Introduction to Programs Data Types and Variables

Writing a basic program. Basics of data types, variables and conditional statements

What I want to do in this video is to expose you

and introduce you to the idea

to what a computer program is.

And just in case you want to follow along

I highly recommend you do that

because the real way to learn computer science

is to really fiddle with things yourself.

This is a Python environment

so I'm going to be doing a lot of the programming in Python.

And right here, this environment is called PyScripter.

P-Y-S-C-R-I-P-T-E-R.

It's free. It's an open-source piece of software.

And I'm using Python..Python...

I believe I'm using Python 2.6 or 2.7.

As long as you are using Python 2

your examples will be the same as mine,

they work the same way.

But If you're using Python 3

you are going to have to use slightly different variations

every now and then to make it work properly.

I'll try to make notes for those when they occur.

So let's just start writing ourselves a computer program.

What's cool about this is,

we can write our computer program right here.

And really we are just editing text in a file.

That's all it is.

It's a set of instructions

and the computer is going to start for the most part.

At the top of this file and just go down

and read these instructions.

Although you will later on

that there's a way to tell the computer

to jump around and to loop around within the instructions,

so that it can do things over and over again or skip other thing.

With that said, let's write ourselves a simple program

and while we do this,

we will expose ourselves to some of the core concepts

that exist within a computer program.

Let me write a very very simple computer program.

So one very simple computer program

would literally just be an expression.

So let me just write 'print 3+7',

so it's literally just going to take 3+7 and print it.

It's going to pass it to the print function

which comes with Python.

Maybe I will write it like this: print(3+7)

Let's save this file. So there's literally only one..

if you think about it, only one command

here on the top line here. That says print 3+7.

Actually, let's add another command,

just so you can see that it's going to go top down.

Let me add another one: print(2-1)

and then let's do: print("this is a chunk of text")

Now let's see what this computer program right here

is going to do.

So let me save it.

So let me save it.

I saved it as the file "testarea.py". Tells a... a...That's a...

The .py extension signifies it is a Python file.

Now let me run the program.

What's nice about this development environment,

this IDE or Integrated Development Environment, is that

you can kind of type and run your program in the same place.

It also color-codes your text,

so you can see what's a function, what's not a function,

the different data-types...

we will talk about more data types in the future.

Let's run this program to see what happens.

So there we go, we ran it!

So it printed 10, then it printed 1,

then it printed "this is a chunk of text"

So it did exactly what we told it to do.

And it did it in the order.

It started up here, it evaluated 3+7 as equal to 10

and it printed it, it printed 10 here.

and then it printed 2-1,

and then it printed "this is a chunk of text"

Now one thing I want to introduce you to, fairly early on,

it's the idea of data types.

So even when you saw this example,

you might have the gut feeling that

look, there is something kind of different

about a 3 or 2 or 1 or 7 and this chunk of text.

This is a number. I can just kind of add numbers.

They're representing some type of quantity.

While this over here is representing a chunk of text.

And your intuition would be right.

These are different data types.

The 3 and 7 and 1... these are numerical literals.

In this particular case, they are integers.

And you can..in this one over here,

this is actually a String,

which is a word you hear a lot in computer science.

this is really..referring to a string...of characters.

and in Python we can actually ask

what are the types of these things.

So you can pass them to the function "type"

so now it should print the type of 3+7, not just 10.

Let's try that. I'll just print 2-1 to show you the difference.

Then I'll print the type of this chunk of text.

The type of this chunk of text.

And let's save it. I just type CTRL+S,

That's a shortcut to save this.

and then I'll try to run this program.

So there you go.

It evaluated this statement. It starts at the inner parenthesis.

3+7 is 10. Then it tries to take the type of 10,

which is a type int, then it prints that type int.

You see it right here. It says type 'int'.

int is short for Integer.

Then it says print(2-1).

It does that on this line right here,

prints 1,

and then it prints the type of this whole thing right over here.

So instead of printing the text,

it prints its type. And its type is a String.

The next thing I want to introduce you to

as we just fiddle our way experimenting with programs

is the idea of a Variable.

Because one of the things is we are going to want

to store these things in different places.

We will learn in future videos that in Python

it's more like we will have labels for these things,

and the labels can change.

Let's see, or we can put them different types of labels.

So let's write a completely different program using variables.

What's cool about Python

some people don't like it, is that

you can put any type of data in any type of variable.

So you can say a=3+5,

then we can say b=a*a-a-1

[note: * means "times", it is used for multiplication.]

and then you can say c=a*b

Then you can have something like...

I will put some space here to make it a little bit cleaner.

c = a*b

Then we can say, let's print c.

So if you want, you can go ahead

and try to figure out what c is going to look like

or we can just run this program.

So let's run the program first

and then we can go back to see if it did the right thing.

So I'm going to save the program,

and now I'm going to run it.

We got 440 for c. Let's see if that makes sense.

So 3+5 is 8. So the label "a" will refer to 8.

So any place in the program, until we redefine "a",

any time you use "a", it's going to say: a is 8. a is referring to 8.

So when you go down over here to define "b"

it'll say OK, a*a. It uses order of operations.

So order of operations, you do your multiplications first.

Especially when you're comparing against a subtraction.

So a*a that's going to be 64.

Then we have 64 - a is 64 - 8, which is 56. Minus 1 is 55.

So "b" is 55. And "c" is going to be a...which is 8.... times 55...

And 8 times 55 is indeed 440.

So it all worked out.

So maybe you want to see what happens

when you get different "a"s.

You can try that out.

you can just change what happens here for different a's.

So maybe we'll have a is equal to ...

Let's make it equal to -6

and now let's run our program to see what happens.

We get -246. And you can verify it by yourself.

You go line by line, and have these variables refer to

what they are defined to be referring to,

and see if you get this response right over here.

Now, if programs were just a bunch of commands

and you just always go straight through,

you wouldn't be able to do really interesting things.

So to do really interesting things you are going to

start seeing things like Conditionals and Loops.

And Conditionals and Loops are something like

Let's do it like this

So...if.... I will leave that stuff over there.

And we'll say "if (a<0):". Maybe we will print(c)

If or "else:", print ... or otherwise ...we'll print (c-a).

So this is interesting. You might already have a gut

for what's going to happen here. Let's save it.

It's amazing how much you can get done

with just these conditionals. So this is saying

if "a" is less than 0, do this,

Otherwise if "a" is not less than 0, do this over here.

So notice we are not going just straight down.

Depending on whether "a" is less than 0 or not,

it's going to either execute this line,

or it's going to execute this line.

And the way that Python knew to only execute this line,

if "a" was less than 0 is it's indented here.

And the indent is part of this clause.

The way it knows that there are new clauses for him

is this colon right over here.

And the way to know what to execute

If none of these happens

If "a" is not less than 0, then it executes this "else" clause.

If you want to do something else after this,

regardless of whether "a" is less than 0 or not,

You can just take it out of the clause by getting rid of the indentation.

So now we can just print "we are done with the program".

Actually, let's add some other stuff in this clause.

Let's print here "a<0".

So notice: this is not going to be evaluated.

We have this inside of a string, so it is just going to

print that thing. Over here we will say

print("a is not less than 0")

This is an interesting program. Let's just run it now.

Let's hope it runs. I save it. Now let's run the program.

And it printed "a<0", so it shows we are inside of this clause.

It printed this. Then it printed "c", which is -246.

It does not execute this,

because this would only be executed if a was not less than 0.

But then it breaks out of this clause and prints

"we are done with the program" no matter what.

Let's now change "a" to see if we can get this other

clause to run. Let's make "a" greater than 0.

Let's make "a" equal to 9 and run the program.

So there. "a" is 9. It checks "is a less than 0?",

well, "a" is not less than 0, so it's not going to execute this,

it's going to go to the else clause.

So it's going to print "a is not less than 0"

which it did over here, then it printed c-a

which is 630. It breaks out of that clause,

and regardless of whether "a" is less than 0 or not,

it prints "we are done with the program".