麻烦与数学公式 - Java

Question

I'm making a program for my java class that calculates the population for a year given the start year (2011) and increases the population by 1.2% every year. The population for 2011 is 7.000 (I'm using decimals, instead of billions). I currently have this code.

int startYear = 2011;
int endYear = user_input.nextInt();
double t = 1.2; //Population percent increase anually
double nbr = (endYear - startYear); //Number of years increased
double pStart = 7.000; //Starting population of 2011
double pEnd = pStart * Math.exp(nbr * t); // Ending population of user input
DecimalFormat nf = new DecimalFormat("2");
System.out.println("Population in " + endYear + ": " (nf.format(pEnd)));

There's no errors in the code, everything works, but I'm having troubles with the pEnd equation. Currently when I enter 2016 for the end year, i get 22824. I've tried googling a formula, but i can't find anything. Do any of you guys have an idea of the formula? If you enter 2016 for the end year, it should be around 7.433

Answer 1

You're incrementing by a factor of 1.2, which would represent 120% instead of 1.2%. I think what you want is :

double t = 0.012;

This change gives me an exact value of 7.4328558258175175 from 2011 to 2016.

EDIT : here's the code as requested by the author :

public static void main(String args[]){
    int startYear = 2011;
    int endYear = 2016;
    double t = 0.012; //Population percent increase anually
    double nbr = (endYear - startYear); //Number of years increased
    double pStart = 7.000; //Starting population of 2011
    double pEnd = pStart * Math.exp(nbr * t); // Ending population of user input
    System.out.println("Population in " + endYear + ": " + pEnd);
}

Answer 2

使用Math.pow(1 + t / 100, nbr)而不是Math.exp(nbr * t)因为你需要(1+t/100)^nbr （即在nbr次自身乘以1 + t / 100 ），而不是exp^(nbr*t) ：

double pEnd = pStart * Math.pow(1 + t / 100, nbr); // Ending population of user input

Answer 3

试试这个。

double pEnd = pStart * Math.pow(1.0 + t / 100, nbr);