dot-plus_to_c function

Handle + in formula LHS

Given a formula like cyl + gear ~ sum_code, this function recursively replaces the + operators with c so that the LHS becomes c(cyl, gear).

.plus_to_c(plus_expr)

Arguments

• plus_expr: Expression

Returns

A new expression where + is replaced with c()

Details

This function doesn't recurse into other function calls, for example: cyl + gear + factor(1 + 2) evaluates to c(cyl, gear, factor(1+2)).