When timetabling, we are subject to restrictions and these restrictions can make our task quite difficult. When you place a lesson manually on the construction grid, it will be fixed in place and that lesson(s) cannot move during the construction of the timetable. This is quite restrictive, especially if a lot of lessons are fixed in place.
Sometimes, it might be wiser to use a less restrictive approach and to tell the solver where you don't want these lessons to be placed. We can use the software to identify specific teaching groups and prevent these lessons using prevented grid periods.
A principal might host a weekly meeting with Assistant Principals in the school. Instead of specifying exactly where you want this meeting to be placed, it would be less restrictive to tell the software where it can't be placed. Then, instead of restricting the meeting to one period, you are giving the solver a choice of many other periods.
Consider another example - Four Transition Year Base Class Groups have a period of Political Education each week and they want these lessons to be held on Friday as they use a facility that is only available to the school on Friday.
The teaching groups for Political Education are 4Pol-A, 4Pol-B, 4Pol-C and 4Pol-D
They could be manually placed on the grid as in the screenshot below.
Then, the other Transition Year lessons will be placed on the remaining free periods on the grid but these manually placed lessons can not be moved during construction. By doing this, we are telling the solver that Transition Year Group 4A must have Political Education on Monday at Period 1 and that Group 4B will have Political Education on Period 2 etc.
This does not have to be done this way as the only condition is that Political Education must take place on Friday. Therefore, 4A group could have a choice of six different periods and should be free to move to any of these during the construction of the timetable.
Instead, we could tell the software that 4Pol* can not be placed on Monday, Tuesday, Wednesday or Thursday and therefore the solver must place these lessons on Friday. Which period on Friday does not matter in this case as long as it is placed on some Friday period.
To do this, we use the Actions button on the Construction Screen. We select 'Restrictions'
We then click on 'Select a new Restriction Type' and select 'Exclude teaching groups on period'
Enter '4Pol*' (as this will identify 4Pol-A and the other Political Education Groups) and then click on 'Select Periods'
We can very quickly select a complete day using the day tick-box.
Click 'Select' and then 'Save' and this 'Rule' will be noted by the solver.
Please Note that you will not see any evidence of this on the Construction Screen Grid but the solver will remember to apply it.
You can see from the screenshot below that the 4 lessons of Political Education are placed on Friday but in periods which suited the construction of the overall timetable.