Introduction to Programs Data Types and Variables

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

In this video I want to introduce you

to what a computer program is.

I highly recommend you follow along

because the real way to learn computer science

is to really fiddle with things yourself.

This is a Python environment. I'm going to be doing a lot

of the programming in a language called Python.

This environment is called PyScripter. It's free,

it's an open-source piece of software and 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 will work the same way. But If your 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 lets just start writing

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. The computer will start

at the top of this file and go down

reading the instructions. Although you will later see

that there are ways to tell the computer to jump

around and loop around within the instructions, so that

it can do things over and over again or skip parts of the program.

With that said, lets write 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.

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

3+7 to the print function which comes with Python.

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

Lets save this file. So there's literally only one command

here on the top line 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 line: print(2-1)

and lets do: print("this is a chunk of text")

Now let's see what this computer program is going to do.

Let me save it. I saved it as the file "testarea.py".

The .py extension signifies it is a Python file.

Now let me run the program. What's nice about these

IDE or Integrated Development Environments is that

you can type and run the 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...

Let's run the 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.

Then it printed 2-1, finally it printed "this is a chunk of text"

Now one thing I want to introduce you to,

fairly early, it's the idea of data types.

So maybe when you saw this example,

you had the gut feeling that there is something different

between the 3, 7, 2 or 1 and this chunk of text.

This is a number. I can add numbers. They represent

some type of quantity. While this down here is representing

a chunk of text. And your intuition would be right.

These are different data types. The 3, 7, 1... they are

numerical literals. They are integers.

And this down here is actually a String, which you hear

often in computer science, referring to a string of characters.

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.

Let's save it. I press CTRL+S, which is the shortcut for save.

Then I 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 int

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 String.

The next thing I want to introduce to you

as we experiment with programs is

the idea of a Variable. Because 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 write a completely different program

using variables. What's cool about Python, even

some people doesn'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

and then you can say c=a*b

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

Finally lets print c. 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 and then

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

I'm going to save the program, and run it.

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

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

Any place in the program, until we redefine "a",

any time you use "a" it will know it means 8.

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

a*a. It uses order of operations, so it does

multiplications first. a*a is 64.

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

So "b" is 55. And "c" is going to be a * b,

which is 8 * 55, indeed 440. It all worked out.

Maybe you want to see what happens when

you get different "a"s. You can try that out.

You can just make "a" equal to -6

and 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 here.

Now, if programs were just a bunch of commands

and you just always went straight through,

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

To do really interesting things you are going to

start seeing things like Conditionals and Loops.

Let's see a conditional. I will leave the first lines unchanged.

And type "if (a<0):". In that case, we will print(c)

then type "else", and in this case we will print (c-a).

So this is interesting. You might already have a gut

for what's going to happen here. But let's save it.

It's amazing how much you can get done just

with 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

"print(c)" if "a" was less than 0 is because

print(c) is indented. The indent is part of this clause.

The way it knows that there are new clauses for him

is because of this colon right here.

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 get rid of the clause by getting rid of

the indentation. So now we can just print a string.

Actually, let's add some more stuff to this clause.

Let's print here "a<0".

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.

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".