增加，减少百分比

Question

I'll try to explain this problem the best way i can with code:

double power = 5000;
//picked up 5 power ups, now need to increase power by 10% per powerup
power += 5 * (power * .10);


//later on...ran into 5 power downs need to decrease power back to initial hp
power -= 5 * (power * .10);//7500 - 3750 -- doesn't work

So what i need is a scaleable solution that gets back to the original value using only the count. Any ideas?

Answer 1

The best way to do this is using a function. It doesn't have to look exactly like this, but:

class Whatever
{

    private double basePower = 5000;

    public int numPowerUps = 5;

    public double GetActualPower()
    {
        return basePower + (numPowerUps * basePower * 0.1);
    }
}

Just change numPowerUps back to 0 when they run out. This way, it looks a whole lot neater.

An aside:
The reason it's not working is because of the fact that adding and then subtracting percentages doesn't work. For instance:

1. What is 10% of 100?    --> 10
2. Add that to the 100    --> 110
3. What is 10% of 110?    --> 11
4. Subtract that from 110 --> 99

You'll always end up with 99% of your original value. If you really want to take a shortcut, you could instead do this:

1. What is 10% of 100?    --> 10
2. Add that to the 100    --> 110
3. What is (100/11) = 9.09090909...% of 110?    --> 10
4. Subtract that from 110 --> 100

But then you're potentially susceptible to floating point errors. The function way of doing it is not only neater and clearer, but potentially less error-prone.

Answer 2

To reverse a %age increase, you must divide by the original %age, not subtract .

ie:

100 + 5% = 100 * 1.05 = 105

to reverse it:

105 / 1.05 = 100

The more usual '5% off' formula would instead give you:

105 - 5% = (105 * 0.95) = 99.75

Answer 3

To power up:

power <- power * (1 + count * percent);
eg: 5000 * (1 + 5 * 0.1)
    5000 * 1.5
    7500

To power back down:

power <- power / (1 + count * percent)
eg: 7500 / (1 + 5 * 0.1)
    7500 / 1.5
    5000

Let's take a more complicated example, 17 power ups, each giving 3% to an intial 1234 power:

  1234 * (1 + 17 * 0.3)
= 1234 * (1 + 5.1)
= 1234 * 6.1
= 7527.4

  7527.4 / (1 + 17 * 0.3)
= 7527.4 / (1 + 5.1)
= 7527.4 / 6.1
= 1234

It actually looks pretty simple when you write it out like that.

Answer 4

This doesn't work because the two percentages are not taken from the same number. They're taken from the same variable, but not the same number.

The first time, power * 0.10 is 500, and 5*500=2500 so the power will be 5000+2500=7500. Now, the power is 7500, so power * 0.10 is 750. 5*750 = 3750 and 7500-3750=3750 and not 5000 like you started out with.

So apparently, what you want is not really to in/decrease by a percentage of the current power. Perhaps it would be better to set a base power (let's say 5000) and an actual power. Then when you in/decrease, you use actualPower = actualPower + 5*0.1*basePower; or something. Or you just accept that five power downs after five power ups does not get you back to initial hp.

Answer 5

I'm going to suspect that what you mean by "doesn't work" is that the value for power does not end up to be exactly 3750 .

This is due to floating-point rounding errors , as floating point values such as double and float are not able to be represented exact values.

If exact values are needed, then using decimal or int would be a better solution, as they are designed to handle exact values.

Edit The actual issue here is not a floating-point rounding error, but an issue noted in Smashery's answer .