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c++ - 表达式必须有常量值，变量不能用作定义数组大小的常量

问题描述

我正在尝试生成此代码，但在double从double b[n];. 我得到的错误是说“表达式必须有一个常量值，变量'n'不能用作常量。您可以提供的任何帮助将不胜感激。 

//Get inputs from user

double V = 0;    // shear load applied
int n;
double H_total = 0;
double A_total = 0;
double a = 0;
double I = 0;
double t = 0;
double e = 0;
double y_bar = 0;

cout << "Input the shear load applied in [N]: " << endl;
cin >> V;

cout << "Input number of sections: " << endl;
cin >> n;

double b[n];
double h[n];
double A[n];
double y[n];
double Q[n];
double Tau[n];
for (int i = 1; i <= n; i++) { // Calculates variables to find shear stress

    cout << "Width of section " << i << " in [mm]: " << endl;
    cin >> b[i];

    cout << "Height of section " << i << " in [mm]: " << endl;
    cin >> h[i];

    H_total += h[i];
    A[i] = b[i] * h[i];
    A_total += A[i];
    y[i] = H_total - 0.5 * h[i];
    a += A[i] * y[i];
    y_bar = a / A_total;

}

cout << "Applied shear force, V = " << V / 1000 << " kN" << endl;
cout << "Y coordinate of the centroid for given cross section, Y_Bar = " << y_bar << " mm" << endl;

for (int i = 1; i <= n; i++) { // Finds moment of inertia

    double d = (y[i] - y_bar);

    I += (b[i] * pow(h[i], 3.0) / 12.0) + (A[i] * pow(d, 2.0));

}

cout << "Moment of Inertia, I = " << I << " mm^4" << endl;

for (int i = 1; i <= n; i++) { // Calculates first moment of inertia

    Q[i] = A[i] * (y[i] - y_bar);

}

for (int i = 1; i <= n - 1; i++) {

    if (b[i] <= b[i + 1]) {

        t = b[i];

    }
    else {

        t = b[i + 1];

    }

    Tau[i] = (abs(V * Q[i]) / (I * t));

}

for (int i = 1; i <= n - 1; i++) {

    if (i <= 2) {

        e += Tau[i];

    }
    else {

        e -= Tau[i];

    }
    cout << "Shear stress between sections " << i << " and " << i + 1 << " = " << e << " MPa" << 
endl;

}
}

标签： c++debugging

解决方案

首先double b[n];不是一个函数，它是一个数组。此错误在二维数组中很常见。但是，您在这里没有使用任何二维数组。此外，除非您提供导致此错误的特定输入，否则您的代码没有错误。您可以看到一些随机输入的输出：

Applied shear force, V = 0.004 kN
Y coordinate of the centroid for given cross section, Y_Bar = 3.29661 mm
Moment of Inertia, I = 322.476 mm^4
Shear stress between sections 1 and 2 = 0.147082 MPa
Shear stress between sections 2 and 3 = 0.231598 MPa

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