# Intro to C++, NEED HELP!!!

• 06-10-2003
romeoz
Intro to C++, NEED HELP!!!
I have to write a program to calculate the diameter, the circumference, and the area of a circle with a radius of 6.75. Please tell me what I am doing wrong...I am going on 5 hours, and I am lost:-( Any help would be greatly appreciated.

//************************************************
//Calculations of a circle program
//This program finds the diameter, circumference
//and the area of a circle with a preset radius.
//************************************************

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

using namespace std;

float pi ;

int main()

{

pi = 3.14159; //Size of PI

cout << "The radius of a circle is : "<< radius << endl;

circumference = diameter * pi;

cout << "The diameter is " << diameter << endl;
cout << "The circumference is " << circumference << endl;
cout << "The area is " << area << endl;

return 0;
}
• 06-10-2003
revelation437
float pi

forgot the ; ?
• 06-10-2003
revelation437
diameter
circumference
area

u dont declare those
• 06-10-2003
romeoz
that was my bad...I put them in..what do u mean I dont declare them...how do I do that? I thought if I put = something then that is declaring them...I think I am lost.. Thanks
• 06-10-2003
Zach L.
To declare a variable, you need to give it a type - i.e.:

double diameter = ...;
float x;
... etc ...
• 06-10-2003
Mister C
try something like

Code:

``` const double RADIUS = 6.75; const double PI = 3.14159; double diameter, circumference, area; etc.```
you need to declare variables before you use them. radius and PI would make better constants since they are not likely to change.
• 06-11-2003
o0obruceleeo0o
or you can do this...
Code:

```double diameter = 2*radius; double circumference = diameter * pi; double area = pi * radius * radius;```
• 06-11-2003
romeoz
I put the following:
const doble PI = 3.14159;

and

double diameter, double circumference, double area and sure enough it worked. Thanks for the help.

I read Chapter 3 over again today and I dont know why I needed to add double in front of those to make it work...could someone explain that to me, I would really appreciate it alot.

Bryan
• 06-11-2003
Zach L.
C++ is a strongly typed language. It makes sure everything has a type, type promotion rules are followed closely (i.e. the result of int+double=double), and no type is used in an improper way.

As a result of all of this, it needs to know the type of a variable when you create it.
• 06-11-2003
romeoz
I know you are probably repaeting yourself, but what does "double" do for the variable? thanks

Bryan
• 06-11-2003
Zach L.
'double' is a data type. It lets the compiler know that that variable will be storing a value to be represented in a specific way. 'double' can hold floating-point numbers (ones with decimals), as can 'float' s (though a float is smaller).
• 06-11-2003
o0obruceleeo0o
double is a type of variable that can store decimals.... and larger/smaller numbers that float can (not sure exactly how many)
• 06-11-2003
romeoz
so anytime I want to store decimals I should put const double in front it the variable...I think I am getting it. I am taking this onlline at UMUC and its very hard doing this online because there is not much help. I appreciate all you guys that gave me help, I hope you can continue to help me throughout the semester:-)

Bryan
• 06-11-2003
Zach L.
Actually, the 'const' modifier just means that its a constant, which cannot be changed in the program. If you want to manipulate the decimal, just use 'double'.

Cheers
• 06-11-2003
o0obruceleeo0o
yeah, the const means that the variable cannot be changed anytime in the program after you declare it. For example:
Code:

```#include <iostream> using namespace std; int main() {     int a=5;     const int b=3;  //Declares a constant integer     cout << a << endl;     cout << n << endl;     a=7;                //no problem, this one isnt const     b=4;                //Will get an error for trying to change the  value of a constant     cout << a << endl;     cout << b << endl; //erase the const infront of int b and it will compile to see that both a and b can change :D     return 0; }```