Original subtitles

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Great, now let's learn numbers. Let me

open an interactive shell here to

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explain this faster and so numbers, you

know what numbers are. To create a number

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just type the number in python and

that's it and you also have operators

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like this is the plus operator and you

can also store numbers in variables

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let's say a equals to 2, b equals to 3

and then if you do a plus b you get 5.

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Something to note is that you know if

you do that in here, a equals to 2 b

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equals to 3, and then if you just do a

plus B and then you execute the script,

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let me open another terminal, you will

not get any output in there. The reason

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is that you are not printing out

anything in your program, so this is

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different from an interactive session.

Here if you simply do an operation or if

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you simply call a variable like that, you'll

get the output promptly, so this is just

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for simplicity when you when you try

things out, so you don't have to pass

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print explicitly, but Python will

understand that you met print. But here

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things are more strict, so you if you

want to print out the output, you want

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to say print a plus b and then execute

and you get the output and yeah basically

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that's the only difference between

executing a program from a Python file

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and running the code in the interactive

shell, so these are numbers and I can

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prove that if I do type 2 for instance.

You'll see that this is an INT

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type which means is an integer, so

basically we have integers and we have

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floats. These are the two main types

of numbers.

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A float is, let's say 2.4 it's a float

and I can prove that 2.4,

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type 2.4. You'll see that this

is a float, however if you do type 2 in

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brackets you'll see that this is a

string, it's not a number and therefore

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you cannot make operations using a

string and a number so Python will give

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you a type error and this is saying

cannot convert integer to string

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implicitly so when you do a string plus

something else Python expects that you

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were adding another string after a plus

sign, so that's why it says I cannot

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convert integer object, so it cannot

convert this to a string implicitly

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because if you were to do two plus three

you'd get 23 because Python treats these

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two as text. So to understand it

better

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let's add some text here b so you get

2a3b, so it's a concatenation operation.

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When the plus operator is used with

strings it is referred to as

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concatenation. Now let's go back to our

sample program that we used earlier

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which was this one in here, so now what

if instead of getting a greeting from

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the user you ask their for their age?

Enter your age, so this is just a message

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you can write anything you want there,

but just try to have a message that

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makes some sense to the user and so also

I'm going to change the variable name to

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something more meaningful, age and then

what I want to do is let's say new age,

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I want to calculate the age plus 50.

Why not?

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I'll print out the new age. Save the

script and let's go and execute this, we are

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going to get an error here I warn you.

But it's good that you see the errors

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that you get because that's how you

learn better.

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So Python my program, execute.

For now it's working fine because what

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happens is Python executes a script

line by line so it goes to the first

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line and if there is no error it

executes that line and in this case we have

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an input function so the program is on

standby mode so it expects from the user

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to enter the input, so let's enter the

input here. Let's say 28 and here is the

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error that we get. So again we saw this

error before. This is saying type error

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cannot convert int object to string

implicitly and you also have the line

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here that the error occurred. So these are

important messages and line two in here.

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So what's happening is that age is a

string it's not a number therefore

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python cannot add up a string with a

number and so why is age a string?

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Because that's what the input function

produces, so the input function get the

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user input and converts it to a string

so even if you pass a number there these

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will always convert everything to a

string.

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Therefore Python you know cannot

concatenate 28 plus 50 so what can you

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do in that case? Well you want to convert

that string into an integer and for

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that you want to use the int function,

so it's a Python function that does the work.

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As you can see it outputs now

the correct output so 28 plus 50 equals

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78 and similarly you can convert

numbers to strings

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and let's say str 50 and I think we'll

get an error here. Yeah, it says type error.

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Unsupported operand type four plus.

So you're adding up an integer with a

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string, so integer with a string. If you

like to convert this to a float, let me

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remove that, save, execute. 28. You get

78 point zero because what

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Python does when you use float is you

know float 28, it converts it to 28.0.

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So floats can be useful in certain scenarios.

And you'll have a lot of scenarios in the

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course, so don't worry about that now

because we have real world

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applications later on where you see the

real benefit of everything that you are

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explaining here in this early sections

that cover the basics. So this is about

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numbers. Try to practice them a little

bit and I'll talk to you in the next

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lecture.