Compiler error : 'area' not declared in this scope - c++

Question

I've just started learning c++ and am trying to write a program for finding the area of a circle. I've written the program and whenever I try to compile it I get 2 error messages. The first is:
areaofcircle.cpp:9:14: error: expected unqualified-id before numeric constant
and the second is:
areaofcircle.cpp:18:5: error: 'area' was not declared in this scope
What should I do? I would post a picture, but I'm a new user, so I can't.
#include <iostream>
using namespace std;
#define pi 3.1415926535897932384626433832795
int main()
{
// Create three float variable values: r, pi, area
float r, pi, area;
cout << "This program computes the area of a circle." << endl;
// Prompt user to enter the radius of the circle, read input value into variable r
cout << "Enter the radius of the circle " << endl;
cin >> r;
// Square r and then multiply by pi area = r * r * pi;
cout << "The area is " << area << "." << endl;
}

Answer 1

The problem is relatively simple. See below.
#define pi 3.1415926535897932384626433832795
int main()
{
// Create three float variable values: r, pi, area
float r, pi, area;
...
Notice you use a macro expansion for pi. This replaces the variable name pi in your declaration with the text 3.1415926535897932384626433832795. This causes the error seen here:
error: expected unqualified-id before numeric constant
Now, since this caused parsing to fail on that statement, area winds up never being declared (since it's after pi). As a result, you also then recieve the following error:
error: 'area' was not declared in this scope
Note that you also actually assign neither pi nor area... you need to do that first.
As a general rule of thumb, don't use macros for constants in C++.
#include <iostream>
#include <cmath>
int main() {
using std::cin;
using std::cout;
using std::endl;
using std::atan;
cout << "This program computes the area of a circle." << endl;
cout << "Enter the radius of the circle " << endl;
float r;
cin >> r;
float pi = acos(-1.0f);
float area = 2 * pi * r;
cout << "The area is " << area << '.' << endl;
return 0;
}

Answer 2

For starters, you never actually assign anything to 'area', so it remains undefined (effectively contains a random number). Try adding the line area = r * r * pi; before the last cout. You also want to remove the float pi from the variable list since it collides with the #define pi at the top. Later, when you learn about #include <math> you may find an M_PI constant inside so that you don't have to do it yourself.

Answer 3

Modified code that builds and works fine :
///--------------------------------------------------------------------------------------------------!
// file: Area.cpp
//
// summary: console program for SO
#include <iostream>
using namespace std;
const double PI = 3.1415926535897932384626433832795;
int main()
{
// Create three float variable values: r, pi, area
double r, area;
cout << "This program computes the area of a circle." << endl;
// Prompt user to enter the radius of the circle, read input value into variable r
cout << "Enter the radius of the circle " << endl;
cin >> r;
// Square r and then multiply by pi area = r * r * pi;
area = PI*r*r;
cout << "The area is " << area << "." << endl;
return 0;
}