如何阻止程序跳过 getline？

发布于 2024-09-30 10:04:38 字数 660 浏览 0 评论 0原文

这是我的主程序，

int main () {

    string command;
    cin>>command;

    if(command == "keyword")
    {
        string str, str2, str3, str4;

        cout << "Enter first name: ";
        getline (cin,str);

        cout << "Enter last name: ";
        getline (cin,str2);

        cout << "Enter age: ";
        getline (cin,str3);

        cout<<"Enter country: ";
        getline (cin,str4);

        cout << "Thank you, " << str <<" "<<str2 <<" "<<str3<<" "<<str4<< ".\n";
    }
}

当输入关键字时，程序立即输出：

输入名字：输入姓氏：

完全绕过输入名字的能力。

原文

This is my main program,

int main () {

    string command;
    cin>>command;

    if(command == "keyword")
    {
        string str, str2, str3, str4;

        cout << "Enter first name: ";
        getline (cin,str);

        cout << "Enter last name: ";
        getline (cin,str2);

        cout << "Enter age: ";
        getline (cin,str3);

        cout<<"Enter country: ";
        getline (cin,str4);

        cout << "Thank you, " << str <<" "<<str2 <<" "<<str3<<" "<<str4<< ".\n";
    }
}

When keyword is entered, the program immediately outputs :

Enter first name: Enter last name:

completely bypassing the ability to enter the first name.

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困倦 2024-10-07 10:04:38

string command;
cin>>command;

在此之后，只需吃掉该行的末尾，

string restOfLine;
getline(cin, restOfLine);

否则，您输入命令的行中的 '\n' 不会被消耗，下一个读取行只会读取它。华泰

string command;
cin>>command;

after this just eat the end of the line

string restOfLine;
getline(cin, restOfLine);

Otherwise the '\n' in the line where you input command is not consumed and the next readline reads just it. HTH

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爱人如己 2024-10-07 10:04:38

cin>>命令不会从输入流中提取换行符（'\n'）；当您调用 getline() 时它仍然存在。因此，您需要对 getline() （或 ignore()）进行额外的虚拟调用来处理此问题。

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久而酒知 2024-10-07 10:04:38

正如其他人所提到的，问题在于，在读取命令时，您将行尾字符留在缓冲区中。除了 @Armen Tsirunyan 提出的替代方案之外，您还可以使用其他两种方法：

使用 std::istream::ignore 来实现：cin.ignore( 1024, '\n' ); （假设行的宽度不超过 1024 个字符。
只需将 cin >> command 替换为 getline( cin , command )。

只需第一个较弱（在行很长的情况下），第二个替代方案修改了语义，因为现在整个第一行（不仅仅是第一个单词）被作为命令处理，但这可能没问题，因为它允许您执行更严格的输入检查（命令按照第一个单词的要求拼写，并且命令行中没有额外的选项。

如果您有不同的命令集，并且有些命令可能需要参数，您可以一次性读取命令行，然后从那里读取命令和参数：

std::string commandline;
std::vector<std::string> parsed_command;
getline( cin, commandline );
std::istringstream cmdin( commandline );
std::copy( std::istream_iterator<std::string>(cmdin), std::istream_iterator(),
           std::back_inserter( parsed_command ) );
// Here parsed_command is a vector of word tokens from the first line: 
// parsed_command[0] is the command, parsed_command[1] ... are the arguments

As mentioned by others, the problem is that while reading the command you are leaving the end of line character in the buffer. Besides the alternative proposed by @Armen Tsirunyan, you can use two other approaches:

Use std::istream::ignore for that: cin.ignore( 1024, '\n' ); (assuming that lines will not be greater than 1024 characters in width.
Just replace cin >> command with getline( cin, command ).

Neither alternative requires creating an extra string, the first is weaker (in the event of very long lines), the second alternative modifies the semantics, as now the whole first line (not just the first word) is processed as the command, but this might be fine as it allows you to perform tighter input checking (the command is spelled as required in the first word, and there are no extra options in the command line.

If you have different set of commands and some might need an argument, you can read the command line in one pass, and then read the command and arguments from there:

std::string commandline;
std::vector<std::string> parsed_command;
getline( cin, commandline );
std::istringstream cmdin( commandline );
std::copy( std::istream_iterator<std::string>(cmdin), std::istream_iterator(),
           std::back_inserter( parsed_command ) );
// Here parsed_command is a vector of word tokens from the first line: 
// parsed_command[0] is the command, parsed_command[1] ... are the arguments

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