创建一个函数以在输入为负数或零时输出字符串.第一次执行用户定义的函数

我删除了这些问题。您混淆了类的实例和函数。函数不必初始化，如果实例在类之外，则必须初始化。我沿着代码写了一些评论。它仍然不漂亮，但至少它有效。

string FUNCTION(double)btw.表示函数只能或应该返回"字符串"。如果不返回任何内容，则函数的编写方式类似于void FUNCTION(double)。如果你的返回是一个字符串，你必须写一些东西来接收返回的字符串，比如：

#include <iostream>
/*
std::string returning_value;
returning_value = FUNCTION(1.0);
*/
//or
std::string FUNCTION(double function_a); //prototype of the function
//you need this if you write the function underneath the main() function
//The main function is returning "return 0" so since "0" is an "int" 
//meaning main is always "int main()" btw. because it is a function,
//just not some function but the "main function" thats called by the OS
int main()
{
std::string returning_value;
double a = 0.1; //initializing with 0.1
returning_value = FUNCTION(a);
std::cout << returning_value << std::endl;
//and in both cases the function would look like:
return 0;
}
std::string FUNCTION(double function_a)
{
std::string returning_value_a = "This is a string that will be returned";
if(function_a == 0.1)
{
returning_value_a = "This is another string";
}
return returning_value_a;
}

你的代码固定的数量最少，至少可以工作您可以测试如何获得您想要的正确输出。玩得开心：(希望我的答案能帮助你：(

#include <iostream>
#include <string>
#include <iomanip>
#include <cmath>
using namespace std;
void fix(double x)
{
string B_error = "B cannot be zero or negative. Please try again: ";
string H_error = "H cannot be zero or negative. Please try again: ";
string h_error = "b cannot be zero or negative. Please try again: ";
string b_error = "h cannot be zero or negative. Please try again: ";
string r_error = "r cannot be zero or negative. Please try again: ";
string y_error;
while (!(cin >> x))
{
if (cin.fail())
{
cout << "Erroneous input. Please try again:n";
cin.clear(); // used to prevent an endless loop if an input type is not an integer
cin.ignore(10000, 'n');
}
}
//warning: comparing floating point with == or != is unsafe
if (x == 'B')
{
y_error = B_error;

if (x <= 0)
{
do
{
cin >> x;
}
while (x <= 0);
}
}
}
int main()
{
int selection;
double I, B, H, b, h, r;
//You need to initialize the variables with a value
I = 1.0;
B = 1.0;
H = 1.0;
b = 1.0;
h = 1.0;
r = 1.0;
//functions don't need to be initialized, thats for Instanzes of classes
//double fix(double);
cout << "Please select the type of beam:n"
<< "1) I-Beamn"
<< "2) Rectangular Beamn"
<< "3) Cylindrical Beamn";
while (!(cin >> selection) || selection < 1 || selection > 3)
{
if (cin.fail() || selection < 1 || selection > 3)
{
cout << "Erroneous input. Please try again:n";
cin.clear(); // used to prevent an endless loop if an input type is not an integer
cin.ignore(10000, 'n');
}
}
switch (selection)
{
case 1:
cout << "You have selected I-beam. All inputs must be in inches.n"
<< "Please input the value for B: ";
fix(B);

cout << "Please input the value for H: ";
fix(H);
if (H <= 0)
{
do
{
cout << "H cannot be zero or negative. Please try again: ";
cin >> H;
}
while (H <= 0);
}
cout << "Please input the value for b: ";
fix(b);
if (b <= 0)
{
do
{
cout << "b cannot be zero or negative. Please try again: ";
cin >> b;
}
while (b <= 0);
}
else if (b > B)
{
do
{
cout << "b cannot be larger than B. Please try again: ";
cin >> b;
}
while (b > B);
}
cout << "Please input the value for h: ";
fix(h);
if (h <= 0)
{
do
{
cout << "h cannot be zero or negative. Please try again: ";
cin >> h;
}
while (h <= 0);
}
else if (h > H)
{
do
{
cout << "h cannot be larger than H. Please try again: ";
cin >> H;
}
while (h > H);
}
I = (B*H*H*H - b*h*h*h)/12.;
cout << "nResults for an I-beam with B = " << B
<< ", H = " << H << ", b = " << b << ", and h = " << h << endl;
cout << setfill('-') << setw(32) << "" << endl;
break;
case 2:
cout << "You have selected rectangular beam. All inputs must be in inches.n"
<< "Please input the value for b: ";
fix(b);
if (b <= 0)
{
do
{
cout << "b cannot be zero or negative. Please try again: ";
cin >> b;
}
while (b <= 0);
}
cout << "Please input the value for h: ";
fix(h);
if (h <= 0)
{
do
{
cout << "h cannot be zero or negative. Please try again: ";
cin >> h;
}
while (h <= 0);
}
I = b*h*h*h/12.;
cout << "nResults for a rectangular beam with b = " << b << " and h = " << h << endl;
cout << setfill('-') << setw(32) << "" << endl;
break;
case 3:
cout << "You have selected cylindrical beam. All inputs must be in inches.n"
<< "Please input the value of r: ";
fix(r);
if (r <= 0)
{
do
{
cout << "r cannot be zero or negative. Please try again: ";
cin >> r;
}
while (r <= 0);
}
I = M_PI*pow(r,4)/4.;
cout << "nResults for a cylindrical beam with r = " << r << endl;
cout << setfill('-') << setw(32) << "" << endl;
break;
}

cout << "The value of the moment of inertia for this beam is: " << I << "in^4" << "nn";
return 0;
}

所以，我不得不添加另一个函数来检查B<b和H<h，并在fix函数中的while循环之后添加一个do-while循环。

这是我的修复代码：

#include <iostream>
#include <string>
#include <iomanip>
#include <cmath>
using namespace std;
void fix(double &x)
{
while (!(cin >> x))
{
if (cin.fail())
{
cout << "Erroneous input. Please try again:n";
cin.clear(); // used to prevent an endless loop if an input type is not a         double
cin.ignore(10000, 'n');
}
}
string return_x = "This cannot be zero or negative. Please try again: ";
while (x <= 0)
{
cout << return_x << endl;
cin >> x;
}
}
void fix2(double &x, double &y)
{
while (x < y)
{
cout << x << " cannot be less than " << y << endl;
fix(y);
}
}
int main()
{
int selection;
double I = 1.0;
double B = 1.0;
double H = 1.0;
double b = 1.0;
double h = 1.0;
double r = 1.0;
cout << "Please select the type of beam:n"
<< "1) I-Beamn"
<< "2) Rectangular Beamn"
<< "3) Cylindrical Beamn";
while (!(cin >> selection) || selection < 1 || selection > 3)
{
if (cin.fail() || selection < 1 || selection > 3)
{
cout << "Erroneous input. Please try again:n";
cin.clear(); // used to prevent an endless loop if an input type is not an integer
cin.ignore(10000, 'n');
}
}
switch (selection)
{
case 1:
cout << "You have selected I-beam. All inputs must be in inches.n"
<< "Please input the value for B: ";
fix(B);
cout << "Please input the value for H: ";
fix(H);
cout << "Please input the value for b: ";
fix(b);
fix2(B, b);
cout << "Please input the value for h: ";
fix(h);
fix2(H, h);
I = (B * H * H * H - b * h * h * h) / 12.;
cout << "nResults for an I-beam with B = " << B << ", H = " << H
<< ", b = " << b << ", and h = " << h << endl;
cout << setfill('-') << setw(32) << "" << endl;
break;
case 2:
cout << "You have selected rectangular beam. All inputs must be in inches.n"
<< "Please input the value for b: ";
fix(b);
cout << "Please input the value for h: ";
fix(h);
I = b * h * h * h / 12.;
cout << "nResults for a rectangular beam with b = " << b
<< " and h = " << h << endl;
cout << setfill('-') << setw(32) << "" << endl;
break;
case 3:
cout << "You have selected cylindrical beam. All inputs must be in inches.n"
<< "Please input the value of r: ";
fix(r);
I = M_PI * pow(r, 4) / 4.;
cout << "nResults for a cylindrical beam with r = " << r << endl;
cout << setfill('-') << setw(32) << "" << endl;
break;
}
cout << "The value of the moment of inertia for this beam is: " << I
<< "in^4" << "nn";
return 0;
}