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Quiz 1.3: Variables and objects

Q1: Passing functions as argument to other functions (callbacks)

Which is the output of the following snippet?:

foo(x)     = x+1
boo(x)     = x+2
zoo(foo,x) = foo(x) + 1
zoo.(boo,[1,2,3])
RESOLUTION

The code first defines two functions, foo and boo, then it defines the function zoo whose first parameter is a function object and then it calls the function with the function boo and the value 1. It doesn't matter that the argument name is foo like the foo function, that is only a local variable within the zoo function. What is actually passed (as an object) is the boo function, so that is the one that will be used to evaluate the result. Note that functions in Julia are First-class citizens of the language: they can be passed as arguments to other functions (as done here), returned by other functions, assigned as objects to any identifier or stored in data structures.

The correct answers are:

  • The vector [4,5,6]

Q2: Effects of different ways of copying objects

Given the following snippet:

foo       = [[1,2],3]
goo       = foo[1]
goo[1]    = 10
zoo       = copy(foo)
zoo[1][1] = foo[1][1]+10
zoo[1]    = 100
doo       = deepcopy(foo)
doo[1]    = foo[1][1]+100

Which is the value of doo[1] ?


RESOLUTION

The object "stored" as first element of the array foo is assigned to the name binding goo too. It is hence the same object that is mutated in the third line command. zoo creates a new array object, but this different object stores the same object as first element. zoo[1][1] = foo[1][1]+10 hence mutates itself (it is equivalent to zoo[1][1] += 10), while the following line reassigns the first element of the (independent) array referenced by zoo to an other totally different object (an integer). foo[1][1] hence is first mutated to 10 using goo and then is further increased of 10 using zoo. When used to compute the value of doo[1] it is hence 120.

The correct answer is: 120