仅将公式应用于当前行和上一行(Q/KDB) [英] Apply formula to current and previous rows only (Q/KDB)
问题描述
我有一个要逐行应用的公式，这样，任何给定行中的当前行和先前行都不会包含在计算中.考虑以下数据:
I have a formula that I'd like to apply row-by-row, such that only the current and previous rows on any given row are included in calculation. Consider this data:
data:([]dt:2017.01.05D19:45:00.238248239 2017.01.05D20:46:00.282382392 2017.01.05D21:47:00.232842342 2017.01.05D22:48:00.835838442 2017.01.05D20:49:00.282382392;sym:`AAPL`GOOG`AAPL`BBRY`GOOG;price:101.20 800.20 102.30 2.20 800.50;shares:500 100 500 900 100)
data:
dt sym price shares
2017.01.05D19:45:00:238248239 AAPL 101.20 500
2017.01.05D20:46:00:282382392 GOOG 800.20 100
2017.01.05D21:47:00:232842342 AAPL 102.30 500
2017.01.05D22:48:00:835838442 BBRY 2.20 900
2017.01.05D20:49:00:282382392 GOOG 800.50 100
公式select sum price from data where i=(last;i)fby sym会产生我需要的结果，但是它只会产生1个数据点.我需要在数据集的每一行进行该计算.
The formula select sum price from data where i=(last;i)fby sym would yield the result I need, however it would only yield 1 datapoint. I need that calculation done at every row of the dataset.
扫描("\")会应用此行为，但是不幸的是，在使用select语句时，我不知道该怎么做.
Scan ("\") applies this behavior, but unfortunately I don't know how to do that when using select statements.
推荐答案
不能完全确定您想要什么，但是以下内容对每个sym使用最新的price来计算总和rp:
Not entirely sure what you want but the following uses the latest price for each sym to calculate the sum rp:
q)update rp:sum each @\[()!();sym;:;price] from data
dt sym price shares rp
-----------------------------------------------------
2017.01.05D19:45:00.238248239 AAPL 101.2 500 101.2
2017.01.05D20:46:00.282382392 GOOG 800.2 100 901.4
2017.01.05D21:47:00.232842342 AAPL 102.3 500 902.5
2017.01.05D22:48:00.835838442 BBRY 2.2 900 904.7
2017.01.05D20:49:00.282382392 GOOG 800.5 100 905
与您在上面给出的最终数据点答案相同.
Which gives the same answer for the final data point as you have given above.
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