Python练习中的高阶函数

Question

I learning Python and during solution an exercise, function filter() returns empty list and i can't understand why. Here is my source code:

"""
Using the higher order function filter(), define a function filter_long_words()
that takes a list of words and an integer n and returns
the list of words that are longer than n.
"""

def filter_long_words(input_list, n):
    print 'n = ', n
    lengths = map(len, input_list)
    print 'lengths = ', lengths
    dictionary = dict(zip(lengths, input_list))
    filtered_lengths = filter(lambda x: x > n, lengths) #i think error is here
    print 'filtered_lengths = ', filtered_lengths
    print 'dict = ',dictionary
    result = [dictionary[i] for i in filtered_lengths]
    return result

input_string = raw_input("Enter a list of words\n")
input_list = []
input_list = input_string.split(' ')
n = raw_input("Display words, that longer than...\n")

print filter_long_words(input_list, n)

Answer 1

Your function filter_long_words works fine, but the error stems from the fact that when you do:

n = raw_input("Display words, that longer than...\n")
print filter_long_words(input_list, n)  

n is a string, not an integer.

Unfortunately, a string is always "greater" than an integer in Python (but you shouldn't compare them anyway!):

>>> 2 > '0'
False

If you're curious why, this question has the answer: How does Python compare string and int?

---

Regarding the rest of your code, you should not create a dictionary that maps the lengths of the strings to the strings themselves.

What happens when you have two strings of equal length? You should map the other way around: strings to their length.

But better yet: you don't even need to create a dictionary:

filtered_words = filter(lambda: len(word) > n, words)

Answer 2

n is a string. Convert it to an int before using it:

n = int(raw_input("Display words, that longer than...\n"))

Python 2.x will attempt to produce a consistent-but-arbitrary ordering for objects with no meaningful ordering relationship to make sorting easier. This was deemed a mistake and changed in the backwards-incompatible 3.x releases; in 3.x, this would have raised a TypeError .

Answer 3

I don't know what your function does, or what you think it does, just looking at it gives me a headache.

Here's a correct answer to your exercise:

def filter_long_words(input_list, n):
    return filter(lambda s: len(s) > n, input_list)

Answer 4

My answer:

def filter_long_words():
     a = raw_input("Please give a list of word's and a number: ").split()
     print "You word's without your Number...", filter(lambda x: x != a, a)[:-1]

filter_long_words()