Can anyone explain how this functional program work?

Question

def apply_twice(func,arg):
   return func(func(arg))

def add_five(x):
   return x+5

print (apply_twice(add_five,10))

The output I get is 20.

This one is actually confusing me like how is it working.Can anybody explain me how this is working by breaking it down

Answer 1

The function apply_twice(func,arg) takes two arguments, a function object func and an argument to pass to the function func called arg .

In Python, functions can easily be passed around to other functions as arguments, they are not treated differently than any other argument type (ie first class citizens ).

Inside apply_twice , func is called twice in the line:

func(func(arg))

Which, alternatively, can be viewed in a more friendly way as:

res = func(arg)  
func(res)  

If you replace func with the name of the function passed in add_five you get the following:

res = add_five(arg)  # equals: 15
add_five(res)        # result: 20

which, of course, returns your expected result.

The key point to remember from this is that you shouldn't think of functions in Python as some special construct, functions are objects just like int s, lists s and everything else is.

Answer 2

Expanding the code it executes as follows, starting with the print call:

apply_twice(add_five,10))

add_five(add_five(10)) # add_five(10) = 15

add_five(15) # add_five(15) = 20

Which gives you the result: 20.

When apply_twice is called, you are passing in a function object and a value. As you can see in the apply_twice definition, where you see func that is substituted with the function object passed to it (in this case, add_five ). Then, starting with the inner func(arg) call, evaluate the result, which is then passed to add_five again, in the outer return func( ... ) call.

Answer 3

What you need to understand here is that

apply_twice(func,arg)

is a higher function which accepts two arguments (another function named func and an argument arg ). The way it works is that it first evaluate the value of the other function, then use the value as an argument inside the higher function.

remember we have a function add_five(x) which add 5 to the argument supply in it... then this function add_five(x) is then passed as an argument to another function called apply_twice_(func,arg) which return func(func(arg)) .

now splitting func(func(arg)) we have func(arg) #lets called it a then func(func(arg))==func(a ) since a = func(agr)

and (a) is our add_five(x) function, after it add 5, then the value we got is re-used as another fresh argument to add another 5 to it, that is why we have 20 as our result.

Another example is:

def test(func, arg):
  return func(func(arg))

def mult(x):
  return x * x

print(test(mult, 2))

which give 16 as result.