Increase, decrease in percentage

Question

I will try to explain this problem in the best way with the 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, I need a scalable solution that returns to its original value using only the count. Any ideas?

Answer 1

The best way to do this is to use a function. It should not 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 to 0 when done. Thus, he looks much neat.

To the side:
The reason it does not work is because adding and then subtracting percent does not work. For example:

 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 will always be 99% of your original value. If you really want to use a shortcut, you can 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 are potentially prone to floating point errors. A functional way to do this is not only more accurate and understandable, but also potentially less error prone.

Answer 2

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

i.e:.

 100 + 5% = 100 * 1.05 = 105 

to cancel it:

 105 / 1.05 = 100 

The more common “5% off” formula will instead give you:

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

Answer 3

To turn on the power:

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

To turn off the power:

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

Let's take a more complex example: 17 bonuses, each of which gives 3% of the main power of 1234:

  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 this.

Answer 4

This does not work, because two percent are not taken from the same number. They are 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, not 5000, as you started with.

Thus, it is obvious that you want to not be / reduced by a percentage of the current capacity. Perhaps it would be better to set the base power (say 5000) and the actual power. Then, when you in / decrease, you use actualPower = actualPower + 5*0.1*basePower; or something like that. Or you just accept that five power outages after five bonuses do not return you to your original speed.

Answer 5

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

This is due to rounding errors with floating point , floating point values such as double and float cannot be represented with exact values.

If exact values ​​are needed, then decimal or int will be the best solution, since they are designed to handle exact values.

Change The actual problem here is not a floating point rounding error, but the problem is noted in Smashery's answer .