un petit scanner pour les stats fournies par benjamin, avec les .tar.gz pour 11-17 et 11-20

as of today:

~/git/flotpython/stats $ python scan.py 11-20
952 students have tried at least once
  5425 ok (successful) trials
+ 16696 ko (unsuccessful) trials
= 22121 total attempts

25 different exercices -> an average of 23.2363445378 attempts per student

affichage 376 ok + 2975 ko = 3351
alternat 2 ok + 1 ko = 3
aplatir 1 ok + 5 ko = 6
carre 363 ok + 2521 ko = 2884
comptage 2 ok + 11 ko = 13
decode_zen 94 ok + 279 ko = 373
diff 139 ok + 575 ko = 714
dispatch1 66 ok + 58 ko = 124
dispatch2 61 ok + 34 ko = 95
distance 1 ok + 2 ko = 3
divisible 864 ok + 1393 ko = 2257
doubler_premier 1 ok + 0 ko = 1
doubler_premier2 1 ok + 0 ko = 1
intersect 4 ok + 16 ko = 20
liste_racines 326 ok + 1176 ko = 1502
merge 239 ok + 1454 ko = 1693
multi_tri 590 ok + 1331 ko = 1921
multi_tri_reverse 488 ok + 1373 ko = 1861
numbers 1 ok + 2 ko = 3
pgcd 2 ok + 1 ko = 3
produit_scalaire 375 ok + 523 ko = 898
spam 579 ok + 2448 ko = 3027
target 846 ok + 512 ko = 1358
validation 1 ok + 0 ko = 1
validation2 3 ok + 6 ko = 9