cog sci 131 section
week 02/07/22
by shreshta (content) + yuan (slides)
agenda
- homework feedback
- recursion + context-free grammar
- python trick: dictionaries
- go over hw3 prompts
homework feedback
- help us grade faster: start each solution on a separate page + include the question number🥺
- help yourself get partial credits: show us your code + comments in each solution, not just a graph
- don't make us guess: use legends whenever necessary (multiple things in one graph) + label axes informatively
- think like a designer: good data viz highlights major patterns in the model/data you want people to see 👉 "affordances" (make the intended function jump out)
recursion
as a programming paradigm: using a function that calls itself until a specified condition is met
def fibonacci(n):
"return the nth fibonacci number"
# base case 1: f1 = 0
if n == 1:
return 0
# base case 2: f2 = 1
elif n == 2:
return 1
# if n > 2, call the function
else:
return fibonacci(n-1) + fibonacci(n-2)F_n = F_{n-1} + F_{n-2}
F_1 = 0, F_2 = 1
base cases:
function:
in natural language: "nonterminal" (e.g., S, NP, VP) is expanded into a set of terminals and nonterminals, which are themselves recursively expanded
[The boy] [believed [the clown laughed]].
DET + N
V
DET + N
V
NP
VP
S
dictionaries
useful data structure for hw3
# store word choices
nouns = {1: "Olivia", 2: "Liam", 3: "baker", 4: "telescope", 5: "animals"}
# initialize an empty dict
sentence_freq = {}
# loop through sentences you generated
for s in sentences:
# if already in dict, increase count by 1
if s in sentence_freq:
sentence_freq[s] += 1
# otherwise, start count at 1
else:
sentence_freq[s] = 1
-
a dictionary consists of a collection of key-value pairs 👉 values are accessed by keys: value = d[key] (indices don't work)
- OrderedDict: remembers the order in which keys were added
- defaultdict: return default values for missing keys
- two use cases in hw2
- store word choices
- count sentence frequencies (prompt says use defaultdict, but ordinary dictionaries work)
hw3 prompts
cogsci131_02_07_section
By Yuan Meng
