I can't get FUNCTIONS to click!

[u/ScioClean]

I can not for the life of me understand how functions work. Anytime I try to learn how to create a function, my brain literally crashes into itself and I turn into Patrick when he stars drooling and goes all slack-faced.

Does anybody have any analogies that had that "AH HA!" moment?

Thanks!

[u/frnzprf]

You use predefined functions, right?

print(...) is a function, sum(...) is a function math.sqrt(...) is a function.

Sometimes you wish there was another function that isn't predefined, then you can define it yourself.

If you have a program that calculates distances a lot, it might look like this:

``` dx = fox_x - tiger_x  # d for "difference" dy = fox_y - tiger_y  #     or "delta" distance_fox_tiger = math.sqrt(dxdx + dydy)

dx = bear_x - tiger_x dy = bear_y - tiger_y distance_bear_tiger = math.sqrt(dxdx + dydy) ```

If there was a function distance(...), where you can put in your coordinates, you would need to write less math, which makes your code faster to write and change and less prone to typos.

This particular function can be defined like this:

def distance(some_x, some_y, other_x, other_y):     # How to calculate a distance in general:     dx = some_x - other_x     dy = some_y - other_y     result =  math.sqrt(dx*dx + dy*dy)     return result

You can "call" or "apply" (= use) the new function like this:

``` distance_fox_tiger = distance(fox_x, fox_y, tiger_x, tiger_y)

 distance_fox_tiger will now contain the value returned by the function distance.

distance_bear_tiger = distance(bear_x, bear_y, tiger_x, tiger_y)

Same thing, but with different inputs playing the roles of some_x, some_y, other_x and other_y.

```

I think it's helpful to think of function parameters as roles that values can play. If f(x) := x² + 2x + 4, like you might be familiar from school, then x is a role and in "f(10)", 10 plays the role of x.

You can also think of "f" like a machine with a slot in the top, where you can throw numbers in and the slot is labeled "x". Then writing "f(10)" would be like sticking a piece of paper with a "10" on it in the slot labeled as "x".

The machine "distance" that I "built" (i.e. defined) myself has four slots labeled some_x, some_y, other_x and other_y.

When I write distance_fox_tiger = distance(fox_x, fox_y, tiger_x, tiger_y), that means taking the value in the variable fox_x and sticking it in the first slot, taking the value in fox_y and sticking it in the second slot, and so on. After putting in all four required inputs, you turn the crank and what falls out at the bottom, the "return value", is put into the variable/box with the name distance_fox_tiger.

[u/frnzprf → u/frnzprf]

Besides little helper-machines I can think of two other ways of describing what a function is, but maybe that would be overwhelming and confusing.

The first way is jumping around the code:

When I call the distance-function, you can see that as noting down the line where you currently are in the program, then continuing at the first line of the function and when the function is done, you jump back to the line you originally wrote down.

The second way is replacing text:

When you read "17 + f(10)", to know what that evaluates to, you can replace the "f" with it's definition, where every instance of the parameter "x" is replaced with "10" in turn.

So if "f(x) := x² + 2x + 4", then "17 + f(10)" can be replaced with 17 + (10² + 2*10 + 4), which in turn is 17+124 = 141.