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 foreloop 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 an

number does if I were to tell you what one factorial

you normally write is as one then an exclamation mark

it basically just means well that would just evaluate to one

If you say two factorial that is going to be equal to two

times one if you say three factorial that is going to be

equal to three times two times one and which is going

to be equal to six and so if you have any number of

factorials 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 one so with that out of the way

lets implement our factorial program and the way I

am going to do it is I am going to take an input from

the user its going to prompt the user to put a number

that they want to take the fatorial of and then it is going

to calculate the factorial and 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 will do it slightly

differnt, acctually I'll talk about that in a second if you are

using Python 3 Python 2 is what most implementation 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 will clarify about that in a future video but anyway

we are going to take an input form 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 get familiarize ourselves with

functions and all we are passing an argument to a function

and the argument that we are passing function, to the input

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

an then you can view it as have 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 whats 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- Intergrated Development Enviornment its free

open source project on a PC but there is many many ways

that you can edit Python you can really just do on any

text editor so anyway, so far all we've done is we've taken

input from the user and we are putting it in the variable

number and whats really cool about Python is we can

just run this program as is and just see what happens

so lets 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 to enter a

non-negative integer take the factorial of so I don't know

we type in 10 and I type ok nothing happened well of

corse nothing happend because I didn't really tell this

to do anything but if we type in number we will see

that it is now assigned to 10 the variable is now refering

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

are 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 am going

to start product at 1 and we are going to do is we are

going to start from 1 up to whatever number this is

and keep multiplying the product by each sucessive 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

but we are going to, within the for loop we are going

to define a variable i this is kind of the clasic variable

to define in forloops and that variable is going to keep

incramenting to larger and larger value as we go through

it so for i in range and forloops in Python are a little bit

differnt 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 and go up to 1 less than that number so over here

we already define 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 one less than then 10 so that the

same thing as range of 10 and what this forloop does

is it asigns the i to each term of the sequence so it starts

with one and then it will go to what ever you tell it to do

in the forloop with i is equal one 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 untill you gone all the way through

the list and so what we can do here is within the forloop

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, an I am going to multiply it times

i plus 1 times i plus one we are going to step through

carefully in the next video but I want you to think about

what it is doing already so right when we start

lets say that number ends up being 3 so product gets

set to 1 and we say for i in range an 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 times

1 is going to be 1 then we are going to assign

i to be 1 this is what the forloop does it itterates

it keeps incramenting, well it dosen't allways have to

incrament and it keeps changeing the value of i

as it goes through this list and so the next time around

our product is one but now i is 1 so 1 + 1 is 2

time 1 is now going to be 2 an 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 times 3 which is 6 and so when we finally done

with this forloop inside of product or I guess what we can

say product will be refering to the acctual factorial of

an number inputed so then we can say print product

and in Python in this case right here the interpreter

knows to break out of this forloop once it runs out of

numbers to assign i to so thats why it doesn't run forever

so lets see if what we did here acctually works lets see

if it acctually works so let me write it like this so lets

see of this things acctually works so I'll save it and then

let me run it and so it is asking me to enter a non-negative

integer to take the factorial of so lets try it with 3

enter it gave me the right answer lets try it again

so enter a non-negative integer to take the factorial

of I don't know lets try something large 10, it worked

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 arbitary number and then next video we are

going to step through much more carefully in case

this kind of confused you what this forloop did

so just hold with me for the next video and watch that

and that has a little bit of a more careful explanation