c++ - Algorithm to calculate the start day of the next month -

i writing simple calendar class. trying overload operator++ use move calendar next month. however, algorithm find next month's start day not quite right.

calendar calendar::operator ++(int) {    int hold;    calendar cal = *this;     month++;    if (month > december)    {       month = january;       year++;       if (year == 0)          year++;    }    previousstartday = startday;    startday = nextstartday;    nextstartday = findnextstartday();    return cal; }  int calendar::findnextstartday() const {    int monthlength,        day = startday;     monthlength = findmonthlength(false);    monthlength -= 28;    day += monthlength;    if (day > saturday)       day -= saturday;    return day; } 

january defined 0, december 11, sunday 0, saturday 6. startday, previousstartday, nextstartday, month, , year private class variables

when test 2013, dates correct until march. @ point gives next start day tuesday instead of monday.

i tried this:

int calendar::findnextstartday() const {    int monthlength,        day = startday;     monthlength = findmonthlength(false);    monthlength -= 28;    day -= monthlength;    if (day < sunday)       day += saturday;    return day; } 

however, gives same results.

edit:

i accounting leap years. here code findmonthlength() determine if or not.

if ((!(year % 4) && (year % 100)) || !(year % 400))    monthlength = 29; else    monthlength = 28; 

problem analysis

let's assume in march , have correct start day (friday, 5).

your findnextstartday algorithm find monthlength equal 3 (31-28), day 2 (8 - 6), tuesday (2) instead of monday (1)...

let's see why wrong running algorithm (first version of findnextstartday):

january: 31-28 = 3, day = 2 (tuesday) + 3 = 5 (friday), correct start date of february.

february: 28 - 28 = 0, day = 5 (friday) + 0 = 5 (friday), correct start date of march.

march: 31-28 = 3, day = 5 (friday) + 3 - 6 (saturday) = 2 (tuesday), wrong start date of april.

bug explanation

the problem when subtract saturday result in overflow (more saturday), leaving 1 day out of count (i.e.: subtract 1 day less want).

think of case in end day == 7. want have sunday (one more saturday - circularly increased), have remove 7, not 6, otherwise monday!

the error in circular increment: in correct algorithm 1 beyond 6 (i.e. 7) must go 0, 2 beyond 6 (i.e. 8) must go 1 , on.

in algorithm 1 beyond 6 (i.e. 7) goes 1, leaving out poor 0 (sunday) , making 1 day of week disappear each time end in case.

if subtract saturday + 1 right day of next month in case of "week days overflow".

bug fixing

in short, change line:

day -= saturday; 

to

day -= (saturday + 1); 

but please, consider reviewing code cleaner version of algorithm!

a small tip use modulo operator circular addition:

day = ((day + monthlength) % (saturday + 1))