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Controlling evaluation

Evaluation is controlled by

  • iterators for iteration
  • conditional evaluation
  • explicit return from a lambda
  • signalling and trapping errors
  • control words


Iterators are the primary means of iterating in q.


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


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.


There is no case or switch control word in q. Use the Case iterator instead. Or a dictionary.

Conditional evaluation

Syntax: ?[x;y;z]

Cond returns z when x is zero; else y.

Two arguments are evaluated: x and either y or z.

Vector Conditional does something similar for lists of arguments, but evaluates all three arguments.

Explicit return

Syntax: :x

The result of a lambda is the last expression in its definition, unless the last expression is empty or an assignment, in which case the lambda returns the generic null ::.

:x has a lambda terminate and return x.

q)foo:{if[0>type x;:x]; x cross x}
q)foo 2 3
2 2
2 3
3 2
3 3
q)foo 3

Signalling and trapping errors

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

q)goo:{if[0>type x;'`type]; x cross x}
q)goo 2 3
2 2
2 3
3 2
3 3
q)goo 3
  [0]  goo 3

Trap and Trap At set traps to catch errors.

Control words


evaluate some expression/s some number of times


evaluate some expression/s if some condition holds


evaluate some expression/s while some condition holds


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

Maps – Each, Each Left, Each Right, Each Parallel, Each Prior
Accumulators – Converge, Do, While

Common errors

Control words are not functions, and return as a result only Identity.

A common error is forgetting to terminate with a semi-colon.

The result of if, do, and while is Identity, (::), which allows one mistakenly to write code such as a:if[1b;42]43 (instead use Cond), or a:0b;if[a;0N!42]a:1b – the sequence is not as intended!