Value conversion integer position to doubling sequence

Question

I have a simple slider (SeekBar) with the fixed range [0, 8]

When I change the value of this slider I want to convert these values as follows

[0, 0.25, 0.5, 1, 2, 4, 8, 16, 32]

It's late and I've confused myself with the maths - what's the simplest way to do this? I convinced myself there should be a simple way to do it using powers ( Math.pow() ) but failed a number of times

edit Just thought I'd add - I know how to get the progress updates using an OnSeekBarChangeListener it's just the actual conversion algorithm I'm struggling with

Answer 1

The simplest way to accomplish it is to reference an array you have assigned somewhere in your code (as comments/other users have pointed out):

// somewhere in your code (possibly static?)
double[] _result = { 0, 0.25, 0.5, 1, 2, 4, 8, 16, 32 };

// then your function
public void onProgressChanged (SeekBar seekBar, int progress, boolean fromUser)
{
    // would need to handle if progress is ever > _result.max
    System.out.format("%f%n", _result[progress]);
}

However, if you want the mathematical way to do it (based on the inputs/outputs you've specified), you'll need to implement some bit shifting:

public void onProgressChanged (SeekBar seekBar, int progress, boolean fromUser)
{
    // get the 'max' of our bar
    double max = (double)seekBar.getMax(); // 8
    // get the 'max' of our 'array' values, this could be a const value
    // or some other formula (like max * 4f)
    double pmax = 32;
    // get our 'mask' by shifting '1' left 'progress' times, then divide by 2
    // to get the divisor to our other formula
    double p = (double)(1 << progress) / 2; 
    // if progress == 0, then just return 0 (since that's our min)
    double val = ((progress == 0) ? 0 : (max / (pmax / p)));

    // the 'one-liner': 
    // ((double)seekBar.getMax()) / (pmax / ((double)(1 << progress) / 2)))

    System.out.format("%f%n", val);
}

Hope that can help.