see
http://code.kx.com/wiki/JB:QforMortals/execution_control#Basic_Conditional_Evaluation
http://code.kx.com/wiki/JB:QforMortals/execution_control#Extended_Conditional_Evaluation
and vector conditional section of
http://code.kx.com/wiki/Reference/QuestionSymbol
n.b. you want ; not ,