我对 java 的百分比变化有疑问

Question

I need to find the percentage change of different numbers, I've got the formula down but the result keep ending up being minus instead of a positive eg 100 to 150 comes out a -50.00% instead of 50.00%. Thanks in advance!

package package2;
import java.util.Scanner;

class Selection3 {

public static void main (String [] args)
{

    perChange();

    
}


public static void perChange() {
    double perCha0, perCha1, perCha2, perCha3, perCha4, perCha5;


    perCha0 = ((38 - 108)*100/38);
    perCha1 = ((35 - 63)*100/35);
    perCha2 = ((4 - 6)*100/4);
    perCha3 = ((3 - 5)*100/3);
    perCha4 = ((20 - 40)*100/20);
    perCha5 = ((100 - 150)*100/100);
    
    System.out.println(perCha0);
    System.out.println(perCha1);
    System.out.println(perCha2);
    System.out.println(perCha3);
    System.out.println(perCha4);
    System.out.println(perCha5);
    
}

output

-184.0 -80.0 -50.0 -66.0 -100.0 -50.0

Answer 1

The problem you have is a math problem, and you have encoded a bad formula into your programming.

A percentage of change is a "difference of change" divided by the original amount. When 30 items become 60 items, it is a difference of +30; but, because you subtract the numbers in the wrong order, you get an incorrect "difference" of -30.

Subtracting is not like addition, changing the order of the numbers doesn't result in the same result.

Answer 2

You have a math problem. For example in you first formula: 38 * 100 / 108 = 35.15%

Answer 3

You have basically 3 options:

1. Use Math.abs()

    perCha0 = (Math.abs(38 - 108)*100/38); // or perCha0 = (Math.abs(x1 - x2)*100/x1);

2. If else condition

    if(x2 > x1) perCha0 = ((x2 - x1)*100/x1); else perCha0 = ((x1 - x2)*100/x1);

3. If you know before that you are calculating for percentage increase ie x2 > x1

    perCha0 = ((x2 - x1)*100/x1);

You can also simplify the above if you want.