Études for Erlang

Étude 6-4: Lists of Lists

Dentists check the health of your gums by checking the depth of the “pockets” at six different locations around each of your 32 teeth. The depth is measured in millimeters. If any of the depths is greater than or equal to four millimeters, that tooth needs attention. (Thanks to Dr. Patricia Lee, DDS, for explaining this to me.)

Your task is to write a module named teeth and a function named alert/1. The function takes a list of 32 lists of six numbers as its input. If a tooth isn’t present, it is represented by the list [0] instead of a list of six numbers. The function produces a list of the tooth numbers that require attention. The numbers must be in ascending order.

Here’s a set of pocket depths for a person who has had her upper wisdom teeth, numbers 1 and 16, removed. Just copy and paste it.

PocketDepths = [[0], [2,2,1,2,2,1], [3,1,2,3,2,3],
[3,1,3,2,1,2], [3,2,3,2,2,1], [2,3,1,2,1,1],
[3,1,3,2,3,2], [3,3,2,1,3,1], [4,3,3,2,3,3],
[3,1,1,3,2,2], [4,3,4,3,2,3], [2,3,1,3,2,2],
[1,2,1,1,3,2], [1,2,2,3,2,3], [1,3,2,1,3,3], [0],
[3,2,3,1,1,2], [2,2,1,1,3,2], [2,1,1,1,1,2],
[3,3,2,1,1,3], [3,1,3,2,3,2], [3,3,1,2,3,3],
[1,2,2,3,3,3], [2,2,3,2,3,3], [2,2,2,4,3,4],
[3,4,3,3,3,4], [1,1,2,3,1,2], [2,2,3,2,1,3],
[3,4,2,4,4,3], [3,3,2,1,2,3], [2,2,2,2,3,3],
[3,2,3,2,3,2]].

And here’s the output:

1> c(teeth).
{ok,teeth}
2> teeth:alert(PocketDepths).
[9,11,25,26,29]

See a suggested solution in Appendix A.