Create an Empty Typed Table

Question

Create an empty table named t with the following schema:

The table should contain no rows.

Solution

Method 1

t:([] a:`symbol$(); b:`float$(); c:`int$())

Result:

a b c
-----

Method 2

t:([] a:"S"$(); b:"F"$(); c:"I"$())

Method 3

t:([] a:0#`; b:0#0n; c:0#0Ni)

Method 4

t:flip `a`b`c!"SFI"$\:()

Method 5

t:flip ([a:0#`; b:0#0n; c:0#0Ni])

All methods create the same empty typed table.

Explanation

When creating an empty table, it is important to define the column types explicitly.

Method 1: Type Casting

An empty list can be cast to a specific type:

`symbol$()
`float$()
`int$()

These typed empty lists are then used as the table columns:

([] a:`symbol$(); b:`float$(); c:`int$())

Method 2: Type Characters

The same table can be created using each type's character code with $ (Tok). Applied to an empty list, it returns an empty list of that type:

"S"$()
"F"$()
"I"$()

where "S", "F" and "I" are the type characters for symbol, float and int respectively.

Method 3: Empty Lists Using Take

A zero-length list can also be created using the Take operator:

0#`
0#0n
0#0Ni

Result:

`symbol$()
`float$()
`int$()

These typed empty lists can be used directly as table columns.

Method 4: Flip a Dictionary

A table is internally a flipped column dictionary.

The expression:

`a`b`c!"SFI"$\:()

creates a dictionary whose values are empty typed lists. The console shows empty values as blanks, but .Q.s1 shows them explicitly:

`a`b`c!(`symbol$();`float$();`int$())

Applying flip converts it into a table:

flip `a`b`c!"SFI"$\:()

Method 5: Dictionary Literal Syntax

Dictionary literal syntax can also be used:

([a:0#`; b:0#0n; c:0#0Ni])

which creates a column dictionary.

Applying flip converts it into a table:

flip ([a:0#`; b:0#0n; c:0#0Ni])

Verifying the Schema

The column names can be inspected using:

cols t

Result:

`a`b`c

The column types can be inspected using:

type each value flip t

Result:

11 9 6h

which are the type numbers of symbol, float and int lists respectively.

Summary

There are several ways to create an empty typed table in q:

• Cast empty lists to the required types.
• Use type characters ("SFI") with $.
• Create zero-length typed lists with 0#.
• Build a column dictionary and flip it.
• Use dictionary literal syntax and flip.

All approaches produce the same table schema with zero rows.