> Sorry, but you have confused me more ;) Can you give an simple example
> of just function() ? Where can it be useful?
>
> And when you say it returns a value, what do you mean by that? return to
> whom and what exactly?
One view that's common is the idea that a function is a box that takes an
input and returns an output:
+----------+
input ----> | function | -----> output
+----------+
The most direct equivalent you've experienced to this is a math function.
For example, if we're traveling in a car at fifty miles an hour, for three
hours, we know that we've traveled 150 miles. (Word problems... aagggh!)
But let's write that as a function:
######
def miles_traveled_at_50_mph_for_3_hours():
return 50 * 3
######
A function definition does not do anything by itself, but it can be
"called" after it's defined. Try writing:
#############################################
def miles_traveled_at_50_mph_for_3_hours():
return 50 * 3
print miles_traveled_at_50_mph_for_3_hours()
#############################################
and run it, and see what you see. We are "defining" the function at the
top, and "calling" the function at the bottom,
(Forgive me for such a long function name: please bear with it for a
moment.)
One natural question we might have here is: how far do we travel if we go
at the same speed, but for 4 and a half miles instead? We could just
calculate it and write it out again:
######
def miles_traveled_at_50_mph_for_4_half_hours():
return 50 * 4.5
######
But we can do a little better, by allowing our function to take "hours" as
an parameter:
######
def miles_traveled_at_50_mph(hours):
return 50 * hours
######
Now we can find out how long we've travelled by computing this function
for the particular hours we'd like. (And we can shorten the function
name. *grin*)
For example, can you guess at what:
print miles_traveled_at_50_mph(4)
print miles_traveled_at_50_mph(3)
print miles_traveled_at_50_mph(2)
gives us? Try it!
Functions let us define a relationship between inputs and outputs. The
idea is that we've made our function take in an input, and it can vary its
answer based on that input.
Our miles_traveled function above changes its answer based on what length
of time we drive. But a different function may define an entirely
different relationship between its input and output. As another quick
example:
######
def square(x):
"""Given x, returns the square of x."""
return x * x
######
or:
######
def height(h):
"""Given someone's height h in feet, tells us if that person is high
or not."""
if h < 4:
return "short"
elif h < 6:
return "medium"
else:
return "high"
######
We now have two other functions that also take in a single input, but they
do different things, and even return different kinds of data. The
square() example, like the miles_traveled example, takes a number and
returns another number. But height() takes a number, and returns a
string.
In general, functions can take anything as an input, and return anything
as an output --- I've been using numbers primarily because I'm trying to
piggyback the knowledge you already should have about math and algebra.
In my work, all the functions I write use more complex inputs --- and
usually not numbers --- but the main ideas are the same.
Do you have questions about this so far?
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