Get the name of the day by number

Question

this is probably a very unique question because I have searched on Google for a while & could not find a solution. It is also interesting.

What I need bascailly is a number, which you can get the day name by it.

For example our number is 7, we need to go through a formula to find out it's day name, but for 7 it's basic, all you do is just get "saturday" in the switch statement.

Basically it's simple to get the name, you just done:

switch (dayNumber) {
    case 1: return "Sunday"; etc...

But my question is pretty complicated I think, and I am not sure if logically there is a possible solution for it.

I want to get the index number of the week, by the number.

examples:

day 7 - Index: 7
day 16 - Index: 2 (because you count the first 2 weeks 7 + 7 = 14 and then + 2 = 16 so the day is monday.
day 24 - index: 3

Why do I need this:

Well I had a question recently in a java course, not related to this, but it has something to do with looping through the month days, so I wanted to be creative & even get the name of the specific day, in a quick way using Mathematics functions and formulas.

So example on how I want to get the name:

for (int i = 1; i <= 31; i++) {
    System.out.println(getDayByInteger(i));
}

public static String getDayByInteger(int day) {
        int dayNumber = 0; //TODO Formula..
        switch (dayNumber) {
            case 1:
                return "Sunday";
            case 2:
                return "Monday";
            case 3:
                return "Tuesday";
            case 4:
                return "Wednesday";
            case 5:
                return "Thursday";
            case 6:
                return "Friday";
            case 7:
                return "Saturday";
            default: return "N/A";
        }
    }

I don't mind if it only will support the number range 1-31, but my question focuses on if it's possible to make it work with any number, because you can always build a formula to work for a specific numbers range I think.

Is it even possible for any number? Or will it require checking statements?

It's hard to figure your question but you seem to be looking for the remainder `%` operator. — devnull, Jan 14 '14 at 11:11
It's not really clear what the input number is meant to be. It sounds like you might be able to get away with just `day % 7` (and then index into an array rather than using switch/case) but your requirements aren't clear. — Jon Skeet, Jan 14 '14 at 11:11
see http://stackoverflow.com/questions/5270272/how-to-determine-day-of-week-by-passing-specific-date — ilya.stmn, Jan 14 '14 at 11:12

score 3 · Answer 1 · answered Jan 14 '14 at 11:11

3

You only have to use modulu and edit your cases:

    switch (dayNumber%7) {
        case 1:
            return "Sunday";
        case 2:
            return "Monday";
        case 3:
            return "Tuesday";
        case 4:
            return "Wednesday";
        case 5:
            return "Thursday";
        case 6:
            return "Friday";
        case 0:                     //change case 7 to case 0
            return "Saturday";
        default: return "N/A";
    }

answered Jan 14 '14 at 11:11

kai

6,702
22
38

2

When does `dayNumber % 7` give a result of 7? – Jon Skeet Jan 14 '14 at 11:12
@Jon Skeet: better now? – kai Jan 14 '14 at 11:13
Wait there is a problem, it will never give result of 7. – Artemkller545 Jan 14 '14 at 11:13
@kai: Yes, although I still wouldn't use a switch statement - just use an array. – Jon Skeet Jan 14 '14 at 11:13
yes, thats better! but i thought user3123545 want to use a switch case. – kai Jan 14 '14 at 11:14

score 1 · Answer 2 · answered Jan 14 '14 at 11:14

1

you can use reminder operator % for this

public static String getDayByInteger(int day) {
    int dayNumber = (day % 7) + 1; 
    switch (dayNumber) {
        // case 1 to 7
    }
}

answered Jan 14 '14 at 11:14

Denim Datta

3,740
3
27
53

1

+1 For being the first person to understand this 'complex' question. – Andrew Thompson Jan 14 '14 at 11:42
1

and +1 for the sarcasm :) – Denim Datta Jan 14 '14 at 11:54

score 0 · Answer 3 · answered Jan 14 '14 at 11:17

Try this out:

public static String getDayByInteger(int day) {
    int dayNumber = day % 7; //  Formula..
    switch (dayNumber) {
    case 0:
        return "Sunday";
    case 1:
        return "Monday";
    case 2:
        return "Tuesday";
    case 3:
        return "Wednesday";
    case 4:
        return "Thursday";
    case 5:
        return "Friday";
    case 6:
        return "Saturday";
    default:
        return "N/A";
    }
}

score 0 · Answer 4 · edited May 23 '17 at 12:10

0

Not clear what you are really looking for, if you wish to figure out the day of the week for a specific date, you might check the following: How to find which day of the week it is - Java

edited May 23 '17 at 12:10

Community

1
1

answered Jan 14 '14 at 11:18

osantos

352
2
18

score 0 · Answer 5 · edited May 23 '17 at 10:28

tl;dr

int yourDayNumber = 9 ;  // 1 = Sunday, 2 = Monday, … , 8 = Sunday, 9 = Monday, …
int isoNumber = ( ( ( yourDayNumber % 7 ) - 1 ) < 1 ) ? ( 7 + ( ( yourDayNumber % 7 ) - 1 ) ) : ( ( yourDayNumber % 7 ) - 1 );
String output = DayOfWeek.of( isoNumber ).getDisplayName( TextStyle.FULL , Locale.US )  // Or Locale.CANADA_FRENCH or Locale.TAIWAN etc.

Details

The Answer by kai is correct for the math part, using % 7. I consolidated that into a single obnoxious line above using a ternary function (see expanded code in last section).

For the other part, getting name of day, you can use new features built into Java using java.time classes.

`DayOfWeek` enum

The DayOfWeek enum provides a singleton instance for each of the sever days of the week.

The days are numbered according to the ISO 8601 standard, 1-7 for Monday-Sunday.

DayOfWeek dow = DayOfWeek.of( 1 ); // Monday is # 1.

The DayOfWeek::getDisplayName method translates the name of the day into any human language specified by a Locale such as Locale.US or Locale.CANADA_FRENCH.

String output = DayOfWeek.MONDAY.getDisplayName( TextStyle.FULL , Locale.CANADA_FRENCH );  // Or Locale.US or Locale.ITALY etc.

lundi

The purpose of such an enum is to use these objects instead of integers. Rather than pass around mere integer numbers, pass around objects such as DayOfWeek. This makes your code more self-documenting, ensures a range of valid values, and provides type-safety.

java.time

The java.time framework is built into Java 8 and later. These classes supplant the old troublesome date-time classes such as java.util.Date, .Calendar, & java.text.SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to java.time.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations.

Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport and further adapted to Android in ThreeTenABP.

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

First day of week

If you must count Sunday as first day of week, adjust to get the ISO 8601 value.

By subtracting one from your code number, we get 1-5 for Monday-Friday. That is just what we want for ISO 8601 numbers. But for Saturday we get -1 and for Sunday we get 0. I take advantage of that fact by adding the negative one or zero to seven to get the desired ISO number of 6 for Saturday and 7 for Sunday. There may be a neater way of doing this, but this approach succeeds according to my limited testing.

int yourDayNumber = 8;  // 1 = Sunday, 2 = Monday, … , 8 = Sunday, 9 = Monday, etc.

int modResult = ( yourDayNumber % 7 );
int isoNumber = ( modResult - 1 );
if ( isoNumber < 1 ) {
    isoNumber = ( 7 + isoNumber );
} else { // Else modResult is 1-5
    // No code needed here.
}

Dump to console.

System.out.println ( "yourDayNumber: " + yourDayNumber + " | isoNumber: " + isoNumber + " | DayOfWeek: " + DayOfWeek.of ( isoNumber ) );