Showing Revision 1 created 06/29/2012 by Amara Bot.

1. Title:
   06-23 Row Plays
2. Description:

   Other units in this course below:
   Unit 1:http://www.youtube.com/playlist?list=PL818D7B4539EED6D3
   Unit 2:http://www.youtube.com/playlist?list=PLB470B7816A914D87
   Unit 3:http://www.youtube.com/playlist?list=PL2C981397711BDD5C
   Unit 4:http://www.youtube.com/playlist?list=PL66FC120367E358C0
   Unit 5:http://www.youtube.com/playlist?list=PL17BE79F86C9283AA
   Unit 6:http://www.youtube.com/playlist?list=PL7B0DE2C0104F814D
   Unit 7:http://www.youtube.com/playlist?list=PL6AE83B25FC53CEEF
   
   Q&A: http://www.youtube.com/playlist?list=PLB8CD9F4159903D84
   
   To gain access to interactive quizzes, homework, programming assignments and a helpful community, join the class at http://www.udacity.com

3. 0 0:00 - 0:04
   There's our whole row, and while I'm at it I might as well define a hand.
4. 4000 0:04 - 0:11
   Now my next target, the next bird to cross off the list is to define a function row_plays,
5. 11000 0:11 - 0:16
   which takes a hand and a row in this format and return a set of legal plays from the row.
6. 16000 0:16 - 0:21
   Now, rather than just return legal words, I'm using this notion of a play,
7. 21000 0:21 - 0:27
   where a play is a pair of location within the row and the word that we want to play.
8. 27000 0:27 - 0:32
   You can imagine it's going to take the same general approach that we've used before,
9. 32000 0:32 - 0:36
   start with an empty set, do something to it, and then return the results that we built up.
10. 36000 0:36 - 0:41
    What is it that we want to do? We want to consider each possible allowable prefix,
11. 41000 0:41 - 0:44
    and to that we want to add all the suffixes, keeping the words.
12. 44000 0:44 - 0:48
    Now, prefixes of what? That's the first thing to figure out.
13. 48000 0:48 - 0:52
    What I'm going to do is enumerate the row--enumerate actually just the good bits.
14. 52000 0:52 - 0:59
    The row from the first position to the last position, and that tells me I don't want the borders.
15. 59000 0:59 - 1:01
    I don't want to consider playing on the borders.
16. 61000 1:01 - 1:05
    I just want to consider playing on the interior of the row.
17. 65000 1:05 - 1:07
    Enumerate that starting from position number 1.
18. 67000 1:07 - 1:09
    One would be where the A is.
19. 69000 1:09 - 1:14
    Now I have an index--a number 1, 2, 3--and I have the square, which is going to be a,
20. 74000 1:14 - 1:17
    and then an anchor and then an anchor and so on.
21. 77000 1:17 - 1:19
    Where do I want to consider my rows?
22. 79000 1:19 - 1:23
    We're going to anchor them on an anchor so I can ask a square an instance of an anchor.
23. 83000 1:23 - 1:26
    If it is an anchor, then there's two possibilities.
24. 86000 1:26 - 1:29
    If it's an anchor like this, there's only one allowable prefix.
25. 89000 1:29 - 1:34
    The prefix which is the letters that are already there just to the left of the anchor.
26. 94000 1:34 - 1:39
    We want to just consider that one prefix and then add all the suffixes.
27. 99000 1:39 - 1:42
    If it's an anchor like this one, then there can be many prefixes.
28. 102000 1:42 - 1:47
    We want all possible prefixes that fit into these spots here,
29. 107000 1:47 - 1:51
    consider each one of those, and for each one of those consider adding on the suffixes.
30. 111000 1:51 - 1:54
    What I'm going to do is define a function, legal _prefix,
31. 114000 1:54 - 2:00
    which gives me a description of the legal prefix that can occur at position i within a row.
32. 120000 2:00 - 2:02
    There are two possibilities.
33. 122000 2:02 - 2:07
    I could combine the possibilities into one, but I'm going to have a tuple of two values returned.
34. 127000 2:07 - 2:12
    I'm going to have legal_prefix return the actual prefix as a string
35. 132000 2:12 - 2:17
    if there is one, like in this case, and return the maximum size otherwise.
36. 137000 2:17 - 2:23
    For this anchor here, this would be legal_prefix of one, two, three, four, five, six--
37. 143000 2:23 - 2:27
    that's for legal_prefix when i = 6.
38. 147000 2:27 - 2:30
    The result would be that there are now characters to the left.
39. 150000 2:30 - 2:34
    It'll be the empty string for the first element of the tuples.
40. 154000 2:34 - 2:38
    The maximum size of the prefix that I'm going to allow is two characters.
41. 158000 2:38 - 2:43
    Now, if I asked here--that's index number one, two, three, four, five, six, seven, eight, nine--
42. 163000 2:43 - 2:49
    when i = 9, the result would be that the prefix is BE,
43. 169000 2:49 - 2:55
    and the maximum size is the same as the minimum size. It's the exact size of 2.
44. 175000 2:55 - 3:01
    I define legal_prefix in order to tell me what to do next based on the two types of anchors.
45. 181000 3:01 - 3:03
    Now, I can go back to row plays.
46. 183000 3:03 - 3:08
    I can call legal_prefix, get my results, and say if there is a prefix,
47. 188000 3:08 - 3:11
    then I want to add to the letters already on the board.
48. 191000 3:11 - 3:16
    Otherwise, I have an empty space to the left, and I want to go through all possible prefixes.
49. 196000 3:16 - 3:19
    Here's what we do if there is a prefix already there.
50. 199000 3:19 - 3:22
    Now we can calculate the start of our position.
51. 202000 3:22 - 3:27
    Remember a row play is going to return the starting location of the word.
52. 207000 3:27 - 3:33
    We can figure that out. It's the i position of the anchor minus the length of the prefix.
53. 213000 3:33 - 3:35
    In fact, let me go and change this comment here.
54. 215000 3:35 - 3:37
    I is not very descriptive.
55. 217000 3:37 - 3:39
    Let's just call that start.
56. 219000 3:39 - 3:41
    Now we know what the starting location is for the word.
57. 221000 3:41 - 3:43
    When we find any words we can return that.
58. 223000 3:43 - 3:47
    Then we go ahead and add suffixes.
59. 227000 3:47 - 3:50
    With the suffixes, some of the letters are going to come out of the hand.
60. 230000 3:50 - 3:54
    We're adding suffixes to the prefix that's already there on the board.
61. 234000 3:54 - 3:57
    Starting in the start location, going through the row,
62. 237000 3:57 - 4:00
    accumulating the results into the result set,
63. 240000 4:00 - 4:02
    and then I needed this one more argument.
64. 242000 4:02 - 4:04
    I actually made a mistake and left this out the first time, and it didn't work.
65. 244000 4:04 - 4:06
    We'll see in a bit what that's there for.
66. 246000 4:06 - 4:09
    Now if we have empty space to the left of the anchor,
67. 249000 4:09 - 4:12
    now we've got to go through all the possible prefixes,
68. 252000 4:12 - 4:14
    but we already wrote that function--find_prefixes. That's good.
69. 254000 4:14 - 4:17
    Looks like we're converging. We're not writing that much new stuff.
70. 257000 4:17 - 4:22
    Now, out of all the possible prefixes for the hand, we only want to look at the ones
71. 262000 4:22 - 4:25
    that are less than or equal to the maximum size.
72. 265000 4:25 - 4:27
    If the prefix is too big, it won't fit into the empty spot.
73. 267000 4:27 - 4:30
    It will run into another word, and we don't want to allow that.
74. 270000 4:30 - 4:32
    We can calculate the start position again.
75. 272000 4:32 - 4:36
    Then we do the same thing. We add suffixes. What do we add them to?
76. 276000 4:36 - 4:39
    We'll the prefix that we just found from the hand.
77. 279000 4:39 - 4:43
    Since the prefix came from the hand, the remaining letters left in the hand
78. 283000 4:43 - 4:45
    we have to subtract out those prefix letters.
79. 285000 4:45 - 4:49
    Here we didn't have to subtract them out, because they prefix letters were already on the board.
80. 289000 4:49 - 4:53
    We're adding to the prefix from the start, from the row, results are accumulated,
81. 293000 4:53 - 4:55
    and we have this anchored equals false again.
82. 295000 4:55 - 5:00
    We're almost there. Just two things left to do--add_suffixes and legal_prefix.
83. 300000 5:00 - 5:04
    Add_suffixes we had before, but it's going to be a little bit more complicated now,
84. 304000 5:04 - 5:06
    because we're dealing with the anchors.
85. 306000 5:06 -
    Legal_prefix is just a matter of looking to the left and see how much space is there.