C++ Primer 英文版答案

##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_4.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 << 2 << "\115\012"; std::cout << 2 << "\t\115\012"; return 0; } ``` ##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):---when you compile the code without the argument "-std=c++11", you will get the warning below: warning: implicit conversion from 'double' to 'int' changes value from 3.14 to 3. ---when you compile the code using "-std=c+11", you will get a error below: error: type 'double' cannot be narrowed to 'int' in initializer list ---conclusion: Obviously, list initialization becomes strict in c++11. ```cpp double i = { 3.14 }; ``` (c): --if you declared 'wage' before, it's right. Otherwise, you'll get a error: 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. The note: Uninitialized objects of built-in type defined inside a function body have a undefined value. Objects of class type that we do not explicitly inititalize 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`, since 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; ``` Legal. Output: ```100 45``` Note: Such naming is considered as bad practise. ##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. 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): valid. but value will be truncated. (d): valid. but value will be truncated. ``` ##Exercise 2.17 >What