stanfordnlp/CoreNLP · error · IllegalArgumentException

Error -- no such thing as zeroth child!

Error message

Error -- no such thing as zeroth child!

What it means

Tregex's IthChildOf relation (>i or >-i) selects the i-th child of a node, with 1-based indexing (negative for counting from the end). i == 0 has no meaning (there is no zeroth child), so the constructor throws IllegalArgumentException.

Solutions

1. Use >1 for the first child (or >-1 for the last)
2. Adjust programmatic index generation to be 1-based
3. Validate user-supplied child indices != 0 before compiling the pattern

Example fix

// before
String pattern = "S >" + childIdx; // childIdx == 0
// after
String pattern = "S >" + (childIdx + 1); // first child is >1

if (i == 0) throw new IllegalArgumentException("child index must be nonzero (1-based), got 0");

boolean validChildIndex(int i) { return i != 0; }

try { TregexPattern.compile(pattern); } catch (IllegalArgumentException e) { /* report bad >i index */ }

When it happens

Trigger: A tregex pattern contains '>0' (e.g. "NP>0"), so the parser constructs IthChildOf(0) and the constructor throws.

Common situations: Zero-based habits from array indexing leak into hand-written tregex queries; generated queries with loop counters starting at 0; users misremembering whether >1 is the first child.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

• Error -- no such thing as zeroth leaf!
• Node cannot be both negated and optional.
• after edge derivative, index() != edgeParamCount()
• after W derivative, index() != beforeOutputWeights()
• after W derivative, index() != x.length()

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/42c408a38efc2b8f. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/trees/tregex/Relation.java:1368

    public int hashCode() {
      int result = super.hashCode();
      result = 29 * result
          + (immediatelyHeads != null ? immediatelyHeads.hashCode() : 0);
      return result;
    }
  }


  private static class IthChildOf extends Relation {

    private static final long serialVersionUID = -1463126827537879633L;

    private final int childNum;

    IthChildOf(int i) {
      super('>' + String.valueOf(i));
      if (i == 0) {
        throw new IllegalArgumentException(
            "Error -- no such thing as zeroth child!");
      } else {
        childNum = i;
      }
    }

    @Override
    Iterator<Tree> searchNodeIterator(final Tree t,
                                      final TregexMatcher matcher) {
      return new SearchNodeIterator() {
        @Override
        void initialize() {
          if (t != matcher.getRoot()) {
            next = matcher.getParent(t);
            if (childNum > 0
                && (next.numChildren() < childNum || next
                    .getChild(childNum - 1) != t)
                || childNum < 0

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