lambda functions as higher-order functions [closed]

Question

I'm learning about higher-order functions, specifically lambda functions in Python. I understand lambda functions to an extent, but a little confused about this lambda higher-order function example.

Why am I getting 6 as the output, I'm just trying to understand this mathematically

>>> high_ord_func = lambda x, func: x + func(x)
>>> high_ord_func(2, lambda x: x * x)
6

I got this problem from Real Python article. Please, I just want to make sense of this, thank you

Answer 1

You can consider high_ord_func as taking 2 arguments, the first is the value of x, and the second is the form of a function to evaluate.

Your second line is evaluating that function for x = 2 and func = x*x

This expands as:

x + func(x) = x + x*x = 2 + 2*2 = 6

Answer 2

Below is simple implementation of what you want to achive with High order function

>>> def func(x: int) -> int:
...     return x * x
...
>>> from collections.abc import Callable
>>>
>>> def high_ord_func(x: int, func: Callable) -> int:
...     return x + func(x)
...
>>> high_ord_func(2, func)
6
>>>

so when you calling high_ord_func by passing 2 and func as argument, you expect 2 + func(2) as return. which on further evaluation of func(2) we got 4, thus making 2+4 and resulting into 6

Answer 3

This is a matter of properly understanding the syntax of the lambda function.
A lambda function is defined as follows: lambda input:output

So in your code

high_ord_func = lambda x, func: x + func(x)

Let's take the part about the lambda function

lambda x, func: x + func(x)

This function takes two inputs

• x

• func

And it returns: x + func(x)

This implies that x is a variable and func is a callable (a function)

Then this lambda function is assigned to the function high_ord_func

This is similar to

def high_ord_func(x, func):
    return x + func(x)

Then, when you call

high_ord_func(2, lambda x: x * x)

The value of x is 2 (the first argument)

And func is the function lambda x: x * x which is a square function (it returns the square of its input).

So the output is: 2 + func(2) = 2 + 2*2 = 6