Need some help with this math code.

[H4X0R46]

Hey guys, alright I'll get right to it, I'm stumped here. Here's my code as it stands.

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

const float Pi = 3.14159;
//Variables
int planets = 1;
int planetchoice;
float percentage;

int main()
{

    cout << "Name: Nicolas Aguirre" << endl;
    cout << "Filename: P3" << endl;
    cout << "Program Objective: To create a program to calculate the surface area of a lune with parameters specified by the user" << endl;

    do {
        cout << planets << endl;
        planets++;
    } while (planets <= 6);
    cout << "1 = Mercury 2 = Mars 3 = Venus 4 = Saturn 5 = Jupiter 6 = Moon." << endl;
    cout << "Choose a planet or the moon: ";
    cin >> planetchoice;
    if (planetchoice <= 6) {
        cout << "Please enter percentage of lune to calculate: ";
        cin >> percentage;
        if (percentage <= 100)
        {
            vector<float> planetr(6);
            planetr[0] = 2440; //Mercury
            planetr[1] = 3397; //Mars
            planetr[2] = 6052; //Venus
            planetr[3] = 60268; //Saturn
            planetr[4] = 71492; //Jupiter
            planetr[5] = 1738; //Moon

            if (planetchoice == 1)
            {
                cout << 2 * pow(planetr[0], 2) * percentage * .01 << endl; //LEFT OFF DECIMAL PLACE IS WRONG


            }
        }
    }
}

The thing is, my answers are coming out with the decimal in the wrong spot, so something is off with my math. Precedence issue maybe? I'm lost here. Thanks guys!

[salem c]

Please use code tags when posting code - you should still be able to edit your post.
http://www.cplusplus.com/articles/jEywvCM9/

> cout << 2 * pow(planetr[0], 2) * percentage * .01
Never mind the decimal point, your formula for surface area is wrong.
https://en.wikipedia.org/wiki/Sphere#Surface_area

[H4X0R46]

Alright, I added code tags to my post. I'm trying to find the surface area of a lune, not the whole sphere. Formula should be S = 2 * r2 * θ. Where θ is the percentage of the lune to be calculated (user inputs this).

[H4X0R46]

Alright nevermind I got it. After using setprecision(0), it now looks right as expected. Weird that that fixed it. Thanks for your help! :)

[lastchance]

No, with that formula, theta is the angle in radians.

When theta is 2.pi you will get 4 pi r^2, the surface area of a sphere (as salem c indicated to you in the link.)

[againtry]

https://en.wikipedia.org/wiki/Spherical_lune.
I get the impression you are trying to do something like this - a special case of a lune.

Of course you'll need to add some error checking to make sure the user selects a planet in the range and the calculation is correct.

#include <iostream>
#include <vector>
#include <string>
#include <cmath>

using namespace std;

const double pi = 3.14159

int main()
{
    vector<double> planet_radius;
    
    planet_radius.push_back(2440);  // Mercury
    planet_radius.push_back(3397);  // Mars
    planet_radius.push_back(6052);  // Venus
    planet_radius.push_back(60268); // Saturn
    planet_radius.push_back(71492); // Jupiter
    planet_radius.push_back(1738);  // Moon
    
    cout << "Name: Nicolas Aguirre" << endl;
    cout << "Filename: P3" << endl;
    cout
    << "Program Objective: To create a program to calculate the surface\n"
    << "area of a lune with parameters specified by the user\n";
    
    int i{0};
    do {
        cout << planet_radius[i] << endl;
        i++;
    } while ( i < planet_radius.size() );
    
    cout
    << "1 = Mercury 2 = Mars 3 = Venus 4 = Saturn 5 = Jupiter 6 = Moon\n"
    << "Choose a planet or the moon: ";
    
    int planet_choice;
    cin >> planet_choice;
    
    cout << "Please enter percentage of lune to calculate: ";
    double percentage;
    cin >> percentage;
    
    cout
    << 2 * pow(planet_radius[planet_choice - 1], 2) * percentage/100 * 2 * pi << '\n';
}

Name: Nicolas Aguirre
Filename: P3
Program Objective: To create a program to calculate the surface
area of a lune with parameters specified by the user
2440
3397
6052
60268
71492
1738
1 = Mercury 2 = Mars 3 = Venus 4 = Saturn 5 = Jupiter 6 = Moon
Choose a planet or the moon: 2
Please enter percentage of lune to calculate: 3
692377
Program ended with exit code: 0

[H4X0R46]

Alright, this looks a bit more correct to me. Yeah the whole idea of calculating a lune and the math involved only makes this assignment harder, I'm terrible at math admittedly.... Tomorrow when I get time, I'm going to add some of this code to my project and see what I can do, I'm also new to C++ so that's double challenging. Thanks guys! Appreciate you all! :)

[chillguy8624]

How did you do on your project? I am currently doing this same project. We have to Use vectors to hold planet names and their corresponding radii. And i am not sure how to do that. Yours has the push back?

[jonnin]

yes, push_back() more or less treats a vector as if it were a stack, adding a new item onto the highest order index / back end of the container for you. That should do what you need.

[Enoizat]

@DaisyDoyle,
please post questions like that in the “Lounge” section of this forum. I think you’ll get better directions there.

“Help about coding” sounds a bit vague, doesn’t it?
This is a forum about C++. Have you been suggested to study C++? It looks very far from sociology.
For data analysis maybe Python could just be what you need? I’m not an expert on the topic.
Python is usually considered a good language for a first approach to programming.

[dutch]

@DaisyDoyle is a stealth spammer. Just delete people like that. (Her only other post was a meaningless "thank you" appended to another thread, a common tactic.)