Array Problem

[vc98]

Hello I am writing a program that gets the minimum(xmin) and maximum value(xmax) of x from the user and then evaluates an equation over the range of values between the minimum value and maximum value.
Equation: f(x) = 0.0572*cos(4.667*x) + 0.0218*pi*cos(12.22*x);
Then the program has to display a table showing the x values and y values. I need to have at least 20 values, so assuming the user enters xmin as -2 and xmax as 2 and the number of values should be 21, the increment will be 0.2.
This is what i have so far:

#include <iostream>
#include <cmath>
using namespace std;

int main()
{
	const int rows = 20;
	const double pi = 3.1416;
	double xmin, xmax, x, y, increment, value, result, a;  
	
	cout << "Enter the minimum value of x";
	cin >> xmin; 
	cout << "Enter the maximum value of x"; n h
	cin >> xmax; 
	
	increment = (xmax - xmin) / rows; 
	
	for ( a = 1; a <= 20; a++);
	{
		y = 0.0572 * cos(4.667 * x) + 0.0218 * pi * cos(12.22 * x);
		result = value[a];
		x+= increment; 
	}
}

I am just having problems storing the 20 numbers in an array because when i execute the program i get the error "invalid types 'double[double]' for array subscript".

Any help is very much appreciated.

[naaissus]

I haven't understood your clearly so I can help you only to compile the program.

a should be declared as int because index of particular element of array can not be double.
You haven't declared value as an array, it should be double ... value[20]
This will fix that error.

Indexing starts from 0 so for loop should look like this for (a = 0; a < 20; a++);

[TheIdeasMan]

Also, variable x is undefined...

This is a golden rule in coding, always make sure variables are assigned some value, preferably upon declaration.

[vc98]

Thanks for the replies. I made the necessary changes and the error is not showing up now. Now I have to enter the 20 pairs of x and y values into a table. This is my updated code:

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

int main()
{
	const int rows = 20;
	const double pi = 3.1416;
	double xmin, xmax, y, increment;
	double value[20], xval[20];
	int a;   
	
	cout << "Enter the minimum value of x: ";
	cin >> xmin; 
	cout << "Enter the maximum value of x: "; 
	cin >> xmax; 
	
	double x = xmin; 
	increment = (xmax - xmin) / (rows - 1); 
	
	for ( a = 0; a < 20; a++);
	{
		y = 0.0572 * cos(4.667 * x) + 0.0218 * pi * cos(12.22 * x);
		y = value[a];
		x = xval[a]; 
		x+= increment;
	}
	
	cout << "X-Value  |  Y-Value" << endl; 
	cout << "_________|__________" << endl; 
	cout << setw(6) << xval[0] << setw(15) << value[0] << endl;
	cout << setw(6) << xval[1] << setw(15) << value[1] << endl;
	cout << setw(6) << xval[2] << setw(15) << value[2] << endl; 
	cout << setw(6) << xval[3] << setw(15) << value[3] << endl; 
	cout << setw(6) << xval[4]<< setw(15) << value[4] << endl; 
	cout << setw(6) << xval[5] << setw(15) << value[5] << endl; 
	cout << setw(6) << xval[6] << setw(15) << value[6] << endl; 
	cout << setw(6) << xval[7] << setw(15) << value[7] << endl; 
	cout << setw(6) << xval[8] << setw(15) << value[8] << endl; 
	cout << setw(6) << xval[9] << setw(15) << value[9] << endl; 
	cout << setw(6) << xval[10] << setw(15) << value[10] << endl; 
	cout << setw(6) << xval[11] << setw(15) << value[11] << endl; 
	cout << setw(6) << xval[12] << setw(15) << value[12] << endl; 
	cout << setw(6) << xval[13] << setw(15) << value[13] << endl; 
	cout << setw(6) << xval[14] << setw(15) << value[14] << endl; 
	cout << setw(6) << xval[15] << setw(15) << value[15] << endl; 
	cout << setw(6) << xval[16] << setw(15) << value[16] << endl; 
	cout << setw(6) << xval[17] << setw(15) << value[17] << endl; 
	cout << setw(6) << xval[18] << setw(15) << value[18] << endl;  
	cout << setw(6) << xval[19] << setw(15) << value[19] << endl;  
	cout << setw(6) << xval[20] << setw(15) << value[20] << endl; 

return 0;  
}

The values in the table are coming out incorrectly. What am I doing wrong?

[TheIdeasMan]

Hi,

Lines 25 & 26 aren't right, the arrays are not initialised, are you sure you didn't mean:

	for ( a = 0; a < 20; a++);
	{
                // would yvalue be a better name?
		value[a] = 0.0572 * cos(4.667 * x) + 0.0218 * pi * cos(12.22 * x);
		//y = value[a];            
		//x = xval[a]; 
                xval[a] = x; // would xvalue be a better name?
		x+= increment;
	}

Are the arguments to the trig functions in radians? Is 4dp in the value of pi sufficient? Try 3.14159265358979323846

Also, consider using a for loop to print the table, and put that into a function.

Had these warnings:

52:53: warning: array subscript is above array bounds [-Warray-bounds] 
52:28: warning: array subscript is above array bounds [-Warray-bounds]

Arrays go from 0 to size-1, in this case 0 to 19. These problems are avoided if you use a for loop correctly.

Also, avoid magic numbers like 20 in your code, make it a const variable instead:

const unsigned int ArraySize = 20; //could use std::size_t here, as the type.

Just noticed you had rows which is the same thing, but you haven't used it as much as you should have.

[vc98]

Hi TheIdeasMan :D

I am required to use the value 3.1416 for pi.
Other than that I followed the things you said but I am still getting incorrect output from my program. Here is the updated version:

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

int main()
{
	const int rows = 20;
	const double pi = 3.1416;
	double xmin, xmax, y, increment;
	double yvalue[rows], xvalue[rows];
	int a;   
	
	cout << "Enter the minimum value of x: ";
	cin >> xmin; 
	cout << "Enter the maximum value of x: "; 
	cin >> xmax; 
	
	double x = xmin; 
	increment = (xmax - xmin) / (rows - 1); 
	
	for ( a = 0; a < rows; a++);
	{
		yvalue[a] = 0.0572 * cos(4.667 * x) + 0.0218 * pi * cos(12.22 * x);
		xvalue[a] = x;
		x+= increment;
	}
	
	cout << "X-Value  |  Y-Value" << endl; 
	cout << "_________|__________" << endl; 
	
	for (int j = 0; j < rows; j++)
	{
		cout << setw(6) << xvalue[j] << setw(6) << yvalue[j] << endl; 
		j++;
	} 

return 0;  
}

[TheIdeasMan]

Ok, just spotted the pesky semicolon on line 22 :+)