How does nltk's concordance work?

Question

Something I discovered while working on this problem was concordance does not like to display the context at the start of a Text :

>>> from nltk.book import *
>>> text3.concordance("beginning",lines=1)
Displaying 1 of 5 matches:
                                   beginning God created the heaven and the ear

Note there is no "In the" in the output above. However concordance has no problem with the end of the Text .

>>> text3.concordance("coffin",lines=1)
Displaying 1 of 1 matches:
 embalmed him , and he was put in a coffin in Egypt .

Interestingly, if you specify a width things work out better (default width=79 , I believe).

>>> text3.concordance("beginning",width=11, lines=1)
Displaying 1 of 5 matches:
In the beginning 

Anyone have an explanation for this? The doc at nltk.org says:

Print a concordance for word with the specified context window. Word matching is not case-sensitive.

Answer 1

Consider this function concordance which i have modified from original source code in the class ConcordanceIndex() from source code HERE .

def print_concordance(self, word, width=35, lines=25):
    """
    Print a concordance for ``word`` with the specified context window.

    :param word: The target word
    :type word: str
    :param width: The width of each line, in characters (default=80)
    :type width: int
    :param lines: The number of lines to display (default=25)
    :type lines: int
    """
    #print ("inside:")
    #print (width)
    half_width = (width - len(word) - 2) // 2
    #print (half_width)
    context = width // 4 # approx number of words of context
    #print ("Context:"+str(context))
    offsets = self.offsets(word)
    if offsets:
        lines = min(lines, len(offsets))
        print("Displaying %s of %s matches:" % (lines, len(offsets)))
        for i in offsets:
            #print(i)
            if lines <= 0:
                break
            left = (' ' * half_width +
                    ' '.join(self._tokens[i-context:i])) #This is were you have to concentrate 
            #print(i-context)
            #print(self._tokens[i-context:i])
            right = ' '.join(self._tokens[i+1:i+context])
            left = left[-half_width:]
            right = right[:half_width]
            print(left, self._tokens[i], right)
            lines -= 1
    else:
        print("No matches")

From the commented area you can observe whenever the value becomes '-ve' then nothing prints out on the console.

You can have ['+ve':'-ve'] but not ['-ve':'+ve']. hence nothing gets printed, in other way null string gets printed.

When self._tokens[i-context:i] initially value will be positive as width increases it tends to negative and hence no output.