【C++】学习笔记二十一——if语句

if (test-condition)
    statement

程序6.1

#include<iostream>
int main()
{
using namespace std;
char ch;
int spaces = 0;
int total = 0;
cin.get(ch);
while (ch != '.')
    {
if (ch == ' ')
            ++spaces;
        ++total;
cin.get(ch);
    }
cout << spaces << " spaces, " << total;
cout << " characters total in sentence\n";
    system("pause");
return 0;
}

1. if else语句

if (test-condition)         //如果测试条件为true或非0，将执行statement1，跳过statement2
    statement1
else                        //如果测试条件为false或0，将执行statement2
    statement2

程序6.2

#include<iostream>
int main()
{
using namespace std;
char ch;
cout << "Type, and I shall repeat.\n";
cin.get(ch);
while (ch!='.')
    {
if (ch == '\n')
cout << ch;
else
cout << ++ch;
cin.get(ch);
    }

cout << "\nPlease excuse the slight confusion.\n";
    system("pause");
return 0;
}

 

if else中的两种操作都必须是一条语句，如果需要多条语句，需要用大括号将它们括起来，组成一个块语句。

2. if else if else结构

程序6.3

#include<iostream>
const int Fave = 27;
int main()
{
using namespace std;
int n;

cout << "Enter a number in the range 1-100 to find ";
cout << "my favorite number: ";
do
    {
cin >> n;
if (n < Fave)
cout << "Too low - - guess again: ";
else if (n > Fave)
cout << "Too high - - guess again: ";
else
cout << Fave << " is right!\n";
    } while (n != Fave);
    system("pause");
return 0;
}