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53 # Programming strategy
67 .indexlink[[Index](index.html)]
71 # Data structures before algorithms
73 * What data do you need?
74 * What operations must you perform on it?
76 These go a long way to defining the algorithms you must use.
78 ## Python built-in types
80 * Numbers (float and integer)
84 * Dictionaries (a.k.a. hashes or maps)
86 Many tasks will fit into these
88 More in the `collections` standard library, such as `namedtuple` and `Counter`
95 * Single or double quotes (beware apostrophies)
96 * Triple-quoted contain newlines
102 "This isn't a problem"
113 * Immutable in Python
114 * Fixed-size collection
115 * Indexed by position
118 neil = ('Neil', 'Smith',
44)
123 * Unpacking with assignment
126 first_name, surname, age = neil
130 for _, surname, age in people:
140 * Index by position and slices
143 members = ['Freddie', 'Brian', 'Roger', 'John']
154 # Lists: beware copy depth!
157 members = ['Freddie', 'Brian', 'Roger', 'John']
158 tour_lineup = members
159 tour_lineup[
0] = 'Paul'
169 tour_lineup = members[:]
173 tour_lineup = list(members)
178 tour_lineup = copy.copy(members)
179 tour_lineup = copy.deepcopy(members)
182 `==` tests value equality; `is` tests object identity
185 members = ['Freddie', 'Brian', 'Roger', 'John']
186 tour_lineup = members[:]
187 tour_lineup == members
188 tour_lineup is members
191 ## Value vs Reference
193 Must be part of your mental model
199 * A set of key-value pairs
201 * Items in arbirary order
202 * In Python, can use any immutable type as the key
203 * Use any type as the value
206 neil = {'first_name': 'Neil', 'surname': 'Smith', 'age':
44}
214 GRID_STRING =
"""08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08
215 49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00
216 81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65
217 52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91
218 22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80
219 24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50
220 32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70
221 67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21
222 24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72
223 21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95
224 78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92
225 16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57
226 86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58
227 19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40
228 04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66
229 88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69
230 04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36
231 20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16
232 20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54
233 01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48"""
236 How many data structures can you think of for this puzzle?
240 # Euler
11 structures
242 * List of lists: either a list of rows or a list of columns
243 * One large list, find a number by row ×
20 + column
244 * Tuple of tuples, one large tuple, list of tuples, tuple of lists...
245 * Dict with keys as (row, column)
246 * Keep it as a string: `int(row *
20 + column) *
3)`
247 * (Beware when joining rows)
248 * List of strings, tuple of strings, dict of strings...
254 Do something to each element of an iterable, returning a sequence of the results
256 * Iterable: anything you can use as `for item in iterable`
257 * Sequence: `tuple`, `list`, `dict`
261 * List of numbers: `[i for i in range(
10)]`
262 * List of squares: `[i **
2 for i in range(
10)]`
263 * List of numbers from a string: `[int(n) for n in str.split()]`
264 * Numbers and their squares: `{i: i **
2 for i in range(
10)}`
265 * Even numbers and their squares: `{i: i **
2 for i in range(
10) if i %
2 ==
0}`
268 # Comprehensions with multiple sequences
270 All the numbers in the Euler
11 grid:
272 [n for row in grid for n in row]
281 for a in range(
1,
21)
282 for b in range(
1,
21)
283 for c in range(
1,
21)
286 if a**
2 + b**
2 == c**
2]
292 for a in range(
1,
21)
293 for b in range(a,
21)
294 for c in range(b,
21)
295 if a**
2 + b**
2 == c**
2]
300 [(a, b, int((a**
2 + b**
2)**
0.5))
301 for a in range(
1,
21)
302 for b in range(a,
21)
303 if int((a**
2 + b**
2)**
0.5) == (a**
2 + b**
2)**
0.5]
312 Turn the Euler
11 grid into:
314 * A list of lists, as a list of rows
315 * A dict with keys of (x, y)
317 Do both with explicit iteration, then using only comprehensions.
321 # Solutions with explicit loops
323 grid_nums = [int(n) for n in GRID_STRING.split()]
328 for rowstart in range(
0, ROWS * COLUMNS, COLUMNS):
329 g1.append(grid_nums[rowstart:rowstart+COLUMNS])
336 for x in range(COLUMNS):
337 for y in range(ROWS):
338 g2[(x, y)] = grid_nums[x + y * COLUMNS]
343 Note the need for the creation of the empty container
347 # Solutions with comprehensions
350 g3 = [grid_nums[row * COLUMNS:(row+
1) * COLUMNS]
351 for row in range(ROWS)]
357 g4 = {(x, y): grid_nums[x + y * COLUMNS]
358 for x in range(COLUMNS)
359 for y in range(ROWS)}
365 g4s = {(x, y): int(GRID_STRING[(x + y * COLUMNS) *
3:(x + y * COLUMNS) *
3 +
2])
366 for x in range(COLUMNS)
367 for y in range(ROWS)}
374 # Other Python data structures
377 Unordered, mutable, no duplicates
379 * Useful for removing duplicates from sequences: `list(set(items))`
382 # collections.defaultdict
383 Like a dict, but doesn't throw an error if you ask for missing key
388 Traceback (most recent call last):
389 File
"<stdin>", line
1, in
<module>
391 >>> import collections
392 >>> d = collections.defaultdict(int)
399 # collections.Counter
400 Counts items in a sequence:
403 collections.Counter(l for l in open('sherlock-holmes.txt').read()
404 if l in string.ascii_letters)
409 # collections.namedtuple
410 Dumb `object` for just storing data in named fields
413 Point = collections.namedtuple('Point', ['x', 'y'])
422 # Data, not algorithms
424 Don't put complex, changable logic into code
426 Build a data structure that describes it, code that reads it.
428 * Euler
11 directions
429 * Multiple choice quiz
433 # Algorithms: trading space for time
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