Why is my Java calculated answer slightly higher from my TI-84 Plus Calculator and Google's calculated answer even though I used the same formula?

Question

Overview

Using DrJava, I have calculated the "Estimated corn plants damaged" based on an alien invasion that left a crop circle x diameter in feet (a user input). I made a calculation to determine an acre being that 43560 feet equals 1 acre. For every acre destroyed, 30000 corn plants are destroyed.

The output I am asking for clarification on why my program is getting a slightly different answer than my calculator is "Estimated corn plants damaged."

My output from my Java program when the user inputs 80

Estimated corn plants damaged: 3462

My calculator's and Google's final calculation of the same diameter

Estimated corn plants damaged: 3450

My Worded Solution

I am getting a different output because the dAcres variable is formatted to round to 3 decimal points for the output but the value it uses for the calculation is not rounded. For example, when the user enters 80 as the diameter, the dAcres variable outputs 0.115. However, the dAcre variable uses .11539 in its calculations instead.

Final Question

How do I make it so my calculation uses only 3 decimal places instead of it's default number of decimal places? Example: 0.115 instead of 0.11539?

My Code

import java.util.*;
import java.text.*;


public class Lab2_Problem2
{
  public static final double dPI = 3.14159;

  public static void main(String[] args )
  {
    // DECLARATIONS
    // Input-capture variables
    double dDiameter;

    // Expression-result variables
    double dArea;
    double dAcres;
    double dCornPlantsDamaged;

    // Other variables
    double dRadius;
    double dRadiusSquared;

    // Instantiations
    Scanner cin = new Scanner(System.in);
    DecimalFormat dfDiameter = new DecimalFormat("#######");
    DecimalFormat dfArea = new DecimalFormat("#######.00");
    DecimalFormat dfAcres = new DecimalFormat("##0.000");
    DecimalFormat dfCornPlantsDamaged = new DecimalFormat("#####");

    // INITIALIZE 
    // INPUT
    System.out.print("Enter crop circle diameter in feet: ");
    dDiameter = cin.nextDouble();

    // PROCESSING AND CALCULATIONS
    dRadius = dDiameter / (double) 2;
    dRadiusSquared = dRadius * dRadius;
    dArea = dPI * dRadiusSquared;
    dAcres = dArea / (double) 43560;
    dCornPlantsDamaged = dAcres * (double) 30000;

    // OUTPUT
    System.out.println("Crop circle diameter: " + dfDiameter.format(dDiameter) + " feet");
    System.out.println("Square feet: " + dfArea.format(dArea));
    System.out.println("Acres: " + dfAcres.format(dAcres));
    System.out.println("Estimated corn plants damaged: " + 
dfCornPlantsDamaged.format(dCornPlantsDamaged));
  }
}

Q&A

Why would you want to calculate by 3 decimal places if your calculator wouldn't do it?
1. Because I would like the output to match the calculations being made.

If I type this in a calculator I get 3461.8... So your program outputs the correct value. Why do you want to introduce rounding errors? And what exactly do you mean when you say Google outputs a different result? Using the calculator in the search field or where? — Markus, Feb 08 '20 at 16:57
Do you really want to make your program bug-for-bug compatible with something less correct? — NomadMaker, Feb 08 '20 at 17:06
@NomadMaker You are correct, but at the same time, I would like to still know the answer to my question. It's for practice and information purposes ore so than accuracy. I posted an answer regarding what you had to say, but I would still like a coded answer. — Nicholas Zachariah, Feb 08 '20 at 17:37
You could write a class similar to BigDecimal, but I seriously suggest finding something productive to do. — NomadMaker, Feb 08 '20 at 20:41

score 0 · Answer 1 · answered Feb 10 '20 at 02:12

Here is the working code in example form from the answer @cricket_007 gave.

dTruncOne = dAcres * 1000;
dTruncTwo = (int) dTruncOne * (double) 30000; // Truncated acres
dCornPlantsDamaged = dTruncTwo / (double) 1000;

dBushelsPerAcre = 211;
dCostPerBushel = 3.90;

OneCricketeer · Accepted Answer · 2020-02-08T17:25:59.703

-1

How do I make it so my calculation uses only 3 decimal places

Multiply it up to a full integer, truncating the decimal, then divide it back down to a decimal

((int) (0.11539 * 1000)) / 1000.0

Alternatively, there's Math.round method

Your calculator wouldn't use 3 decimal places, though, so I'm not sure why you'd want to

And, for simplicity, you can write out the formulas like so

destroyed_acres = ((pi * (d/2)^2) ft) * (acre/ft) 
ans = (corn/acre) * destroyed_acres

FWIW, wolframalpha says 3462

https://www.wolframalpha.com/input/?i=+%28%28pi+*+%2880%2F2%29%5E2%29+square+feet%29+to+acres+*+30000

Google does too (π × (80 ÷ 2)^2) × (1 ÷ 43560) × 30000

edited Feb 08 '20 at 17:25

answered Feb 08 '20 at 16:59

OneCricketeer

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That would truncate the extra decimals. Usually you'd want the result rounded, not truncated, when you reduce the precision of a number. – Andreas Feb 08 '20 at 17:01
`^2 ft` Really?!? – Andreas Feb 08 '20 at 17:07
1

What about it? Square feet – OneCricketeer Feb 08 '20 at 17:08
1

Would you rather see `(Pi*r^2) ft` – OneCricketeer Feb 08 '20 at 17:08
1) `^` is not a exponent operator. 2) Math expressions cannot handle units (i.e. `ft`, `acre`). – Andreas Feb 08 '20 at 17:09
1

It's not actual code... It's an expression. Caret is an exponent. Try it in Google calculator or wolfram alpha – OneCricketeer Feb 08 '20 at 17:09
That is certainly not clear from the answer, especially an answer saying: "And, for simplicity *(of your code)*, you can write out the formulas like so *(in code)*". If not for the simplicity of the code, then what are you simplifying? – Andreas Feb 08 '20 at 17:12
But what help is that? Don't you think OP already has the formula? How did you think OP wrote the code in the first place without having the formula? What were you trying to say with that "for simplicity" statement, if not suggesting writing the *code* differently? – Andreas Feb 08 '20 at 17:15
1

5 variables vs 2? – OneCricketeer Feb 08 '20 at 17:21
This technique fails in 90% of cases for four decimal places. Proof [here](https://stackoverflow.com/questions/153724/how-to-round-a-number-to-n-decimal-places-in-java/12684082#12684082). – user207421 Feb 10 '20 at 03:22
@user207 thanks, was just answering the posted question – OneCricketeer Feb 10 '20 at 06:54

Why is my Java calculated answer slightly higher from my TI-84 Plus Calculator and Google's calculated answer even though I used the same formula?

2 Answers2