The code $$lls columns $1,2,...,$ n $1$ in that order. For each column, the code starts at row $0,$ and descends straight down the column to $$ll each slot one$-$by$-$one, including the location that lies directly above the zero in the rows diagonal. Then it starts afresh in the next column. You do not need any algera to $$gure out the data entries. They are just the integers in normal consecutive order. Your author had to use a little counting to typeset the example below, but your code can compute the number by incrementing the value just inserted at the latest step