I'm working my way through the NLTK book by Bird, Klein, and Loper and I'm stuck on a problem. I'm working through the book for my own personal enrichment, and not for a class.
The problem I'm stuck on is 4.29:
Write a recursive function that pretty prints a trie in alphabetically sorted order, eg:
chair: 'flesh' ---t: 'cat' --ic: 'stylish' ---en: 'dog'
I'm using this code from the book to create the trie:
def insert(trie, key, value):
if key:
first, rest = key[0], key[1:]
if first not in trie:
trie[first] = {}
insert(trie[first], rest, value)
else:
trie['value'] = value
trie = {}
insert(trie, 'chat', 'cat')
insert(trie, 'chien', 'dog')
insert(trie, 'chair', 'flesh')
insert(trie, 'chic', 'stylish')
I modified an answer from this discussion for a function that recursively goes through the trie and pulls out the complete keys and values:
def trawl_trie(trie):
unsorted = []
for key, value in trie.items():
if 'value' not in key:
for item in trawl_trie(trie[key]):
unsorted.append(key + item)
else:
unsorted.append(': ' + value)
return unsorted
But I'm not able to use recursion to make an alphabetical list, nor can I figure out how to use recursion to replace the duplicate parts of the keys. The best that I can do is to create a helper function that goes through the results of the function above:
def print_trie(trie):
# sort list alphabetically
alphabetized = list(sorted(set(trawl_trie(trie))))
print(alphabetized[0])
# compare the 2nd item ~ to the previous one in the list.
for k in range(1, len(alphabetized)):
# separate words from value
prev_w, prev_d = (re.findall(r'(\w+):', alphabetized[k - 1]), re.findall(r': (\w+)', alphabetized[k - 1]))
curr_w, curr_d = (re.findall(r'(\w+):', alphabetized[k]), re.findall(r': (\w+)', alphabetized[k]))
word = ''
# find parts that match and replace them with dashes
for i in range(min(len(prev_w[0]), len(curr_w[0]))):
if prev_w[0][i] == curr_w[0][i]:
word += prev_w[0][i]
curr_w[0] = re.sub(word, '-' * len(word), curr_w[0])
print(curr_w[0] + ": " + str(curr_d[0]))
This would be the output:
print_trie(trie)
chair: flesh
---t: cat
--ic: stylish
---en: dog
Does anyone know if it would be possible to get the same result with one recursive function? Or I am stuck using a recursive function to go through the trie, and a second helper function to make everything look nice?
Cheers,
def insert(trie, key, value):
"""Insert into Trie"""
if key:
first, rest = key[0], key[1:]
if first not in trie:
trie[first] = {}
insert(trie[first], rest, value)
else:
trie['value'] = value
def display(trie, s = ""):
"""Recursive function to Display Trie entries in alphabetical order"""
first = True
for k, v in sorted(trie.items(), key = lambda x: x[0]):
# dictionary sorted based upon the keys
if isinstance(v, dict):
if first:
prefix = s + k # first to show common prefix
first = False
else:
prefix = '-'*len(s) + k # dashes for common prefix
display(v, prefix) # s+k is extending string s for display by appending current key k
else:
print(s, ":", v) # not a dictionary, so print current # not a dictionary, so print current string s and value
# Create Trie
trie = {}
insert(trie, 'chat', 'cat')
insert(trie, 'chien', 'dog')
insert(trie, 'chair', 'flesh')
insert(trie, 'chic', 'stylish')
#Display Use Recursive function (second argument will default to "" on call)
display(trie)
Output
chair : flesh
---t : cat
--ic : stylish
---en : dog
I modified DarrylG's answer ( thanks again! ) by adding a list of accepted keys that gets passed through the recursion and a pair of for
-loops that iterate through this list to see if there are any common elements at the beginning of the string that can be replaced.
Edit 11/2/19: This modification has a bug that I hadn't noticed. It will run correctly the first time, but on subsequent runs it will replace too many characters will dashes, ie:
----r: flesh
---t: cat
---c: stylish
----n: dog
def insert(trie, key, value):
"""Insert into Trie"""
if key:
first, rest = key[0], key[1:]
if first not in trie:
trie[first] = {}
insert(trie[first], rest, value)
else:
trie['value'] = value
def display(trie, s = "", final = []):
"""Recursive function to Display Trie entries in alphabetical order"""
for k, v in sorted(trie.items(), key = lambda x: x[0]):
# dictionary sorted based upon the keys
if isinstance(v, dict):
display(v, s + k, final) # s+k is extending string s for display by appending current key k
else:
# replace common elements at beginning of strings with dashes
i = sum([any([f.startswith(string[:j]) for f in final]) for j in range(1, len(string))])
string = '-' * i + s[i:]
print(string + ":", v) # not a dictionary, so print current edited s and value
final.append(s)
# Create Trie
trie = {}
insert(trie, 'chat', 'cat')
insert(trie, 'chien', 'dog')
insert(trie, 'chair', 'flesh')
insert(trie, 'chic', 'stylish')
#Display Use Recursive function (second argument will default to "" on call)
display(trie)
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