调用后缺少函数值

正如你的教授所说，请不要使用全局变量。在你的代码中，你有不同的变量volume，我怀疑你知道你什么时候访问哪个变量。让我们浏览一下您的代码:

 double volume;    // global variable, never written, read in function charge
 int main() {
    ...
    double volume; // local variable, only visible in function main, but never used
    ...
 }
 double calculate(double length, double width, double height) {
    double volume; // local variable, only visible in calculate
    volume = ...;
    return volume;
 }
 double charge(ifstream & inFile, ofstream & prt) {
    ...
    if(volume > 7) // no local variable volume defined, so using global variable volume
    ...
 }

同样，你声明了一个函数calculate，但你从不调用它。

请考虑启用尽可能多的编译器警告。

试试这样做。问题是你需要在调用它之前声明你的函数原型，但是如果你把你的主函数放在。c文件的底部，你不需要声明原型函数，希望它是清晰的

#include <iostream> 
#include <iomanip> 
#include <cmath> 
#include <fstream> 

using namespace std;  

void header(int lab_number, char lab_part)
{ 
    cout << "Kevin Schultzn"; 
    cout << "Lab" << lab_number << lab_part << endl << endl; 
} 

double length, width, height, x, weight, total = 0, shipping_cost, volume;
double calculate(double length, double width, double height)
{ 
    double volume; 
    volume = length * width * height; 
    return volume; 
} 
double charge(ifstream & inFile, ofstream & prt) 
{
            const double basic_charge = 12;
            const double Vsurcharge = 5;
            const double Dsurcharge = 4;
            const double Wsurcharge = 2;
            double netWeight = 0;
            while (!inFile.eof())  
            { 
                shipping_cost = basic_charge; 
                if (volume > 7)  
                    shipping_cost += Vsurcharge; 
                if (length > 3 || width > 3 || height > 3) 
                    shipping_cost += Dsurcharge; 
                if (weight > 50) 
                {
                    netWeight = weight - 50;
                    shipping_cost += netWeight * Wsurcharge; 
                }
                total += shipping_cost; 
                prt << setw(4) << right << setprecision(1) << length << setw(6) << right << setprecision(1) << width << setw(6) << right << setprecision(1) << height << setw(8) << right; 
                prt << weight << setw(5) << right << setprecision(2) << fixed << "$" << shipping_cost << endl; 
                inFile >> length; 
                inFile >> width; 
                inFile >> height; 
                inFile >> weight; 
            }
     return total;
}
int main () 
{ 
    double volume = calculate(length, width, height);
    double chargeAmount;

    ifstream inFile; 
    ofstream prt("lab7new_out.txt"); 
    header(7, 'A'); 
    prt << "        S & S Global Servicesn";
    prt << "    Shipping Cost Analysis Reportnn"; 
    prt << "Length Width Height Weight Shippingn"; 
    prt << "                             Costnn"; 
    inFile.open("c:\lab7\pkg.txt"); 
        if (!inFile) 
        cout << "Error opening the filen"; 
    inFile >> length; 
    inFile >> width; 
    inFile >> height; 
    inFile >> weight; 
    chargeAmount = charge(inFile, prt);

    prt << "------------------------------------" << endl;
    prt << "nTotal cost: $" << total; 
    system("pause"); 
    return 0; 
}