Select Statement :Select aexp
A :Select structure is used to execute alternative blocks of code depending upon the value of an array. For example, the following displays 'I is 1' if the variable I has the value 1, 'I is 2' if it is 2, or 'I is neither 1 nor 2' if it has some other value.
[1] :Select I
[2] :Case 1
[3] 'I is 1'
[4] :Case 2
[5] 'I is 2'
[6] :Else
[7] 'I is neither 1 nor 2'
[8] :EndSelect
In this case, the system compares the value of the array expression to the right of the :Select statement with each of the expressions to the right of the :Case statements and executes the block of code following the one that matches. If none match, it executes the code following the :Else (which is optional). Note that comparisons are performed using the ≡ primitive function, so the arrays must match exactly. Note also that not all of the :Case expressions are necessarily evaluated because the process stops as soon as a matching expression is found.
Instead of a :Case statement, you may also use a :CaseList statement. If so, the enclose of the array expression to the right of :Select is tested for membership of the array expression to the right of the :CaseList using the ∊ primitive function.
Note also that any code placed between the :Select and the first :Case or :CaseList statements are unreachable; future versions of Dyalog APL may generate an error when attempting to fix functions which include such code.
Example
[1] :Select ?6 6
[2] :Case 6 6
[3] 'Box Cars'
[4] :Case 1 1
[5] 'Snake Eyes'
[6] :CaseList 2⍴¨⍳6
[7] 'Pair'
[8] :CaseList (⍳6),¨⌽⍳6
[9] 'Seven'
[10] :Else
[11] 'Unlucky'
[12] :EndSelect