Chapter04 Processing and Summarizing Data

• In statistics nomenclature, the act of summarizing data is known as descriptive statistics. In data science nomenclature such activities take the names of analytics and dash-boarding, while the process of manipulating and pre-processing data is sometimes called data cleansing, or data cleaning.

Mutability, References, Shallow Copies and Deep Copies in Julia

• Two different mechanisms for passing variables to functions.
  • Call by value f() gets a copy of the variable x. As f() executes, even if the code appears to modify x, it is actually modifying a local copy.
  • Call by reference f() obtains a memory reference (pointer) to x. In such a case, as f() executes, if it modifies x, then it actually modifies values in the original memory location of x.
• In Julia, both mechanisms exist under a unified umbrella called pass by sharing. This means that variables are not copied when passed to funcitons. However, if a value is about to a changed within a function then depending on the mutability attribute of its types.
  • If the variable type is immutable then a local copy is made and the behavior follows the "call by value" type.
  • If the variable type is mutable then the called function does not create a local copy. Instead, it can modify the original variable according to "call by reference" mechanisms.
• In Julia, primitive type such as Int64 or Float32 are immutable. But arrays are mutable.

f(z::Int) = begin z=0 end
f(z::Array{Int}) = begin z[1]=0 end

x = 1
@show typeof(x)
@show isimmutable(x)
println("Before call by value: ", x)
f(x)
println("After call by value: ", x, '\n')

x = [1]
@show typeof(x)
@show isimmutable(x)
println("Before call by reference: ", x)
f(x)
println("After call by reference: ", x)

• copy() creats a "shallow copy" of the vairables and copies all entries but does not do it recursively.
• deepcopy() recursively produces a copy until a completely independent copy of the variable is created.

println("Immutable: ")
a = 10
b = a
b = 20
@show a

println("\nNo copy:")
a = [10]
b = a
b[1] = 20
@show a

println("\nCopy: ")
a = [10]
b = copy(a)
b[1] = 20
@show a

println("\nShallow copy: ")
a = [[10]]
b = copy(a)
b[1][1] = 20
@show a

println("\nDeep copy: ")
a = [[10]]
b = deepcopy(a)
b[1][1] = 20
@show a;