Sort months ( with strings ) algorithm

Question

I have this months array:

[\"January\", \"March\", \"December\" , \"October\" ]

And I want to have it sorted like this:

[\"         


        

Answer 1

Have an array with the proper sort, and sort based on it.

Another solution (if your language supports it) is to have an associative array from month names to numbers (1..12) and use a custom comparator running sort on your array.

Solution in Perl :D

my @mon = qw( January February March April May June July August September October November December );
my $mon;
@{$mon}{@mon} = (0..$#mon);

sub by_month {
    $mon->{$a} <=> $mon->{$b};
}

sort by_month @data_to_sort

(although I'm sure a golfer could do that in < 30 characters)

And here's a solution in plain C : http://www.pnambic.com/CPS/SortAnal/html/MonOrder.html

Answer 2

Thanks all for the suggestions, I would like to mark you all as accepted.

Here's the resulting code:

// correct order 
months as String[] = ["jan", "feb", "mar", "apr", "may", "jun",
                      "jul", "aug", "sep", "oct", "nov", "dec"]
// my unsorted months 
myMonths as String[] = ["mar", "dec", "jul", "jan", "sep"]

// poor substitute for Map
mappedIndex as Int[]

// create an array with the corresponding index 
for each m in myMonths do
    i as Int = 0;
    for each month in months do 
       if m == month then 
           mappedIndex[] = i // no break, so I should use "else"
       else
          i = i + 1
       end
    end
end

// At this point mapped index has the same order as my "unsorted" array
// in this case [2,11,5,0,8]

// Fortunately this language has "sort" otherwise I would jump out of the window
mappedIndex.sort()

// create a new array to hold the sorted values
myMonthsSorted as String[]

// and fill it with the correct value
for each i in mappedIndex do 
   myMonthsSorted[] = months[i]
end

Answer 3

If I had a way to supply a custom sorting order, I'd create a list defining the correct order:

correct = List("January", "February", "March", ...)

And then sort by the position in that list, something like:

toSort.sort(a, b) => compare(correct.index(a), correct.index(b))

Answer 4

Talking from a Java POV, I'm going to pimp (as I often do) google-collections (soon to be replaced with Guava):

Arrays.sort(myMonths, Ordering.explicit("Jan", "Feb", "Mar", ....));

... and you're done.

Don't write it yourself if someone else has done it, probably more efficiently and with a nicer API than you probably would.

Not helpful in the general case, but just in case any Java folk have the same problem...

Answer 5

Colleagues,

I see the issue/business problem lasts more than 2 year. I decided to write comparator for sorting months' names (stored as strings) properly. It also holds names of the months for desired locale ============== Comparator =======================

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;

import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;
import java.util.Map;

/**
 *
 * @author akashtalyan
 */
public class MonthNamesComparator implements Comparator {

    private static Map<Locale, List> monthMap = new HashMap<Locale, List>();
    private final Locale locale;

    public MonthNamesComparator(Locale locale) {
        if (locale == null) {

            throw new NullPointerException("MonthNamesComparator cannot accept null value for Locale parameter.");
        }
        List months = new ArrayList(12);
        Calendar cal = Calendar.getInstance(locale);
        SimpleDateFormat dateFormat = new SimpleDateFormat("MMMM", locale);
        this.locale = locale;
        if (!monthMap.containsKey(locale)) {
            for (int i = 0; i < 12; i++) {
                cal.set(Calendar.MONTH, i);
                months.add(dateFormat.format(cal.getTime()).toLowerCase());
            }
            monthMap.put(locale , months);
        }
    }

    @Override
    public int compare(Object month1, Object month2) {
        List months = monthMap.get(this.locale);
        if (months == null) {
            throw new NullPointerException("MonthNamesComparator cannot perform comparison - internal data is not initialized properly.");
        }
        return (months.indexOf(((String) month1).toLowerCase()) - months.indexOf(((String) month2).toLowerCase()));

    }
}

And simple test class to POC:

import java.util.Locale;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

/**
 *
 * @author akashtalyan
 */
public class TestMonths {
    public static void main(String[] args){
        Locale en = Locale.ENGLISH, ru = new Locale("ru","RU");
        String[] monthsToTestEn = new String[] {"FebRUary", "maY", "sepTember", "january", "december"};
        String[] monthsToTestRu = new String[] {"АпреЛь", "январь", "Март", "Август"};

        Map map = new TreeMap(new MonthNamesComparator(en));
        int i = 0;
        System.out.println("En Map original:");
        for (String month : monthsToTestEn) {
            System.out.print(month + " ");
            map.put(month, new StringBuilder(String.valueOf(++i)).append(" position in source array").toString());
        }
            System.out.println();
            System.out.println("En Map sorted:");
        for (String month : (Set<String>)map.keySet()) {
            System.out.println(month + " " + map.get(month));
        }
        i = 0;
        map = new TreeMap(new MonthNamesComparator(ru));
        System.out.println("Ru Map original:");
        for (String month : monthsToTestRu) {
            System.out.print(month + " ");
            map.put(month, new StringBuilder(String.valueOf(++i)).append(" position in source array").toString());
        }
            System.out.println();
        System.out.println("Ru Map sorted:");
        for (String month : (Set<String>)map.keySet()) {
            System.out.println(month + " " + map.get(month));
        }
    }

}

Enjoy it, works like a charm.

Answer 6

Create a table with name->index, then sort the array based on its value in the table.

A couple of examples may be useful, in C# arr.sort(myCompare), in Java Collections.sort(arr, myCompare), in Python arr.sort(myCompare), in PHP usort($arr, 'myCompare'), in C++ sort(vec.begin(), vec.end(), myCompare).