): valid. let d equal 3.14159. - (b): valid. automatic convert will happen. - (c):