chpater 4 update

LearnJulia/src/Chapter4.jl

@@ -1,7 +1,34 @@
 using DataFrames
+using CSV
+
+# Julid for Data Sciences makes use of functions to keep variable scooping under control
 
 function grades_2020()
   name = ["Sally", "Bob", "Alice", "Hank"]
   grade_2020 = [1, 5, 8.5, 4]
   DataFrame(; name, grade_2020)
 end
+
+function write_grades()
+  path = joinpath("data", "grades.csv")
+  CSV.write(path, grades_2020())
+end
+
+
+filter(:name => ==("Alice"), grades_2020())
+
+function complex_filter(name, grade)::Bool
+  name = startswith(name, "A") || startswith(name, "B")
+  grade = 6 < grade
+  name && grade
+end
+
+filter([:name, :grade_2020] => complex_filter, grades_2020())
+
+function salaries()
+  names = ["John", "Hank", "Karen", "Zed"]
+  salary = [1_900, 2_800, 2_800, missing]
+  DataFrame(; names, salary)
+end
+
+subset(salaries(), :salary => ByRow(>(2_000)); skipmissing = true)