Q Language - Type Casting

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It is often required to change the data type of some data from one type to another. The standard casting function is the “$” dyadic operator.

Three approaches are used to cast from one type to another (except for string) −

  • Specify desired data type by its symbol name
  • Specify desired data type by its character
  • Specify desired data type by it short value.

Casting Integers to Floats

In the following example of casting integers to floats, all the three different ways of casting are equivalent −

q)a:9 18 27

q)$[`float;a]     / Specify desired data type by its symbol name, 1st way
9 18 27f

q)$["f";a]        / Specify desired data type by its character, 2nd way
9 18 27f

q)$[9h;a]         / Specify desired data type by its short value, 3rd way
9 18 27f

Check if all the three operations are equivalent,

q)($[`float;a]~$["f";a]) and ($[`float;a] ~ $[9h;a])
1b

Casting Strings to Symbols

Casting string to symbols and vice versa works a bit differently. Let’s check it with an example −

q)b: ("Hello";"World";"HelloWorld")    / define a list of strings

q)b
"Hello"
"World"
"HelloWorld"

q)c: `$b                               / this is how to cast strings to symbols

q)c                                    / Now c is a list of symbols
`Hello`World`HelloWorld

Attempting to cast strings to symbols using the keyed words `symbol or 11h will fail with the type error −

q)b
"Hello"
"World"
"HelloWorld"

q)`symbol$b
'type

q)11h$b
'type

Casting Strings to Non-Symbols

Casting strings to a data type other than symbol is accomplished as follows −

q)b:900               / b contain single atomic integer

q)c:string b          / convert this integer atom to string “900”

q)c
"900"

q)`int $ c            / converting string to integer will return the
                      / ASCII equivalent of the character “9”, “0” and
                      / “0” to produce the list of integer 57, 48 and
                      / 48.
57 48 48i

q)6h $ c              / Same as above
57 48 48i

q)"i" $ c             / Same a above
57 48 48i

q)"I" $ c
900i

So to cast an entire string (the list of characters) to a single atom of data type x requires us to specify the upper case letter representing data type x as the first argument to the $ operator. If you specify the data type of x in any other way, it result in the cast being applied to each character of the string.

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