Question

我不知道我在这里犯了什么错误。但是sscanf并没有填满我的双数组（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，那就是所有..

输出结果为：

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，但是正在玩代码。解决这个问题后，我得到了这个输出：

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，事实是，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，您需要使用％lf而不是％f

Answer 2

sscanf doubles使用的格式错误，%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()按照指示解析浮点数，并将它们作为floats存储到程序实际用作doubles的内存中。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变量中存储无意义的值。更准确地说，它只修改double变量的一半字节。 floats和doubles通常在内存中有不同的表示形式。

此外，此sscanf()调用的返回值应与5进行比较，而不是1。sscanf返回成功转换的字段数。这就是您的计划输出value was not ok。

的原因

关于输出，数字输出像整数一样，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()。存在一些更复杂的IMHO错误的API来控制iostream和<<运算符的转换。我会强烈反对这些因为它们对cout的副作用不容易回复。

sscanf在将字符串拆分为double值时失败

2 个答案: