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Group up consecutive numbers in a list

Sandro Tosi, 10 September 2006

As output from a command (webMethods scheduler output), I recieve a list of number (in this case hours and minutes) that could be really long, let's say every hour from 5 AM to 9 PM is a 17 items list, that could be grouped up into 5..21 .

The Java code provided does exactly this work: takes an int array as input and returns a StringBuffer with the sequential numbers grouped up as shown above.

Here is the main code (highlighted with java2html), the full source code is available at this link:

    //loop on hours; i it's the index of current element
    while (i<hours.length) {
      //set it to current scanned element in hours
      current=hours[i];
      //the next element index
      j=i+1;

      //This is the main hack: I fix the current element, hours[i],
      //then scan until elements are consecutive, incrementing j
      while (j<hours.length && current+j-i==hours[j]) { j++; }

      //if j was not modified, than print only current
      if (j==i+1) {
        output.append(separator+current);
        separator=", ";
        i=j;
      //if it's modified, than print a set
      } else {
        output.append(separator+current+".."+hours[j-1]);
        separator=", ";
        i=j;
      }
    }

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