problem related to functions application

Hello, I'm Iris- a new learner in PP and hope to get your help in learning PP.

I met a problem: when i read an excel with  A column which are numbers, i want to calculate the sqrt of numbers in column A, I used 'custom manipulator' component with the expression sqrt(A), but it showed "Statement doesn't appear to do anything" when running. How can I  solve it?

Many thanks in advance.