What's the use of index in speller.c and where it is declared? I can't understand how it is used in the speller.c.
Index is used to keep track of which character is being iterated over in the word to spell checked. It is also used to verify that the word being spell-checked is a word meeting the standard of, a certain length. i.e not longer than the longest word in the English Language. Index is used wherever we are dealing with words to be spell-checked, checking if a word is the correct length and iterating over the alphabets in the word to be spell-checked.