c++ - sscanf 无法将字符串拆分为 double 值

Question

我不知道我在这里犯了什么错误。但 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;
}

我暂时只运行一次循环。这意味着 i=0 就这样了..

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，但不是 %f。我是这样做的，但正在玩代码。解决此问题后，我得到以下输出:

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

我首先这样做了，问题是，当我打印这些 double 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

您在 sscanf 中使用了错误的格式对于 doubles ，应该是%lf而不是%f 。在启用警告的情况下进行编译，例如 g++ -Wall -W会优雅地捕获这个语法错误。 sscanf()按照指示解析 float 并将其存储为 floats进入程序实际使用的内存 doubles 。这会调用未定义的行为。

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

此外，这个sscanf()的返回值调用应与 5 进行比较，不是1 。 sscanf返回成功转换的字段数。这就是你的程序输出 value was not ok 的原因。 .

关于输出，数字输出像整数，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，因为您使用 sscanf()以及。存在一些更复杂且恕我直言构思不良的 API 来控制 iostream 和 << 的转换。运算符(operator)。我强烈建议不要使用这些，因为它们对cout有副作用。不容易恢复。