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Help with writing function

chunkymonkey11 (24)
I am having some more troubles with functions. This function will calculate and return the X-coordinate of the vertex of the parabola. It takes two double arguments: the a-coefficient for the parabola and the b-coefficient for the parabola. It returns a double: the X-coordinate of the vertex of the parabola.

The formula for calculating the X-coordinate of the vertex is:

X-Vertex = -b-coefficient / ( 2 * a-coefficient)


xVertex = calcXvertex( aCoeff, bCoeff );

Here is what I have done so far (not sure if I'm right)
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double calcXvertex()
calcXvertex = -bCoeff / (2 *aCoeff)
xVertex = calcXvertex( aCoeff, bCoeff );
helios (17574)
chunkymonkey11 (24)
Thanks for the link (I read over it, and the C++ book), but that does not help me with the question above. I still have no clue how to tackle this problem.
helios (17574)
that does not help me with the question above
It should. The problem is that you're trying to define a function in a way that makes no syntactical sense.
chunkymonkey11 (24)
ok, so something like this:

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double calcXvertex ( double, double );
	{
	xVertex = -bCoeff / ( 2 * aCoeff);
	
	return xVertex;	
	}


helios (17574)
The parameter names are not optional.

EDIT: Also, you're not declaring xVertex.
Last edited on
chunkymonkey11 (24)
I declared xVertex prior to this chunk of code (sorry for not informing you).

I remember reading in the article that the parameter names are the values that will be involved in the function.

so I am just going to hit you with my code. So you can see the whole code (in full).
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#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

//*********************************************************************
//*
//*       Function Prototypes
//*
//*********************************************************************
double getA ();
double calcXvertex ( double, double );
double calcYvertex ( double, double, double );
void printConcavity ( double );
double calcDiscriminant ( double, double, double );
void printRoots ( double, double, double );



int main()
{
//*********************************************************************
//*
//* Variable Dictionary
//*
//* aCoeff, bCoeff, cCoeff  -  used to hold the coefficients
//*
//* xVertex  -  used to hold the X-coordinate of the vertex
//*
//* yVertex  -  used to hold the Y-coordinate of the vertex
//*
//*********************************************************************

double aCoeff, bCoeff, cCoeff;
double xVertex, yVertex;




//Setup the formatting

cout << fixed << setprecision(3);


//Get the A, B, and C coefficients from the user


double getA();
{
cout << "Enter the a coefficient (non -zero value:\n";
cin >> aCoeff;

while (aCoeff < 0)
	{
	cout << "Error: The a-value MUST be non-zero. Try Again:";
	cin >> aCoeff;
	} 
	
	return aCoeff;
	}
	

//Calculate the X and Y coordinates for the vertex of the parabola

double calcXvertex ( aCoeff, bCoeff );
	{
	xVertex = -bCoeff / ( 2 * aCoeff);
	
	return xVertex;	
	}

double calcYvertex ( double, double, double );
	{
	yVertex = ( aCoeff * (xVertex^2) ) + ( bCoeff * xVertex ) + cCoeff;
	
	return yVertex;	
	};


//Display the coefficients and vertices

cout << endl << "*******************************************"
     << endl << "        Quadratic Equation Analyzer"
     << endl << "*******************************************"
     << endl << "a Coefficient" << setw(30) << aCoeff
     << endl << "b Coefficient" << setw(30) << bCoeff
     << endl << "c Coefficient" << setw(30) << cCoeff
     << endl << "-------------------------------------------"
     << endl << "X Coordinate" << setw(31) << xVertex
     << endl << "Y Coordinate" << setw(31) << yVertex
     << endl << "-------------------------------------------";


//Display the concavity of the parabola

void printConcavity ( double );
	{
	if ( aCoeff > 0 )
		{
		cout << "Parabola opens UPWARD." << endl;	
		}	
	else 
		{
		cout << "Parabola opens DOWNWARD." << endl;	
		}	
		
	return 0;
	}


//Display the roots

cout << endl << "-------------------------------------------";

double calcDiscriminant ( double, double, double );
	{
	double discriminant;
	
	discriminant = (bCoeff^2 - 4) * (aCoeff * cCoeff);
	
	return discriminant;	
	}

void printRoots ( double, double, double );
	{
	if ( calcDiscriminant >	0 )
		{
		cout << "There are two real roots.";	
		}
	if else ( calcDiscriminant == 0 )
		{
		cout << "It has one root with a multiplicity of 2.";	
		}	
	else 
		{
		cout << "There are no real roots";	
		}	
	return 0;	
	}


cout << endl << endl;
system("pause");
return 0;
}


Do you think what I did in line 66 should be the status quo for every place on this script that has something like (double, double)? Also since I have given you the whole code, I might as well tell you that I have had some troubles compiling the thing, any clues why (except for the things you have already addressed)?

I really appreciated your help in dealing with me helios. I have spent hours writing this code, and I just want to go to bed feeling like I have accomplished something.
Last edited on
helios (17574)
Okay, well, there's many things wrong, here.

1. You can't define functions inside of other functions. main() is a function.
2. Nearly every procedural programming language makes a distinction between formal parameters and actual parameters (look it up). I can very well do this:
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int times2(int x){
    return 2*x;
}
//...
int foo=5;
int bar=times2(foo);
foo=times2(bar);
There doesn't need to be any relation between the expression you pass to a function and the name of the corresponding formal parameter. In fact, most of the time there won't be.
What this means is that the parameter name, as I said before, is not optional.
There is one exception, though: if you don't use the parameter in the function, you can leave it anonymous:
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int foo(int){
    return 2;
}
The parameter type is always mandatory.
3. There are many semicolons where they don't belong. The semicolon terminates statements. If you misplace a semicolon, you can radically change the meaning of a piece of code:
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int foo(int)
{
    return 2;
}
[code]Means something very different from
[code]
int foo(int);
{
    return 2;
}
Depending on the surrounding context, one or the other may compile and the other not. However, you'll practically never want the latter version.
4. In C/++, ^ is not exponentiation.
5. Logic error: The discriminant is b*b - 4*a*c, not (b*b-4)*a*c.
6. if else is not a valid sequence of tokens. The valid form is else if
7. If a function has return type void, the return statement should either not be there, or not be followed by an expression.
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void foo(){
}

void bar(){
    return;
}


I'll clean the code up for you, so you can see how it should be done:
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#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

// I'll omit the function declarations for stylistic reasons. Feel free to add
// them back if you want.

double getA(){
	double aCoeff;
	cout << "Enter the a coefficient (non -zero value:\n";
	cin >> aCoeff;

	while (aCoeff < 0){
		cout << "Error: The a-value MUST be non-zero. Try Again:";
		cin >> aCoeff;
	} 
	return aCoeff;
}

double calcXvertex(double aCoeff,double bCoeff){
	return -bCoeff / ( 2 * aCoeff);	
}

double calcYvertex(double aCoeff,double bCoeff,double cCoeff){
	double xVertex=calcXvertex(aCoeff,bCoeff);
	return aCoeff*xVertex*xVertex + bCoeff*xVertex + cCoeff;	
}

void printConcavity(double aCoeff){
	if (aCoeff>0)
		cout << "Parabola opens UPWARD." << endl;
	else
		cout << "Parabola opens DOWNWARD." << endl;
}

double calcDiscriminant(double a,double b,double c){
	return b*b - 4*a*c;	
}

void printRoots(double a,double b,double c){
	//The function has to actually be called to take effect:
	double discrimitant=calcDiscriminant(a,b,c);
	if (discrimitant>0)
		cout << "There are two real roots.";	
	else if (discrimitant==0)
		cout << "It has one root with a multiplicity of 2.";	
	else 
		cout << "There are no real roots";	
}

int main()
{
	double aCoeff, bCoeff, cCoeff;
	double xVertex, yVertex;

	cout << fixed << setprecision(3);

	//The function has to actually be called to take effect:
	aCoeff=getA();
	
	//TODO: get the other coefficients.
	
	xVertex=calcXvertex(aCoeff,bCoeff);
	yVertex=calcYvertex(aCoeff,bCoeff,cCoeff);

	//Display the coefficients and vertices
	cout << endl << "*******************************************"
		<< endl << "        Quadratic Equation Analyzer"
		<< endl << "*******************************************"
		<< endl << "a Coefficient" << setw(30) << aCoeff
		<< endl << "b Coefficient" << setw(30) << bCoeff
		<< endl << "c Coefficient" << setw(30) << cCoeff
		<< endl << "-------------------------------------------"
		<< endl << "X Coordinate" << setw(31) << xVertex
		<< endl << "Y Coordinate" << setw(31) << yVertex
		<< endl << "-------------------------------------------";


	//Once again, the function has to be called, or nothing happens.
	printConcavity(aCoeff);
	
	cout << endl << "-------------------------------------------";
	printRoots(aCoeff,bCoeff,cCoeff);
	
	cout << endl << endl;
	return 0;
}
Last edited on
chunkymonkey11 (24)
Thank you helios, I am left speechless. I took 30min+ to read this code over and over again to insure that I did not waste your time in teaching me. I never knew that functions could even be placed before main() like that.

I only have one thing to say: YOU ARE THE BEST :).
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