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Fast array formula for running sum of a column for consecutive identical values in another

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    Question Fast array formula for running sum of a column for consecutive identical values in another

    I have a column (column E) with integer values, and another column (F) with arbitrary values. I want to compute, in column G, the running total of F, but limited to the segment of consecutive rows where the current value of [ICODE]F[/ICODE] is constant. The description may be easier to understand with formulas:
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    I don't know in advance how long the sequence will be and I would like to use an array formula to automatically adjust the length of the output, instead of repeating the formula in G2 in the G column.
    The following formula works:
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    but is much slower than the G2 formula above: it becomes slow when the sequence has a length in the thousands, while the G2 formula can easily handle hundreds of thousands of values.

    How can I make the array formula as fast as the non-array formula?
    (and BTW, why is the array formula so slow? All operations inside it should be very fast.)

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    Re: Fast array formula for running sum of a column for consecutive identical values in ano

    crossposted: https://www.mrexcel.com/board/thread...olumn.1210428/

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    Re: Fast array formula for running sum of a column for consecutive identical values in ano

    Sorry, I wrote the cross-posting reference in the first version of the post, but as a new user I am not allowed to include links, and so I removed it.

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    Re: Fast array formula for running sum of a column for consecutive identical values in ano

    Forum rule #3:

    3. Questions that are cross-posted to other web forums must contain links to those posts on those forums or a comment to tell us where else the question has been asked. (A, Z*)
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    Re: Fast array formula for running sum of a column for consecutive identical values in ano

    Click * below if this answer helped

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    Re: Fast array formula for running sum of a column for consecutive identical values in ano

    After many efforts I finally found an array formula that is almost as fast (i.e. about 3 times slower) as the non-array version:



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