Writing a Simple Factorial Program. (Python 2)

Writing a simple factorial program with a "for" loop

What I want to do in this video
is introduce you to a “for loop.”

We are going to do this by writing a little program
that calculates the factorial of a number.

And in case you don't remember what
a factorial of a number does –

you normally write it as “1 factorial” –
as 1, then an exclamation mark. ( 1! ) Like that:

That will just evaluate to 1.

If you say “2 factorial,” ( 2! ),
that’s going to be equal to 2 × 1.

If you say “3 factorial,” ( 3! )
that is going to be equal to

3 × 2 × 1, which is going to be equal to 6.

So, if you have any number’s factorial,
it is just going to be that number,

times one number less than that,
times one number less than that,

all the way down to 1.

So, with that out of the way,
let’s implement our factorial program.

The way I am going to do it is
I am going to take an input from the user.

It’s going to prompt the user to put
a number that they want to take the factorial of;

and then it is going to calculate the factorial.

So what I am going to do is, I am going to use
a function that Python provides for us.

And I'm using Python 2 in this video.

You'd have to do it slightly different —
actually, I'll talk about that in a second —

if you are using Python 3.

Python 2 is what most implementations
of Python are built in at the time of this video.

But there is a Python 3 out there
that is kind of a newer version.

I clarify that a bit in a future video.
But anyway –

We are going to take an input from the user,
and we are going to prompt them with a message.

We are going to say, ”Enter a
non-negative integer to take the factorial of.”

And just so that we familiarize
ourselves with functions and all,

we are passing an argument to the function.

And the argument that we are
passing to the function – to the input function

right over here is a string.

And “string” sounds like a very complicated thing;
but it really is just a string of characters.

You can view it as a bunch of text.

And it will present this text to the user,
and it will give them a little input box.

And the user will put something into the input box.

It will evaluate what is in that input box.

And then, you can either view it as
having the variable number refer to it,

or put it in the variable number, depending
on how you want to visualize the variable.

If you are using Python 3,
you will want to call “eval”

on what’s returned from this function.

This is if you are doing Python 3.

I am running Python 2 right now.

And if you want to do it
exactly the way I'm doing it,

I am using the PyScripter IDE —
Integrated Development Environment.

It’s a free open-source project on a PC.

But there are many, many ways
that you can edit Python.

You can really just do it
on any text editor — so anyway.

So, so far, all we've done
is we've taken input from the user

and we're putting it in the variable number.

And what’s really cool about Python
is we can just run this program as is

and just see what happens.
So let’s do that.

I'm going to save it and I'm going to run it.
This little play button runs it in this IDE.

And just like we said,
it prompts the user:

“Enter a non-negative integer
to take the factorial of.”

So I don't know — le me type in 10.
And I [click] OK. Nothing happened.

Well of course nothing happened!
Because I didn't really tell this to do anything.

But if we type in a number, we'll
see that it is now assigned to 10.

The variable is now
referring to the number 10

So now we can use this number
and take the factorial of it.

So what I'm going to do —
and we're going to do it iteratively. —

And we are going to talk more
about what that means in the future.

So I'm going to define
another variable called product.

And I'm going to start product at 1.

And what we are going to do
is start from 1 up to whatever number this is,

and keep multiplying the product
by each successively larger number

So we are going to start –

So I am going to set up a for loop here —
And in the next video I'm going to really step through

exactly what a for loop does.

Within the for loop, we are
going to define a variable i.

This is kind of the classic
variable to define in for loops.

And that variable is going to keep incrementing
to larger and larger values as we go through it.

So “for i in range.” And for loops
in Python are a little bit different

than if you were doing
it in Javascript or Java.

And I will do a future video where we do that.

But range of number —

And just to understand what
range of number does is,

It provides a list of numbers
starting with 0,

up to 1 less than that number.

So over here we already defined
what number is — number is 10.

So if I type in range of 10 —
or let me do range of number.

It gives me a list starting
with 0 all the way to 9.

All the way to 1 less than than 10

So that the same thing as range of 10.

And what this for loop does is
it assigns this i to each term of the sequence –

So it starts with 1 — or each term of this list.
It starts with 1, and then it'll –

whatever you tell it to do.In the for loop,
with i is equal to 1,then it will assign i to to be 1.

Sorry. It will start with i equals 0,
then it will go to i equals 1,

then i equals 2 then i equals 3 –
so on and so forth – all the way

until you have gone all
the way through the list.

And so what we can do here is — within the for loop — we can redefine

product to be equal to the
product – the former product –

So our new product is going to be
our old product times –

and I am going to multiply it
times i + 1.

We are going to step through
it carefully in the next video.

But I want you to think
about what it is doing already.

So right when we start, let’s
say that number ends up being 3.

So product gets set to 1.

And we say for i in range.

And then, if we do range of 3,
then i is first going to be 0.

And so we are going to take our old product –
so 1 – times i + 1.

Well i is going to be 0 so 0 + 1 is 1.

So it is going to be 1 × 1,
[which] is going to be 1.

Then we are going to assign i to be 1.

This is what the for loop does.

It iterates.

It keeps incrementing,

(Well it doesn't always have to increment.)

It keeps changing the value of i
as it goes through this list.

And so the next time
around our product is 1.

But now i is 1. So 1 plus 1 is 2, times 1
is now going to be 2.

And then it will go – it will set i to be 2.

So it is going to be 2+1 times what product
was from the last iteration – which was 2.

And so it is going to be 2 × 3 which is 6

And so when we are finally done
with this for loop, inside of product –

or I guess we can say product
will be referring to

the actual factorial of the number inputted.

So then we can say print product.

And in Python – in this case right here –
the interpreter knows to break out

of this for loop once it runs
out of numbers to assign i to.

So, that’s why it doesn't run forever.

Let’s see if what we
did here actually works.

Let’s see if it actually works.

So let – actually – let me write it like this.
So let’s see if this things actually works.

So I'll save it.
And then let me run it.

And so it’s asking me, “Enter a
non-negative integer to take the factorial of.”

So let’s try it with 3 – Enter.

It gave me the right answer.

Let’s try it again.

So, “Enter a non-negative
integer to take the factorial of.”

I don't know, let’s
try something large: 10.

It worked. [I'm] assuming that
you can verify this for yourself.

So what we have done here
is write a very simple program.

But, it already does something neat –
it takes a factorial of an arbitrary number.

In the next video, we're going to
step through it much more carefully,

in case this kind of confused you –
what this for loop did.

So, just hold with me for
the next video, and watch that.

That has a little bit of a
more careful explanation.