Lecture 2: Data Type & Formatted Input/Output (2)

細部拆解一個 C++ 程式碼

Debugger 逐行解釋

#include <iostream>

int main(void)
{
    int a = 0, b = 0, c = 0;
    std::cout << "input a:";
    std::cin >> a;
    std::cout << "input b:";
    std::cin >> b;
    c = a + b;
    std::cout << "a: " << a << std::endl
              << "b: " << b << std::endl
              << "a + b = " << c << std::endl;
    return 0;
}

為何要寫 #include <> ?

Include 了許多已經先寫好的 function，有些是跟系統有關，我們看不到原始碼的，我們只要拿來用就好。 可以打開 /usr/include/c++/9/iostream 看原始碼。

為何 int main(void) ?

main 的定義

1. int main () { body }
2. int main (int argc, char *argv[]) { body }

The main function is called at program startup after initialization of the non-local objects with static storage duration. It is the designated entry point to a program that is executed in hosted environment (that is, with an operating system). The entry points to freestanding programs (boot loaders, OS kernels, etc) are implementation-defined.

Another example:

#include <iostream>
#include <sstream>

int main(int argc, char *argv[])
{
    int a = 0, b = 0, c = 0;
    std::stringstream char_array_to_int;
    std::cout << argc << std::endl;
    char_array_to_int << argv[1] << ' ' << argv[2];
    char_array_to_int >> a;
    char_array_to_int >> b;
    c = a + b;
    std::cout << "a: " << a << std::endl
              << "b: " << b << std::endl
              << "a + b = " << c << std::endl;
    return 0;
}

Coding Style & 變數名稱規則

好的 coding style 很重要

Bad example:

#include <iostream>
#include <sstream>
int main(int argc, char *argv[]){int a = 0, b = 0, c = 0;std::stringstream char_array_to_int;std::cout << argc << std::endl;char_array_to_int << argv[1] << ' ' << argv[2];char_array_to_int >> a;char_array_to_int >> b;c = a + b;std::cout << "a: " << a << std::endl << "b: " << b << std::endl << "a + b = " << c << std::endl;return 0;}

intend & spacing

Bad example:

#include <iostream>
#include <sstream>
int main(int argc, char *argv[]){int a = 0, b = 0, c = 0;
  std::stringstream char_array_to_int;
    std::cout<<argc<<std::endl;
    char_array_to_int<<argv[1]<<' '<<argv[2];
char_array_to_int>>a;
    char_array_to_int>>b;
 c=a+b;
    std::cout<<"a: "<<a<<std::endl
              <<    "b: "      << b <<      std::endl
    << "a + b = "<< c << std::endl;
    return 0;
}

fomatter

In vscode:

• Windows: Shift Alt F
• macOS: Shift Option F
• Linux: Ctrl Shift I

Demo

變數名稱規則

GNU

#include <iostream>
#include <sstream>
int main(int argc, char *argv[])
{
  int a = 0, b = 0, c = 0;
  std::stringstream char_array_to_int;
  std::cout << argc << std::endl;
  char_array_to_int << argv[1] << ' ' << argv[2];
  char_array_to_int >> a;
  char_array_to_int >> b;
  c = a + b;
  std::cout << "a: " << a << std::endl
            << "b: " << b << std::endl
            << "a + b = " << c << std::endl;
  return 0;
}

Hungarian notation

#include <iostream>
#include <sstream>
int main(int argc, char *argv[])
{
  int a = 0, b = 0, c = 0;
  std::stringstream charArrayToInt;
  std::cout << argc << std::endl;
  charArrayToInt << argv[1] << ' ' << argv[2];
  charArrayToInt >> a;
  charArrayToInt >> b;
  c = a + b;
  std::cout << "a: " << a << std::endl
            << "b: " << b << std::endl
            << "a + b = " << c << std::endl;
  return 0;
}

變數格式 & 型態轉換

int, long, long long

unsigned int & overflow

#include <iostream>
int main(void)
{
  int i = 2147483647;
  unsigned int j = 4294967295U;
  std::cout << i << ' ' << i + 1 << ' ' << i + 2 << std::endl;
  std::cout << j << ' ' << j + 1 << ' ' << j + 2 << std::endl;
  return 0;
}

float, double, double double

floating point precision

#include <iostream>
#include <iomanip>
int main(void)
{
  float x = 1.23456789;
  std::cout << std::setprecision(3) << x << std::endl;
  std::cout << std::setprecision(4) << x << std::endl;
  std::cout << std::fixed;
  std::cout << std::setprecision(5) << x << std::endl;
  std::cout << std::setprecision(9) << x << std::endl;
  return 0;
}

cast

#include <iostream>
int main(void)
{
  double f = 3.14;
  unsigned int n1 = (unsigned int)f; // C-style cast
  std::cout << f << std::endl;
  std::cout << n1 << std::endl;
  return 0;
}

char

ASCII

Hex = Hexdecimal Value, Char = Character

#include <iostream>
int main(void)
{
  char char_five = '5';       // char_five = 53
  char char_diff = '5' - '0'; // char_diff = 53-48 = 5
  char c = 'B' + 32;          // c = 98 = 'b'
  std::cout << char_five << std::endl;
  std::cout << (int)char_five << std::endl;
  std::cout << (int)char_diff << std::endl;
  std::cout << c << std::endl;
  return 0;
}

others

UTF-8, UTF-16, Big5, Shift_JIS, etc.

Reference:

• htchen/i2p-nthu by 陳煥宗老師
• Main function - cppreference.com
• 【VSCode】如何在 VSCode 上自定義 C++ 的 coding style | by Zam Huang | Medium
• Naming convention (programming) - Wikipedia
• GNU Coding Standards
• Fundamental types - cppreference.com
• setprecision - C++ Reference
• Explicit type conversion - cppreference.com
• ASCII Table
• What is a buffer in C?