added ex2.1~ex2.8

ch02/ex2_4.cpp

@@ -0,0 +1,15 @@
+#include <iostream>
+
+int main()
+{
+  unsigned u = 10, u2 = 42;
+  std::cout << u2 - u << std::endl; // 32
+  std::cout << u - u2 << std::endl; // 4294967264
+  int i = 10, i2 = 42;
+  std::cout << i2 - i << std::endl; // 32
+  std::cout << i - i2 << std::endl; // -32
+  std::cout << i - u << std::endl; // 0
+  std::cout << u - i << std::endl; // 0
+
+  return 0;
+}

docs/ch02/ex2.1_2.2.md

@@ -0,0 +1,51 @@
+##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 `unsigned` and `signed` type have the **same size**. for example, sizeof(unsigned short) == sizeof(short).
+
+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)

docs/ch02/ex2.3_2.8.md

@@ -0,0 +1,91 @@
+##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](https://github.com/pezy/Cpp-Primer/blob/master/ch02/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.
+
+##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's integer 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;
+}
+
+```

docs/index.md

@@ -17,6 +17,8 @@ C++ Primer 5th Answer Note
 
 - Part I: The Basics
   - Chapter 2. Variables and Basic Types
+    - [Exercise 2.1 ~ 2.2](/Cpp-Primer/ch02/ex2.1_2.2)
+    - [Exercise 2.3 ~ 2.8](/Cpp-Primer/ch02/ex2.3_2.8)
   - Chapter 3. Strings, Vectors, and Arrays
   - Chapter 4. Expressions
   - Chapter 5. Statements