Very simple 'programs'
Introduction
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. Lets give it a
go.
Opening IDLE
Go to the start menu, find
Python, and 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)
|
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. define a few
variables, and then test to see if
a certain line will work. That
will come in a later lesson,
though.
Math in Python
Now try typing the stuff in
bold. You should get the output
shown in blue. I've given
explainations in brackets.
|
Code Example 2 - Maths |
>>> 1 + 1
2
>>> 20+80
100
>>> 18294+449566
467860
(These are additions)
>>> 6-5
1
(Subtraction)
>>> 2*5
10
(Multiply, rabbits!)
>>> 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)
>>> 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 seperating each
section are a way of
seperating clearly different
things that you are printing)
>>> 21/3
7
>>> 23/3
7
>>> 23.0/3.0
7.6666...
(division, 2nd time ignoring
remainder/decimals,
3rd time including decimals)
>>> 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 |
19%3
|
5
|
|
**
|
Exponent |
2**4
|
16
|
Remember that thing called
order of operation that they
taught in maths? Well, it applies
in python, too. Here it is, if you
need reminding:
- parentheses ()
- exponents **
- multiplication *, division
\, and remainder %
- addition + and subtraction -
Order of Operations
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 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.
Comments, Please
The final thing you'll need to
know to move on to multi-line
programs is the comment. Type the
following (and yes, the output is
shown):
|
Code Example 5 - comments |
|
>>> #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,
thankyou 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 bits of code by putting a
# in front of it - if you don't
want it to compile, but cant
delete it because you might need
it later.
|
