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$ Cond

Conditional evaluation

Syntax: $[x;y;z]

Where x evaluates to zero, returns z, otherwise y.

q)$[0b;`true;`false]
`false
q)$[1b;`true;`false]
`true

Only the first argument is certain to be evaluated.

q)$[1b;`true;x:`false]
`true
q)x
'x

For brevity, nested triads can be flattened: $[q;a;$[r;b;c]] is equivalent to $[q;a;r;b;c]. An example of Cond in a signum-like function:

q){$[x>0;1;x<0;-1;0]}'[0 3 -9]
0 1 -1

$[q;$[r;a;b];c] is not the same as $[q;r;a;b;c].

Cond with many arguments can be translated to triads by repeatedly replacing the last three arguments with the triad: $[q;a;r;b;s;c;d] is $[q;a;$[r;b;$[s;c;d]]]. So Cond always has an odd number of arguments. (Until V3.6 2018.12.06 – see below.)

These two expressions are equivalent:

q)$[0;a;r;b;c]
q)    $[r;b;c]

Assigning a local variable within a code branch

Good style avoids using Cond to control side effects, such as amending variables. (Using if is a clearer signal to the reader that a side effect is intended.)

Also, setting local variables in a code branch can have unintended consequences.

Even numbers of arguments

Since V3.6 2018.12.06 an even number of arguments does not signal 'cond but will return either a result or the generic null.

q)$[1b;`true;1b;`foo]
`true
q)$[0b;`true;1b;`foo]
`foo
q)$[0b;`true;0b;`foo]           / return generic null
q)$[0b;`true;0b;`foo]~(::)
1b

$ dollar
Basics: Evaluation control