# Controlling evaluation

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?v1v2v3               / values
v1v1v3v2v3v2v3v3v2v1
q)r1r2r3 v1v2v3?v             / Find
r1r1r3r2r3r2r3r3r2r1
q)(v1v2!r1r2) v                 / dictionary: implicit default
r1r1r2r2r2r1
q)r1r2default v1v2?v           / explicit default
r1r1defaultr2defaultr2defaultdefaultr2r1

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;::) v1v2?v)@'v
abcv1
abcv1
v3
"v2"
v3
"v2"
v3
v3
"v2"
abcv1

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.

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: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.

## 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.