问题
Consider the following table:
myTable:
a b
-------
1
2
3 10
4 50
5 30
How do I replace the empty cells of b with a zero? So the result would be:
a b
-------
1 0
2 0
3 10
4 50
5 30
Right now I'm doing:
myTable: update b:{$[x~0Ni;0;x]}'b from myTable
But I am wondering whether there is a better/easier solution for doing this.
回答1:
Using the fill operator (^)
Example Table:
q) tbl:flip`a`b!(2;0N)#10?0N 0N 0N,til 3
a b
---
0 2
1 1
1 1
1
1
Fill nulls in all columns with 0:
q)0^tbl
a b
---
0 2
1 1
1 1
0 1
1 0
Fill nulls only in selective columns with 0:
q)update 0^b from tbl
a b
---
0 2
1 1
1 1
1
1 0
回答2:
In some cases when you want to fill the null values from the previous non-null value, you can use fills functions.
q)tbl:flip`a`b!(2;0N)#10?0N 0N 0N,til 3
a b
---
2 1
1
1 2
0
update fills a, fills b from tbl
q)a b
---
2 1
2 1
2 1
1 2
0 2
using fills with aggregation
q)tbl:update s:5?`g`a from flip`a`b!(2;0N)#10?0N 0N 0N,til 3
a b s
-----
1 a
2 a
0 0 g
2 2 g
0 a
q)update fills a, fills b by s from tbl
a b s
-----
1 a
1 2 a
0 0 g
2 2 g
0 2 a
来源：https://stackoverflow.com/questions/30016789/kdb-replace-null-integer-with-0