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KFoldIterator - Splits wrongly, last fold is usually to small or even zero #5974

boschhd opened this issue Jul 26, 2018 · 2 comments


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commented Jul 26, 2018

Expected behavior: KFoldIterator should split the dataset into k folds as evenly as possible.

Observed behavior: The last batch is often very small, in the range of 0..(k-1), which could explain the high variance of results in issue #5343

If the dataset size (n) is divisible by the desired number of folds (k) without remainder, it splits the dataset evenly.
However, if there is a remainder, it divides only into (k-1) folds and assigns the remainder to the last fold. This creates a last fold of only up to (modulo k-1) elements! This means that that the test set of the last fold will be extremely small and probably create NaN results in evaluation due to missing classes.

In the case that n is divisible by k and k-1 at the same time, this even creates an empty fold which will cause Exceptions later on.

Related lines in the file

Thanks to @RajaniVM for noticing the problem!


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commented Jul 27, 2018

Thanks for the issue (and @RajaniVM for flagging) - easy to confirm with size 99, 10 splits:

    public void test5974(){
        DataSet ds = new DataSet(Nd4j.linspace(1,99,99).transpose(), Nd4j.linspace(1,99,99).transpose());

        KFoldIterator iter = new KFoldIterator(10, ds);

            DataSet fold =;
java.lang.IllegalArgumentException: NDArrayIndex is out of range. Beginning index: 99 must be less than its size: 99
	at org.nd4j.linalg.indexing.NDArrayIndex.validate(
	at org.nd4j.linalg.indexing.NDArrayIndex.resolve(
	at org.nd4j.linalg.api.ndarray.BaseNDArray.get(
	at org.nd4j.linalg.dataset.DataSet.getRange(
	at org.nd4j.linalg.dataset.api.iterator.KFoldIterator.nextFold(
	at org.nd4j.linalg.dataset.KFoldIteratorTest.test5974(

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commented Sep 21, 2018

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.

@lock lock bot locked and limited conversation to collaborators Sep 21, 2018

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