How to make this for loop in Python work?

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予麋鹿
予麋鹿 2021-01-19 16:55

I want to spit out a list of palindromes until a certain letter I give.

It\'s about this part:

def pyramid_palindrome(last_letter):
    for letter in         


        
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  • 2021-01-19 17:29

    Your problem was when calling your two palindrome functions and on how you were changing the value for last_letter-

    I tried to modify your code as little as possible:

    def pyramid_palindrome(last_letter):
        for letter in range(97,last_letter):
            print(call_first_part_palindrome(letter) + call_second_part_palindrome(letter-2))
    

    And this remains the same:

    def call_first_part_palindrome(last_letter):
    
        letters_a_to_y = ""             
    
        for letter in range(97,last_letter):
            letters_a_to_y += chr(letter) 
    
        return(letters_a_to_y)
    
    def call_second_part_palindrome(last_letter): 
    
        letters_y_to_a = ""
    
        for letter in range(last_letter,96,-1): 
            letters_y_to_a += chr(letter)
    
        return(letters_y_to_a)
    
    pyramid_palindrome(112)
    

    Which outputs:

    a
    aba
    abcba
    abcdcba
    abcdedcba
    abcdefedcba
    abcdefgfedcba
    abcdefghgfedcba
    abcdefghihgfedcba
    abcdefghijihgfedcba
    abcdefghijkjihgfedcba
    abcdefghijklkjihgfedcba
    abcdefghijklmlkjihgfedcba
    abcdefghijklmnmlkjihgfedcba
    
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  • 2021-01-19 17:37

    Your this entire logic could be simplified using string slicing along with string.ascii_lower as:

    import string
    alphs = string.ascii_lowercase   # returns string of lower case characters
    last_letter = 'f'
    
    for i in range(len(alphs)):
        print alphs[:i]+alphs[i::-1]
        if alphs[i] == last_letter:  # break the loop when `last_letter` is found
            break
    

    which will generate output as:

    a
    aba
    abcba
    abcdcba
    abcdedcba
    abcdefedcba    
    

    Edit: If you do not want to import string, you may get the string of lowercase characters via using:

    alphs = ''.join(chr(i) for i in range(97,123))
    
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  • 2021-01-19 17:40

    Your code is more complicated than it needs to be, and does unnecessary work generating each string of letters from scratch.

    I'm not sure why you don't want to import the letter string from the string module, but you can easily generate the string of letters once and then slice it to get the substrings required to build each palindrome. The code below works on Python 2 or 3.

    def pyramid_palindrome(last_letter):
        letters = ''.join([chr(i) for i in range(97, last_letter)])
        for i in range(last_letter - 97):
            print(letters[:i] + letters[i::-1])
    
    pyramid_palindrome(102)
    

    output

    a
    aba
    abcba
    abcdcba
    abcdedcba
    

    Alternatively, keep letters as a list and use .join on the sliced lists:

    def pyramid_palindrome(last_letter):
        letters = [chr(i) for i in range(97, last_letter)]
        for i in range(last_letter - 97):
            print(''.join(letters[:i] + letters[i::-1]))
    

    It's theoretically faster to add two lists than two strings, although there are optimizations in CPython for small strings so you may not notice the difference unless the strings are longer than 1000 or so. OTOH, calling .join once on letters is probably better than calling it for each palindrome.


    Here's a minor variation of the first version. We save all the palindromes into a list of strings. Then the caller can join that list of strings into one string and print it with one print call.

    def pyramid_palindrome(last_letter):
        letters = [chr(i) for i in range(97, last_letter)]
        return [''.join(letters[:i] + letters[i::-1])
            for i in range(last_letter - 97)]
    
    print('\n'.join(pyramid_palindrome(102)))
    
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