stanfordnlp/CoreNLP · error · IllegalArgumentException

Illegal request for fold

Error message

Illegal request for fold ${fold} of ${numFolds} on data set of size ${size}

What it means

Thrown by GeneralDataset.splitOutFold when the requested fold index is out of the valid range [0, numFolds) for a data set of the given size. It is a range-validation guard protecting k-fold partitioning from bad fold arguments.

Solutions

1. Use fold indices in 0..numFolds-1
2. Verify numFolds matches the value used to create the cross-validation iterator
3. Clamp or wrap fold indices before calling splitOutFold

When it happens

Trigger: Thrown at src/edu/stanford/nlp/classify/GeneralDataset.java:290 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/b29339262cba50c6. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/classify/GeneralDataset.java:290

   *  @param fractionSplit The first fractionSplit of datums (rounded down) will be the second split
   *  @return A Pair of data sets, the first being the remainder of size ceiling(this.size() * (1-p)) drawn
   *          from the end of the dataset and the second of size floor(this.size() * p) drawn from the
   *          start of the dataset.
   */
  public abstract Pair<GeneralDataset<L, F>, GeneralDataset<L, F>> split (double fractionSplit);

  /** Divide out a (devtest) split of the dataset versus the rest of it (as a training set).
   *
   *  @param fold The number of this fold (must be between 0 and (numFolds - 1)
   *  @param numFolds The number of folds to divide the data into (must be greater than or equal to the
   *                  size of the data set)
   *  @return A Pair of data sets, the first being roughly (numFolds-1)/numFolds of the data items
   *         (for use as training data_, and the second being 1/numFolds of the data, taken from the
   *         fold<sup>th</sup> part of the data (for use as devTest data)
   */
  public Pair<GeneralDataset<L, F>, GeneralDataset<L, F>> splitOutFold(int fold, int numFolds) {
    if (numFolds < 2 || numFolds > size() || fold < 0 || fold >= numFolds) {
      throw new IllegalArgumentException("Illegal request for fold " + fold + " of " + numFolds +
              " on data set of size " + size());
    }
    int normalFoldSize = size()/numFolds;
    int start = normalFoldSize * fold;
    int end = start + normalFoldSize;
    if (fold == (numFolds - 1)) {
      end = size();
    }
    return split(start, end);
  }

  /**
   * Returns the number of examples ({@link Datum}s) in the Dataset.
   */
  public int size() {
    return size;
  }

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