c++ - 尝试读取失败后 cin 读数为零

Question

我希望用户输入 vector<double> a_ 的值直到没有 double 值被读取。我想制作一个名为 b_ 的 double vector 它的大小取决于 a_的大小，其实是一模一样的。 b_ 的初始化必须靠近a_在里面。但是 b_ 有一些问题，我不明白。 b_ 的所有值都设置为 0。

代码:

#include <iostream>
#include <vector>

using namespace std;

int main() {
    vector<double> a_, b_;
    double tmp;
    while(cin >> tmp) {
        a_.push_back(tmp);
    }
    cout << "A size = " << a_.size() << endl;
    for(int i = 0; i < a_.size(); i++) {
        cin >> tmp;
        b_.push_back(tmp);
    }
    for(int i = 0; i < a_.size(); i ++) { 
        cout << "a[" << i <<"] = " <<a_[i];
        cout << " " <<"b[" << i <<"] = " <<b_[i] << endl;
    }
    return 0;
}

输入是:

4
5
6
e
7
8
9

输出是:

A size = 3
a[0] = 4 b[0] = 0
a[1] = 5 b[1] = 0
a[2] = 6 b[2] = 0

Answer 1

我通过评论添加了一些解释:

while(cin >> tmp) {
    a_.push_back(tmp); // read 4, 5, 6 and push_back them
}                      // the loop ends when extraction fails for reading e

...

for(int i = 0; i < a_.size(); i++) {
    cin >> tmp;        // extraction fails for reading e everytime, 
                       // and tmp will be set to 0.0
    b_.push_back(tmp); // push_back 0.0 everytime
}

注意根据std::basic_istream::operator>>的行为,

If extraction fails, zero is written to value and failbit is set. If extraction results in the value too large or too small to fit in value, std::numeric_limits::max() or std::numeric_limits::min() is written and failbit flag is set.

设置failbit后，如果想继续阅读，需要清除failbit并丢弃导致错误的字符。如:

// ...
cin.clear();
cin.ignore();
for(int i = 0; i < a_.size(); i++) {
    cin >> tmp;
    b_.push_back(tmp);
}
// ...

然后输入:

4
5
6
e
7
8
9

你会得到

A size = 3
a[0] = 4 b[0] = 7
a[1] = 5 b[1] = 8
a[2] = 6 b[2] = 9

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