详解C++编程中的条件判断语句if-else与switch的用法

  if ( expression )
 statement1
[else
 statement2]

// if_else_statement.cpp
#include <stdio.h>

int main() 
{
 int x = 0;
 if (x == 0)
 {
  printf_s("x is 0!\n");
 }
 else
 {
  printf_s("x is not 0!\n"); // this statement will not be executed
 }

 x = 1;
 if (x == 0)
 {
  printf_s("x is 0!\n"); // this statement will not be executed
 }
 else
 {
  printf_s("x is not 0!\n");
 }

 return 0;
}

x 是 0！
x 不是 0!

 switch ( expression )
case constant-expression : statement
[default : statement]

// switch_statement1.cpp
#include <stdio.h>

int main() {
 char *buffer = "Any character stream";
 int capa, lettera, nota;
 char c;
 capa = lettera = nota = 0;

 while ( c = *buffer++ ) // Walks buffer until NULL
 {
  switch ( c )
  {
   case 'A':
   capa++;
   break;
   case 'a':
   lettera++;
   break;
   default:
   nota++;
  }
 }
 printf_s( "\nUppercase a: %d\nLowercase a: %d\nTotal: %d\n",
  capa, lettera, (capa + lettera + nota) );
}

// switch_statement2.cpp
// C2360 expected
#include <iostream>
using namespace std;
int main(int argc, char *argv[])
{
 switch( tolower( *argv[1] ) )
 {
  // Error. Unreachable declaration.
  char szChEntered[] = "Character entered was: ";

 case 'a' :
  {
  // Declaration of szChEntered OK. Local scope.
  char szChEntered[] = "Character entered was: ";
  cout << szChEntered << "a\n";
  }
  break;

 case 'b' :
  // Value of szChEntered undefined.
  cout << szChEntered << "b\n";
  break;

 default:
  // Value of szChEntered undefined.
  cout << szChEntered << "neither a nor b\n";
  break;
 }
}