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Issue with xkey
Hi,
I am learning about tables for the first time and have been working through both the Tables and Tables Exercises notebooks.
I encountered a strange occurrence with xkey which I cannot figure out the cause of.
example is:
a b c ---------------------- 1 mini 3.14 2 example 2.72 3 table 2.997925e+08
complexTab is:
c d ---------- 1 2 more 3 4 complex 5 6 table
The question (in the Tables notebook) asks us to create an unkeyed table called bigExample which combines both the tables example and complexTab to have the column order c d a b c.
I am unable to figure out why show bigExample:() xkey complexTab!example results in column c in complexTab taking the place of what should be column c in example, even after the reading the documentation for xkey:
c d a b c ------------------------- 1 2 more 1 mini 1 2 3 4 complex 2 example 3 4 5 6 table 3 table 5 6
The solution provided is show bigExample: 0!complexTab!example which results in what I believe is required:
c d a b c ---------------------------------- 1 2 more 1 mini 3.14 3 4 complex 2 example 2.72 5 6 table 3 table 2.997925e+08
It seems that xkey cannot be used to unkey a table here. I am uncertain as to why this is the case.
(It may be worth noting that example only had columns a and b initially. Column c was later created via
example[`c]: 3.14 2.72 299792458)
This issue does not occur for another question (in the Tables Exercises notebook) which involves unkeying a table.
The question is as follows:
tradeKey is:
id | sym price size side ----| ------------------- 1001| IBM 79 117 B 1002| GE 88 123 S 1003| JPM 83 112 B 1004| BP 74 166 S 1005| JPM 87 136 B 1006| BP 54 137 S 1007| JPM 95 144 B 1008| ASD 91 128 B 1009| JPM 52 120 B 1010| IBM 89 130 B
The question asks us to unkey the tradeKey table and assign it to the variable unkeyedTrade.
The solution provided is:
unkeyedTrade:0!tradeKey
//or
unkeyedTrade:() xkey tradeKey
show unkeyedTradeEither one results in:
id sym price size side ------------------------ 1001 IBM 79 117 B 1002 GE 88 123 S 1003 JPM 83 112 B 1004 BP 74 166 S 1005 JPM 87 136 B 1006 BP 54 137 S 1007 JPM 95 144 B 1008 ASD 91 128 B 1009 JPM 52 120 B 1010 IBM 89 130 B
(It may be worth noting that there was one modification made to tradeKey as part of an earlier question which resulted in what is seen now. The side for tradeKey id 1009 was modified to `B via
tradeKey[([]id:enlist 1009);`side]:enlist `B
That side used to be `S.)
I am uncertain as to why it seems that xkey cannot be used to unkey a table in the first question but it can be utilized in the second question.
I would greatly appreciate any assistance or clarification with this issue.
Thank you very much.
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1 Reply
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The phenomenon you are seeing is because there are two columns named c in complexTab!example. In general duplicated column names in a table are a bad idea, and some functions even check for this and throw an error. But xkey just uses the # operator, which takes the first occurrence of the column to determine its value, so the first c column gets duplicated. ! is a built-in operator so it can use a different implementation to rearrange some pointers in memory that doesn’t rely on the column names.
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