java - Math error switch always ends up zero -

i writing small program find day of week using gregorian math. following code outputs 0 switch statement resulting in same output each time.

this code:

    isum = (centcode + iyear + (iyear /4) + monthcode + iday);     ioutput %= isum;      switch (ioutput)     {         case 0:             sday = "sunday";         break;          case 1:             sday = "monday";         break;          case 2:              sday = "tuesday";         break;          case 3:              sday = "wednesday";         break;          case 4:              sday = "thursday";         break;          case 5:             sday = "friday";         break;          case 6:             sday = "saturday";         break; } 

ioutput %= isum; 

is short form of

ioutput = ioutput % isum; 

which not want. guess want like

ioutput = isum % 7;