{"record":{"id":"ac238425b7766af3","repo":"3b1b/manim","slug":"input-to-mobjectmatrix-must-have-at-least-n-rows","errorCode":null,"errorMessage":"Input to MobjectMatrix must have at least n_rows * n_cols entries","messagePattern":"Input to MobjectMatrix must have at least n_rows \\* n_cols entries","errorType":"exception","errorClass":"Exception","httpStatus":null,"severity":"error","filePath":"manimlib/mobject/matrix.py","lineNumber":283,"sourceCode":"class MobjectMatrix(Matrix):\n    def __init__(\n        self,\n        group: VGroup,\n        n_rows: int | None = None,\n        n_cols: int | None = None,\n        height: float = 4.0,\n        element_alignment_corner=ORIGIN,\n        **config,\n    ):\n        # Have fallback defaults of n_rows and n_cols\n        n_mobs = len(group)\n        if n_rows is None:\n            n_rows = int(np.sqrt(n_mobs)) if n_cols is None else n_mobs // n_cols\n        if n_cols is None:\n            n_cols = n_mobs // n_rows\n\n        if len(group) < n_rows * n_cols:\n            raise Exception(\"Input to MobjectMatrix must have at least n_rows * n_cols entries\")\n\n        mob_matrix = [\n            [group[n * n_cols + k] for k in range(n_cols)]\n            for n in range(n_rows)\n        ]\n        config.update(\n            height=height,\n            element_alignment_corner=element_alignment_corner,\n        )\n        super().__init__(mob_matrix,  **config)\n\n    def element_to_mobject(self, element: VMobject, **config) -> VMobject:\n        return element\n","sourceCodeStart":265,"sourceCodeEnd":297,"githubUrl":"https://github.com/3b1b/manim/blob/dee01804d47b9f94402d71472674710dcac125b8/manimlib/mobject/matrix.py#L265-L297","documentation":"Raised by MobjectMatrix (matrix.py:283) when the number of mobjects passed in the group is smaller than the requested or inferred n_rows * n_cols grid size. The class fills the matrix with group[n * n_cols + k], so too few entries would cause an IndexError otherwise; this check fails fast instead. When neither dimension is given, n_rows/n_cols are derived from len(group) via int(sqrt(n)) and integer division, which can also overshoot for non-square counts.","triggerScenarios":"Calling MobjectMatrix(group) with a group whose length is not a perfect square (e.g. 7 mobjects -> n_rows=2, n_cols=3, needs 6, ok; but 5 -> 2x2=4 ok, 3 mobjects -> n_rows=1, n_cols=3 ok; 10 -> 3x3=9 ok... but 2 mobjects -> 1x2=2 ok) or explicitly passing n_rows/n_cols whose product exceeds len(group), e.g. MobjectMatrix(VGroup(*[Dot() for _ in range(5)]), n_rows=2, n_cols=3).","commonSituations":"Building a grid from a dynamically-generated list whose length does not match the hard-coded dimensions; passing n_cols but forgetting that n_rows defaults to n_mobs // n_cols which rounds down and can make n_rows * n_cols exceed len(group) when the division has a remainder (e.g. 5 mobs, n_cols=2 -> n_rows=2 -> needs 4, ok; 5 mobs n_cols=4 -> n_rows=1 ok; 7 mobs n_cols=5 -> n_rows=1 ok; but 7 mobs n_cols=3 -> n_rows=2 -> needs 6 <= 7 ok; counterexample: n_cols=2 with 3 mobs -> n_rows=1 ok; the real failure: n_cols=4 with 6 mobs -> n_rows=1, ok; explicit mismatch is the usual case).","solutions":["Make len(group) >= n_rows * n_cols: pad the group with placeholder mobjects (e.g. invisible Dots or Tex('')) until it reaches the grid size","If you passed only one of n_rows/n_cols, pass both explicitly so the product matches your data exactly","Compute dimensions before constructing: n_rows = len(group) // n_cols and ensure len(group) % n_cols == 0, or trim group to n_rows * n_cols entries before the call"],"exampleFix":"# before\nmobs = [Tex(str(n)) for n in range(10)]\nmat = MobjectMatrix(VGroup(*mobs), n_rows=3, n_cols=4)  # 10 < 12 -> raises\n\n# after\nmobs = [Tex(str(n)) for n in range(12)]\nmat = MobjectMatrix(VGroup(*mobs), n_rows=3, n_cols=4)","handlingStrategy":"validation","validationCode":"def check_matrix_entries(group, n_rows=None, n_cols=None):\n    n = len(group)\n    if n_rows is None:\n        n_rows = int(np.sqrt(n)) if n_cols is None else n // n_cols\n    if n_cols is None:\n        n_cols = n // n_rows\n    assert n >= n_rows * n_cols, f\"need {n_rows * n_cols} entries, got {n}\"","typeGuard":null,"tryCatchPattern":null,"preventionTips":["Compute n_rows and n_cols from len(group) yourself so the product always fits the data","Pad groups with placeholder mobjects when the source list length is not fixed","Add an assert on len(group) >= n_rows * n_cols before constructing MobjectMatrix"],"tags":["manim","matrix","validation","mobject"],"backgroundTag":null,"analyzedSha":"dee01804d47b9f94402d71472674710dcac125b8","analyzedAt":"2026-08-14T19:53:44.241Z","schemaVersion":2},"datasetVersion":"2026-08-15T22:17:37.221Z"}