Why can't I use sourcecode = "myFile.txt" f = open(sourcecode, mode='rb') to open my file and compress it? This is all pretty new to me. I would be glad if some of you could give me some advice on how to solve the problem.
def compress(uncompressed):
"""Compress a string to a list of output symbols."""
sourcecode = "myFile.txt"
f = open(sourcecode, mode='rb')
# Build the dictionary.
dict_size = 256
dictionary = dict((chr(i), chr(i)) for i in xrange(dict_size))
# in Python 3: dictionary = {chr(i): chr(i) for i in range(dict_size)}
w = ""
result = []
for c in uncompressed:
wc = w + c
if wc in dictionary:
w = wc
else:
result.append(dictionary[w])
# Add wc to the dictionary.
dictionary[wc] = dict_size
dict_size += 1
w = c
# Output the code for w.
if w:
result.append(dictionary[w])
return result
def decompress(compressed):
"""Decompress a list of output ks to a string."""
# Build the dictionary.
dict_size = 256
dictionary = dict((chr(i), chr(i)) for i in xrange(dict_size))
# in Python 3: dictionary = {chr(i): chr(i) for i in range(dict_size)}
w = result = compressed.pop(0)
for k in compressed:
if k in dictionary:
entry = dictionary[k]
elif k == dict_size:
entry = w + w[0]
else:
raise ValueError('Bad compressed k: %s' % k)
result += entry
# Add w+entry[0] to the dictionary.
dictionary[dict_size] = w + entry[0]
dict_size += 1
w = entry
return result
compressed = compress(f)
print (compressed)
decompressed = decompress(compressed)
print (decompressed)
I found this code at: http://rosettacode.org/wiki/LZW_compression#Python
One problem is the fact that you open the file inside the compress function. That means that f is not visible outside the function, and therefore calling compress(f) will give you an error that it does not find f. The correct syntax would be to move the lines sourcecode = "myFile.txt" and f = open(sourcecode, mode='rb') there, so that you have:
sourcecode = "myFile.txt"
f = open(sourcecode, mode='rb')
compressed = compress(f.read())
f.close() # don't forget to close open files
Notice the f.read() when calling compress. f by itself is a file descriptor, f.read() is the one that returns the contents of the file as a string to be given as an argument to compress()
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