Controlling evaluation¶

' ': /: \: each peach prior $[test;et;ef;…] Cond \ / scan over do if while

.[f;x;e] Trap : Return exit @[f;x;e] Trap-At ' Signal

Evaluation is controlled by

iterators (maps and accumulators) for iteration
conditional evaluation
explicit return from a lambda
signalling and trapping errors
control words
exit

Iterators¶

Iterators are the primary means of iterating in q.

Maps¶

The maps Each, Each Left, Each Right, Each Parallel, and Each Prior are iterators that apply values across the items of lists and dictionaries.

Accumulators¶

The accumulators Scan and Over are iterators that apply values progressively: that is, first to argument/s, then progressively to the result of each evaluation.

For unary values, they have three forms, known as Converge, Do, and While.

Case¶

Case control structures in other languages map values to code or result values. In q this mapping is more often handled by indexing into lists or dictionaries.

q)show v:10?`v1`v2`v3               / values
`v1`v1`v3`v2`v3`v2`v3`v3`v2`v1
q)`r1`r2`r3 `v1`v2`v3?v             / Find
`r1`r1`r3`r2`r3`r2`r3`r3`r2`r1
q)(`v1`v2!`r1`r2) v                 / dictionary: implicit default
`r1`r1``r2``r2```r2`r1
q)`r1`r2`default `v1`v2?v           / explicit default
`r1`r1`default`r2`default`r2`default`default`r2`r1

The values mapped can be functions. The pseudocode

for-each (x in v) {
    switch(x) {
    case `v1:
        `abc,x;
        break;
    case `v2:
        string x;
        break;
    default:
        x;
    }
}

can be written in q as

q)((`abc,;string;::) `v1`v2?v)@'v
`abc`v1
`abc`v1
`v3
"v2"
`v3
"v2"
`v3
`v3
"v2"
`abc`v1

and optimized with .Q.fu.

Control structures¶

Conditional evaluation¶

$[test;et;ef;…]

Cond evaluates and returns ef when test is zero; else et.

In the ternary form, two expressions are evaluated: test and either et or ef. With more expressions, Cond implements if/then/elseif… control structures.

Cond

Vector Conditional

The Vector Conditional operator, unlike Cond, can be used in query templates.

Vector Conditional is an example of a whole class of data-oriented q solutions to problems other languages typically solve with control structures. Data-oriented solutions are typically more efficient and parallelize well.

Control words¶

do: evaluate some expression/s some number of times
if: evaluate some expression/s if some condition holds
while: evaluate some expression/s while some condition holds

Control words are not functions. They return as a result the generic null.

Common errors with control words

a:if[1b;42]43               / instead use Cond
a:0b;if[a;0N!42]a:1b        / the sequence is not as intended!

Control words are little used in practice for iteration. Iterators are more commonly used.

Explicit return¶

:x has a lambda terminate and return x.

Explicit return

Signalling and trapping errors¶

Signal will exit the lambda under evaluation and signal an error to the expression that invoked it.

Trap and Trap At set traps to catch errors.

`exit`¶

The exit keyword terminates kdb+ with the specified return code.