On Datorama when trying to get the length of an array I'm able to get it using a calculated dimension by:
a = SPLIT([Media_Buy_Name],'_')
return EXTRACT(a[a.length-1],'&',0)
But when using the formula editor inside the mapping tool, the same code is not working : class not found: java.lang.Object$length
a
? Even if it's not syntactically correct, it would be problematic.