def undo_inverted_index(inverted_index):
    
    """
    The purpose of the function is to 'undo' and inverted index. It inputs an inverted index and
    returns the original string.
    """

    #create empty lists to store uninverted index
    word_index = []
    words_unindexed = []
    
    #loop through index and return key-value pairs
    for k,v in inverted_index.items(): 
        for index in v: word_index.append([k,index])

    #sort by the index
    word_index = sorted(word_index, key = lambda x : x[1])
    
    #join only the values and flatten
    for pair in word_index:
        words_unindexed.append(pair[0])
    words_unindexed = ' '.join(words_unindexed)
    
    return(words_unindexed)

#create 'original_abstract' feature
ai_search['original_abstract'] = list(map(undo_inverted_index, ai_search['abstract_inverted_index']))