Question

Not sure how to word this algorithm question, actually. I have toggle buttons in an android app, each corresponding to a numbered channel. Channels are (1,n) but the toggle button ids are some unknown, unsequential (to the buttons) integer. I need to collect the channel numbers and build a command string. My 1995-based Java skills have given me this:

String ch;
int i = 1;
for (ToggleButton toggle : toggles) {
    if (toggle.isChecked()) {
        ch = String.format("+%d", i+1);
        channels = channels.concat(ch);
    }
} 

If toggle buttons 1,2,4,5,6,7,11,13,21,22,23,24,25 are checked, this code snippet successfully gives me the string "+1+2+4+5+6+7+11+13+21+22+23+24+25"

However, what I would more like to do is have the string "+1/2, +4/7, +11, +13, +21/25"

I wonder if there’s an easier way to do this than multiple if statements:

String ch;
int it = 0;
int last = 1;
for (ToggleButton toggle : toggles ) {
    if (toggle.isChecked()) {
    if (it == last + 1) {
            // somehow continue or note that we're only adding the "/"
        } else {
            // If we're not the next one, then build the last string
            // of "n/last" and then add ", +" to start building next string
        }
    }
    last++;
}

That seems like a bit of brute force type of algorithm, so I don’t know if there’s a more elegant solution that Java might have in it’s bag of tricks (Or, more likely, that I should have)

Thanks.

Answer 1

On java.util.BitSet

I’d use java.util.BitSet both for the representation and the span search algorithm. Here’s the basic idea (see on ideone.com):

    import java.util.*;
    //...

    BitSet bs = new BitSet();
    int[] onBits = new int[] { 1,2,4,5,6,7,11,13,21,22,23,24,25 };
    for (int onBit : onBits) {
        bs.set(onBit);
    }
    System.out.println(bs);
    // {1, 2, 4, 5, 6, 7, 11, 13, 21, 22, 23, 24, 25}

    StringBuilder sb = new StringBuilder();
    for (int begin, end = -1; (begin = bs.nextSetBit(end + 1)) != -1; ) {
        end = bs.nextClearBit(begin) - 1;
        if (sb.length() > 0) sb.append(", ");
        sb.append(
            (begin == end)
                ? String.format("+%d", begin)
                : String.format("+%d/%d", begin, end)
        );
    }
    System.out.println(sb);
    // +1/2, +4/7, +11, +13, +21/25

The nextSetBit and nextClearBit methods are really handy in finding the spans.

---

On StringBuilder instead of String with +=/concat

The StringBuilder joining algorithm is a standard one. Here it is when refactored out:

    StringBuilder sb = new StringBuilder();
    for (Element e : elements) {
        if (sb.length() > 0) sb.append(SEPARATOR);
        sb.append(e);
    }

You should use a StringBuilder or StringBuffer instead of String with +=/concat for building long strings.