rohitg00/ai-engineering-from-scratch · error · ValueError

bleu4 requires at least one reference

Error message

bleu4 requires at least one reference

What it means

Error "bleu4 requires at least one reference" thrown in rohitg00/ai-engineering-from-scratch.

Source

Thrown at phases/19-capstone-projects/63-multimodal-eval/code/main.py:130

    return [tuple(seq[i:i + n]) for i in range(len(seq) - n + 1)]


def _count(ngrams: list[tuple[int, ...]]) -> dict[tuple[int, ...], int]:
    out: dict[tuple[int, ...], int] = {}
    for g in ngrams:
        out[g] = out.get(g, 0) + 1
    return out


def bleu4(generated: list[int], references: list[list[int]],
          smoothing: bool = True) -> float:
    """BLEU-4 against multiple reference captions.

    Uses Chen and Cherry "method 1" smoothing when any n-gram precision is 0
    and `smoothing` is True.
    """
    if not references:
        raise ValueError("bleu4 requires at least one reference")
    if not generated:
        return 0.0

    weights = [0.25, 0.25, 0.25, 0.25]
    log_p_sum = 0.0
    for n in range(1, 5):
        gen_ngrams = _ngrams(generated, n)
        gen_counts = _count(gen_ngrams)
        ref_max_counts: dict[tuple[int, ...], int] = {}
        for ref in references:
            ref_counts = _count(_ngrams(ref, n))
            for g, c in ref_counts.items():
                if c > ref_max_counts.get(g, 0):
                    ref_max_counts[g] = c

        clipped = 0
        for g, c in gen_counts.items():
            clipped += min(c, ref_max_counts.get(g, 0))

View on GitHub (pinned to 39ea8a1c6d)

When it happens

Trigger: Thrown at phases/19-capstone-projects/63-multimodal-eval/code/main.py:130 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of rohitg00/ai-engineering-from-scratch@39ea8a1c6d (2026-08-26). Data as JSON: /api/errors/a73a443cce15fcec. Report an issue: GitHub.