Question

所以我试图从C中的函数输出结构。到目前为止的代码是：

//Step 1: Create a volume function that outputs a volume structure
//Step 2: Input boat dimensions from main
//Step 3: Use outputted volume values to calculate centre of gravity/buoyancy
//Step 4: Use values 

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

//Struct to fill with the values calculated in the submerged_volume function
struct vol {
    double V, Uc, Vc;
    };

struct vol submerged_volume(double L1, double L2, double Lavg, double H) {

    double C, delta, a, b, d;
    double theta, theta_rad, theta_min, theta_lim, theta_lim2, theta_lim_deg;
    double Ug1, Ug2, Vg1, Vg2, V1, V2;
    double pi; 

    pi = 4*atan(1);
    C = sqrt(L1*L1 + L2*L2);
    delta = acos(L1/C);

    theta_lim = asin(H/L1);
    theta_lim_deg = (theta_lim/pi) * 180.0;
    theta_min = asin(H/C) - delta; 
    theta_lim2 = 0;

    //Calling the structure to fill with values
    struct vol volume;


    double V_sub, Uc_sub, Vc_sub;
    V_sub = 0;

    //Volume calculations

    for (theta == 0; theta <= 10; theta = theta + 0.5) {
        theta_rad = (theta * pi)/180.0;
        //if (H > L2) { 
        printf(" H > L2\n");                          //Case where H > L2
            if (theta_rad > theta_lim) {
                V_sub = Lavg * L2 * (L2/(2.0 * tan(theta_rad)) + L1 - H/sin(theta_rad));
                //Case of triangle plus rectangle
                V1 = Lavg * L2 * L2/(2.0 * tan(theta_rad));     
                V2 = Lavg * L2 * (L1 - H/sin(theta_rad));
                Ug1 = -(H/sin(theta_rad) - L2/tan(theta_rad) + L2 * (cos(theta_rad)/(3.0*sin(theta_rad/2.0))));
                Vg1 = -(L2 - sin(theta_rad/2.0) * (L2/(3.0 * sin(theta_rad/2.0))));
                Ug2 = -(L1 + H/sin(theta_rad))/2.0;
                Vg2 = -L2/2.0;           //b 
                }
            else if (theta_rad > theta_min) {
                V_sub = Lavg * tan(theta_rad)/2.0 * pow((L1 - L2 * tan(theta_rad) - ((H - L2/cos(theta_rad))/sin(theta_rad))), 2);
                //Case of a triangle only 
                V1 = V_sub;
                V2 = 0; 
                Ug1 = -1.0/3.0 * (2.0 * L1 + L2 * tan(theta_rad) + (H - L2/cos(theta_rad))/sin(theta_rad));
                Vg1 = -(L2 - tan(theta_rad)/3.0 * (L1 - (H - L2/cos(theta_rad))/sin(theta_rad) - L2 * tan(theta_rad)));
                }
            else {
                V_sub = 0; 
                }
            //}

            if (V_sub != 0) {
            Uc_sub = Ug2 - V1/(V1 + V2) * (Ug2 - Ug1);
            Vc_sub = Vg2 - V1/(V1 + V2) * (Vg2 - Vg1);
            //moment = m * g * (b*sin(theta_rad) - a*cos(theta_rad)) + (Uc * cos(theta_rad) - Vc * sin(theta_rad)) * Fa - d * Fm;
            //fN = -(f * Fm * cos(theta_rad));
            //friction = m * g - Fa - Fm * sin(theta_rad);
            }
        }
    volume.V = V_sub;
    volume.Uc = Uc_sub;
    volume.Vc = Vc_sub;



    /*
    volume.V = 110;
    volume.Uc = 10;
    volume.Vc = 10;
    */

    return volume;
    }

int main() {

    double L1, L2, Lavg, H;
    struct vol volume; 

    printf("Enter L1: \n");
    scanf("%lf", &L1);
    printf("Enter L2: \n");
    scanf("%lf", &L2);
    printf("Enter Lavg: \n");
    scanf("%lf", &Lavg);
    printf("Enter H: \n");
    scanf("%lf", &H); 

    volume = submerged_volume(L1, L2, Lavg, H);

    printf("V = %lf\nUc = %lf\nVc = %lf\n", volume.V, volume.Uc, volume.Vc);

    return 0;
    }

我首先尝试使用theta（theta == 5）的一个设定值，这非常有效。我不知道我是否能做到这一点但是我基本上想要为theta经历for循环时为每个θ值设置V，Uc和Vc的值。这可能吗？我希望这可以解释好，如果需要更多细节，请告诉我。提前致谢！

Answer 1

我认为实现这一目标的正确方法是通过引用或通过指针将输出结构传递给函数：

// Declare
void submerged_volume(double L1, double L2, double Lavg, double H, struct vol& out);

// Use
struct vol volume;
submerged_volume(L1, L2, Lavg, H, volume);

或通过指针：

// Declare
void submerged_volume(double L1, double L2, double Lavg, double H, struct vol *out);

// Use
struct vol volume;
submerged_volume(L1, L2, Lavg, H, &volume);

Answer 2

第1点：您的代码至少有一个错误，这可能是导致问题的原因。快速汇编给了我以下警告：

 In function 'submerged_volume':
40: warning: statement with no effect
40: warning: 'theta' is used uninitialized in this function
35: warning: 'Vc_sub' may be used uninitialized in this function
35: warning: 'Uc_sub' may be used uninitialized in this function
19: warning: 'Vg2' may be used uninitialized in this function
19: warning: 'Ug2' may be used uninitialized in this function

特别是第40行：

for (theta == 0; theta <= 10; theta = theta + 0.5) {
           ^^
           want "=" here

第2点：顺便说一句，您应该忽略有关submerged_volume使用指针的建议而不是返回struct，原因有两个。

这里的结构非常小，没有区别。
编译器可能会消除不必要的复制（例如，通过隐式使用指针）。

第3点：OP明确表示他/她希望返回结构的数组。在这种情况下，我们做需要添加指针，因为数组不是C中的“第一类对象”。例如：

在main()：

 struct vol volume[21];
 // ...
 submerged_volume(L1, L2, Lavg, H, volume);
 // ...

在submerged_volume()：

 void submerged_volume(double L1, double L2, double Lavg, double H, struct vol out[])
 // ...
 int i = theta * 2;
 // ...
 out[i].V  = V_sub;  // at bottom of loop
 out[i].Uc = Uc_sub;
 out[i].Vc = Vc_sub;

或者使用或许更常见的习语：

 void submerged_volume(double L1, double L2, double Lavg, double H, struct vol *out)
 // ...
 out->V  = V_sub;  // at bottom of loop
 out->Uc = Uc_sub;
 out->Vc = Vc_sub;
 out++;

（注意我在这里使用硬编码的21是一个坏主意。）

输出从函数返回结构

2 个答案: