lastTokenIndex

LastTokenIndex is a variable name used in natural language processing, parsing, and tokenization to denote the position of the final token in a sequence of tokens. It is not a built-in language feature but a common convention in software that manipulates token streams.

In tokenization, lastTokenIndex usually equals len(tokens) - 1 in zero-based indexing. If a tokenizer adds padding or special tokens, the meaningful last token may occur before the end of the list, in which case lastTokenIndex is less than len(tokens) - 1.

In sequence modeling, lastTokenIndex marks the boundary for processing, evaluation, or generation. It helps to slice inputs to fixed lengths or to align token positions with attention masks, segment IDs, or other alignment data. Some systems treat end-as-exclusive bounds, in which case the end index is len(tokens).

Edge cases include empty inputs and off-by-one risks. With an empty token list, lastTokenIndex is often defined as -1 or can be considered undefined. Always clarify whether lastTokenIndex is inclusive (points to the last token) or exclusive (points to one past the last token).

Related terms include tokenCount, tokenStart, tokenEnd, and tokenSpan. An example: tokens = ["Hello", "world"]; lastTokenIndex = 1; len(tokens) = 2; end-exclusive boundary would be 2.