applyComponents with boolean array for checkbox

Question

Hello! 
 I'm trying to show some sections on one form with applyComponents() function. 
 Suppose I have an array of data type MyData. And I want to show sections for each array element one by one with section labels like "Data #1", "Data #2". I'm trying to use 
 applyComponents(rule!mySection( 
 data: myData[_], 
 n: _ 
 ), 
 {1, 2, 3} 
 ) 
 But I get error "Interface Definition: Expression evaluation error at function a!applyComponents [line 8]: Invalid index (-2147483647) for list: valid range is 1..2" 
 Looks like I can use "_" only once and have to pass the whole array into the internal rule. May be there are other way? 
 
 Second question is that I need a checkbox in each section that will make this section editable and later I need this data. Unfortunately a single checkbox will have "true" value for selected state and null otherwise. To get the state of checkboxes I'm trying to create an array of boolean with null values with repeat() function which throws error "Expression evaluation error at function 'repeat': ". The only thing I can think of is to pass true/false value and translate it into true/null in the checkbox and back in the saveInto. Are there more clear ways to do this?

Carlos Santander · Accepted Answer

For your first question, _ is used in function/rule calls, not for array indexing. Yes, you'll need to pass your entire array to the rule. If you're in a more recent Appian version, you can use a!foreach and just access fv!item, fv!index, etc. 
 
For your second question, yes, converting in value and in saveInto is the way to go.

Josh · Answer

For your first question, you can have multiple deferred inputs (_), but in your example is evaluating to: {rule!mySection(data: myData[1], n: 2), rule!mySection(data: myData[3], n: error}. 
 
For your second question I usually create a checkbox wrapper rule that takes in true/false but converts the false value to display null instead.