Get all days in a month that start in a certain day of the week

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6 replies
  1. japanfever
    japanfever says:

    Its WRONG in somre corner cases:

    > get_days_from_weekday_and_month(6, 7)
    [ 8, 15, 22, 29 ]
    > //BAD !!!, in 2017 must be: [ 1, 8, 15, 22, 29 ]

    > get_days_from_weekday_and_month(7, 7)
    //Infinite loop

    //If you want to do it right it’s harder than it looks
    function get_days_from_weekday_and_month_GOOD(weekday, month, year) {
    if (!year) year = (new Date()).getFullYear();
    month = (month-1)%12;

    //japanfever magic here E3839EE383AAE382AAE9B3A9E38388E383ACE383AD
    var monthdays = [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ];
    var monthstep = [ 7, 3, 3, 6, 8, 4, 6, 2, 5, 7, 3, 5 ];
    var days = [];

    //Leap years should be taken into account
    leap = 0;
    if (((year)%4 == 0 && (year)%100 != 0) || (year)%400 == 0) {
    monthdays[1] = 29;
    if (month < 2) leap = 1;
    }
    var inc = 8 – ((monthstep[month] + (year%100) + parseInt((year%100)/4) –
    2*parseInt((year/100)%4) – leap) % 7);

    var d = (weekday + inc)%7;
    while (d get_days_from_weekday_and_month_GOOD(6, 7)
    [ 1, 8, 15, 22, 29 ]

    > get_days_from_weekday_and_month_GOOD(7, 7)
    [ 2, 9, 16, 23, 30 ]

    ¯\_(ツ)_/¯

    • Génesis
      Génesis says:

      I really appreciate your extremely useful comments, I would like to meet you if you visit anytime South of Spain xD

  2. japanfever
    japanfever says:

    Your blog cut my post. I try again

    //If you want to do it right it’s harder than it looks E3839EE383AAE382AAE9B3A9E38388E383ACE383AD
    function get_days_from_weekday_and_month_GOOD(weekday, month, year) {
    if (!year) year = (new Date()).getFullYear();
    month = (month-1)%12;

    //japanfever magic here E3839EE383AAE382AAE9B3A9E38388E383ACE383AD
    var monthdays = [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ];
    var monthstep = [ 7, 3, 3, 6, 8, 4, 6, 2, 5, 7, 3, 5 ];
    var days = [];

    //Leap years should be taken into account
    leap = 0;
    if (((year)%4 == 0 && (year)%100 != 0) || (year)%400 == 0) {
    monthdays[1] = 29;
    if (month < 2) leap = 1;
    }
    var inc = 8 – ((monthstep[month] + (year%100) + parseInt((year%100)/4) –
    2*parseInt((year/100)%4) – leap) % 7);

    var d = (weekday + inc)%7;
    while (d <= monthdays[month]) {
    days.push(d);
    d += 7
    }

    return days;
    }

    • Génesis
      Génesis says:

      Thanks japanfever, you are right, there is an infinte loop and sometimes some days are missing.

      I fixed with:
      if (weekday < 0 || weekday > 6) return ‘Weekday must be between 0 and 6’
      if (month < 1 || month > 12) return ‘Month must be between 1 and 12’

      And
      d.setDate(0)

      This last line is for the missing days.

      Your implementation is quite appeling, I like you oldschool way and your magic :P, your code is valid in any programming language but as you said is a little bit harder so I end up fixing mine!

      • japanfever
        japanfever says:

        //Sorry, i dont speak english, only code, in japanese is my sign: E3839EE383AAE382AAE9B3A9E38388E383ACE383AD
        //i’m 14 old and I’m forbidden to talk to strangers, i visit hundreds of blogs looking for personal challenges to better program

        //Ok, now without magic, using the Javascript Date Object as you
        //Slower than the other, but faster than yours, cause i now use only one loop
        function get_days_from_weekday_and_month_BEST(weekday, month, year) {
        if (!year) year = (new Date()).getFullYear();
        var days = [];

        var inc = 8 – (new Date(year, month-1, 1)).getDay();
        var d = (weekday + inc)%7;
        var monthdays = (new Date(year, month, 0)).getDate();
        while (d <= monthdays) {
        days.push(d);
        d += 7
        }

        return days;
        }

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