sscanf 无法将字符串拆分为双精度值

Question

我不知道我在这里犯了什么错误。但是 sscanf 没有填充我的双数组（第一个索引）中的值。这是代码

int main() {
    int n = 0;
    cout << "Enter the number of equations" << endl;
    cin >> n;

    string coeffData;
    string powerData;
    double m_coeff_X[5] = {0.0,0.0,0.0,0.0,0.0};
    double m_coeff_Y[5] = {0.0,0.0,0.0,0.0,0.0};
    double m_coeff_Z[5] = {0.0,0.0,0.0,0.0,0.0};
    double m_coeff_V[5] = {0.0,0.0,0.0,0.0,0.0};
    double m_coeff_W[5] = {0.0,0.0,0.0,0.0,0.0};
    double m_const[5] = {0.0,0.0,0.0,0.0,0.0};

    for (int i = 0; i < n; i++) {
        cout << "Enter the coefficients for Number " << i+1 <<
                " equation. x,y,z,v,w (separated by commas)" << endl;
        cin >> coeffData;

        if (sscanf(coeffData.c_str(), "%f,%f,%f,%f,%f", 
            &m_coeff_X[i], &m_coeff_Y[i], &m_coeff_Z[i],
            &m_coeff_V[i], &m_coeff_W[i]) == 1) {
            cout << "value was ok";
        } else {
            cout << "value was not ok";
        }
        cout << m_coeff_X[i] << endl;
        cout << m_coeff_Y[i] << endl;
        cout << m_coeff_Z[i] << endl;
        cout << m_coeff_V[i] << endl;
        cout << m_coeff_W[i] << endl;
    }
    return 0;
}

我运行暂时只循环一次。这意味着 i=0 仅此而已..

输出为：

Enter the coefficients for Number 1 equation. x,y,z,v,w (separated by commas)
1.0,2.0,3.0,4.0,5.0
value was not ok
5.26354e-315
5.30499e-315
5.32571e-315
5.34643e-315
5.3568e-315

编辑：感谢您提醒我它是 %lf 而不是 %f。我是这样做的，但正在玩代码。解决这个问题后，我得到这个输出：

Enter the coefficients for Number 1 equation. x,y,z,v,w (separated by commas)
1.0,2.0,3.0,4.0,5.0
value was not ok
1
2
3
4
5

我首先这样做，问题是，当我打印这些双打时，它们打印为整数，我很困惑为什么会这样，我在这里做错了什么？

Answer 1

对于 double 你需要使用 %lf 而不是 %f

Answer 2

您在 sscanf 中为 doubles 使用了错误的格式，应该是 %lf 而不是 %f。在启用警告的情况下进行编译，例如使用 g++ -Wall -W 会优雅地捕获此语法错误。 sscanf() 解析浮点数，按照指示将它们作为floats 存储到程序实际使用的内存中作为doubles。这会调用未定义的行为。

您的程序可能会崩溃。这里似乎在这些 double 变量中存储了无意义的值。更准确地说，它只修改了 double 变量的一半字节。 floats 和 doubles 通常在内存中有不同的表示。

此外，此 sscanf() 调用的 return 值应与 5 进行比较，而不是 1。sscanf returns成功转换的字段数。这就是为什么你的程序输出 value was not ok.

的原因

关于输出，数字像整数一样输出，因为它们具有整数值。您可以使用 snprintf() 控制将数字转换为字符串的方式。这样做是一致的，因为您也使用 sscanf()。存在一些更复杂和恕我直言构思不当的 API 来控制 iostream 和 << 运算符的转换。我强烈建议不要使用这些，因为它们对 cout 有副作用，而且不容易恢复。

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