Univeristy student- I like the idea, but its not practical..?

(Toronto Ontario)

Im a univeristy student and I really like this idea because it has a limit which your debit or credit cards may not...
However, Im not really sure how this would actaully work out it seems very inconvenient: While out and about- if you want to pay for some groceries and then transportation for ex- you are taking from two jars . You wont physically have all the jars on you so this method relies on you planning your purchases ahead - which is very inconvenient. (Maybe this is just because I am a student)?

It actually makes more sense to be able to tally the jars and keep them on paper. In a journal which you need to keep on you at all time , write the heading of each jar and throughout the week (using credit or debit) record purchase prices under each headings and deduct- as you "take from the jar". This seems easier than the jar method to me?...ALthough I understand how it takes a bit more self control..

Any tips on how to make the jar method practical or does it rely heavily on planning purchases ahead (times at which you make them).

Thanks,

Adrienne