# A Beginner's Python Tutorial/Very Simple Programs

## IntroductionEdit

OK! We have Python installed, now what? Well, we program!

And it is that simple (at least for now!). Python makes it easy to run single lines of code—one-liner programs. Let's give it a go.

## Opening IDLEEdit

Run the program labelled IDLE (Stands for Integrated Development Environment).

Now you are in the IDLE environment. This is the place you will be spending most time in. Here you can open a new window to write a program, or you can simply mess around with single lines of code, which is what we are going to do. Type the following and press enter (don't type '>>>' as it should already be there but yes in python 3.0 you have to add(" sin to start after using print command and ") for closing or it will show an error message...

Code Example 1
Hello, world!

>>> print ("Hello, world!")

What happened? You just created a program, that prints the words 'Hello, world'. The IDLE environment that you are in immediately compiles whatever you have typed in. This is useful for testing things, e.g., defining a few variables, and then testing to see if a certain line will work. That will come in a later lesson though.

## Math in PythonEdit

Now try the following examples. I've given explanations in parentheses.

Code Example 2 – Maths
```>>> 1 + 1
2

>>> 20 + 80
100

>>> 18294 + 449566
467860

>>> 6 - 5
1
(Subtraction)

>>> 2 * 5
10
(Multiplication)

>>> 5 ** 2
25
(Exponentials; e.g., this one is 5 squared)

>>> print "1 + 2 is an addition"
1 + 2 is an addition
(The print statement, which writes something onscreen. Notice that 1 + 2 is left unevaluated.)

>>> print "One kilobyte is 2^10 bytes, or", 2 ** 10, "bytes."
One kilobyte is 2^10 bytes, or 1024 bytes.
(You can print sums and variables in a sentence.
The commas separating each section are a way of
separating clearly different things that you are printing.)

>>> 21 / 3
7

>>> 23 / 3
7
(Division; note that Python ignores remainders/decimals.)

>>> 23.0 / 3.0
7.666666666666667
(This time, since the numbers are decimals themselves, the answer
will be a decimal.

>>> 23 % 3
2

>>> 49 % 10
9
(The remainder from a division)
```

As you see, there is the code, then the result of that code. I then explain them in brackets. These are the basic commands of Python, and what they do. Here is a table to clarify them (because tables look cool, and make you feel smarter ;) ):

Table 1 – Python operators
Command Name Example Output
+ Addition 4 + 5 9
- Subtraction 8 - 5 3
* Multiplication 4 * 5 20
/ Division 19 / 3 6
% Remainder (modulo) 19 % 3 1
** Exponent 2 ** 4 16

Remember that thing called order of operations that they taught in maths? Well, it applies in Python, too. Here it is, if you need reminding:

1. parentheses ()
2. exponents **
3. multiplication *, division \, and remainder %
4. addition + and subtraction -

## Order of OperationsEdit

Here are some examples that you might want to try, if you're rusty on this:

Code Example 3 – Order of operations
```>>> 1 + 2 * 3
7
>>> (1 + 2) * 3
9
```

In the first example, the computer calculates 2 * 3 first, then adds 1 to it. This is because multiplication has the higher priority (at 3) and addition is below that (at a lowly 4).

In the second example, the computer calculates 1 + 2 first, then multiplies it by 3. This is because parentheses (brackets, like the ones that are surrounding this interluding text ;) ) have the higher priority (at 1), and addition comes in later than that.

Also remember that the math is calculated from left to right, unless you put in parentheses. The innermost parentheses are calculated first. Watch these examples:

Code Example 4 – Parentheses
```>>> 4 - 40 - 3
-39
>>> 4 - (40 - 3)
-33
```

In the first example, 4 - 40 is calculated,then - 3 is done.

In the second example, 40 - 3 is calculated, then it is subtracted from 4.

The final thing you'll need to know to move on to multi-line programs is the comment. You should always add comments to code to show others who might be reading your code what you've done and why. Type the following (and yes, the output is shown):

```>>> #I am a comment. Fear my wrath!

>>>
```

A comment is a piece of code that is not run. In Python, you make something a comment by putting a hash (#) in front of it. A hash comments everything after it in the line, and nothing before it. So you could type this:

Code Example 6 – Comment examples
```>>> print "food is very nice" #eat me
food is very nice
(A normal output, without the smutty comment,
thank you very much)

>>># print "food is very nice"

(Nothing happens, because the code was after a comment)

>>> print "food is very nice" eat me

(You'll get a fairly harmless error message,
because you didn't put your comment after a hash)
```

Comments are important for adding necessary information for another programmer to read, but not the computer; for example, an explanation of a section of code, saying what it does, or what is wrong with it. You can also comment out bits of code if you don't want them to compile, but can't delete them because you might need them later.