Conditional structures/J

if-then

if. is a simple conditional structure in J. As other control words, if. can be used only within definitions of other syntactic elements.

<lang j> test=: monad define if. 5 > 4 do. 123 end. )

  test

123

  test=: monad define

if. y > 4 do. y * 2 elseif. y = 4 do. 'exactly' elseif. do. y - 10 end. )

  test 5

10

  test 4

exactly

  test 3

_7</lang> The condition may be omitted, or it may be empty (like an array with 0 elements). In this cases the condition is considered to be satisfied.

select-case

select. is another conditional structure. case. match causes the execution of this case. branch and then terminates the further structure execution. fcase. match after the execution of the associated code executes the next branch; if that's with fcase. again, it again executes the next branch, etc.

<lang j> test=: monad define t1=. 'Count to three? ' select. y fcase. 1 do. t1=. t1 , 'one ' fcase. 2 do. t1=. t1 , 'two ' case. 3 do. t1=. t1 , 'three' case. 4 do. 'Just four' end. )

  test 2

Count to three? two three

  test 1

Count to three? one two three

  test 4

Just four

  test 5

Count to three?</lang>

power

Another way to execute code conditionally is power conjunction. In the code u ^: v y verb u is executed with argument y only if v y condition is satisfied.

<lang j> ('magic number'&[) ^: (=&42) 5 5

  ('magic number'&[) ^: (=&42) 6

6

  ('magic number'&[) ^: (=&42) 42

magic number</lang>

agenda

Another way to execute code conditionally is the agenda conjunction. In the code u0`u1`...`uN @. v y result of v y is an index in the range 0..N which determines which of the functions u0..uN will be executed. (Also negative numbers greater than -N are treated as N-(v y).)

<lang j> (2&+)`(3&+)`(5&+) @. * 2 5

  (2&+)`(3&+)`(5&+) @. *  _2

3</lang>

Here, * without a left argument is signum (1 for positive numbers and _1 for negative numbers).

conditions without conditions

Conditional effects can often be obtained without conditional structures. In J, a boolean result is a 1 or a 0, where 1 represents true and 0 represents false. If we have a boolean array B which corresponds in shape to a numeric argument Y, and we have a function F where we want F Y where B is true, and Y where B is false, we can use an expression like:

This also assumes, of course, that F is well behaved (that we can ignore any issues related to side effects), and has the right shape. [The token -. is J's boolean "not" verb.]

If you do not want to pay for the execution of F Y for cases where B is false, and if Y is a simple list, then a variation would be:

For example:

<lang J> Y=: p: i. 5

2 3 5 7 11

  B =: 1 0 1 0 1
  F=: *:
  (Y * -. B) + B * F Y

4 3 25 7 121

  F B # Y

4 25 121

  (Y * -. B) + F&.(B&#) Y

4 3 25 7 121</lang>