Error when performing pattern search on a randomly generated characters:-CodePudding

So I am trying to implement the Knuth-Morris-Pratt algorithm in Python, below is my implementation:

def KMP(Pattern, Chars):
   # compute the start position (number of characters)of the longest suffix that matches the prefix
   # Then store prefix and the suffix into the list K, and then set the first element of K to be 0 and the second element to be 1
    K = [] # K[n] store the value so that if the mismatch happens at n, it should move pattern Pattern K[n] characters ahead. 
    n = -1
    K.append(n) #add the first element, and keep n = 0. 
    for k in range (1,len(Pattern)   1):
        # traverse all the elements in Pattern, calculate the corresponding value for each element. 
        while(n >=0 and Pattern[n] != Pattern[k - 1]): # if n = 1, if n >=1 and the current suffix does not match then try a shorter suffix
            n = K[n]
        n = n   1 # if it matches, then the matching position should be one character ahead
        K.append(n) #record the matching position for k
    
    #match the string Chars with Pattern
    m = 0
    for i in range(0, len(Chars)): #traverse through the list one by one
        while(m >= 0 and Pattern[m] != Chars[i]): # if they do not match then move Pattern forward with K[m] characters and restart the comparison
            m = K[m]
        m = m   1 #if position m matches, then move forward with the next position 
        if m == len(Pattern): # if m is already the end of K (or Pattern), then a fully matched pattern is found. Continue the comparison by moving Pattern forward K[m] characters
            print(i - m   1, i)
            m = K[m]
def main():
        Pattern = "abcba"
        letters = "abc"
        Chars = print ( ''.join(random.choice(letters) for i in range(1000)) ) 
        kmp(Pattern, Chars)
if __name__ == '__main__':
    main()

When I try to run this code for a list of randomly generated letters which are abc I get the following error:

 ---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-25-c7bc734e5e35> in <module>
      1 if __name__ == '__main__':
----> 2     main()

<ipython-input-24-2c3de20f253f> in main()
      3         letters = "abc"
      4         Chars = print ( ''.join(random.choice(letters) for i in range(1000)) )
----> 5         KMP(Pattern, Chars)

<ipython-input-21-edf1808c23d4> in KMP(Pattern, Chars)
     14     #match the string Chars with Pattern
     15     m = 0
---> 16     for i in range(0, len(Chars)): #traverse through the list one by one
     17         while(m >= 0 and Pattern[m] != Chars[i]): # if they do not match then move Pattern forward with K[m] characters and restart the comparison
     18             m = K[m]

TypeError: object of type 'NoneType' has no len()

I am not really sure what I am doing wrong, any help will be greatly appreciated

CodePudding user response：

After I replaced

Chars = print ( ''.join(random.choice(letters) for i in range(1000)) )

Chars =  ''.join(random.choice(letters) for i in range(1000))

it worked for me.