everythingPython

Repetition Structures

2011-05-25T13:00:00.002+09:30

Coming Soon!

Selection Structures

2011-05-25T11:13:00.005+09:30

Selection structures are used to make decisions that allow us to control the path of execution.
This lesson covers the if selection structure.

A simple if statement has the following form

if expression:

true_suite

If the expression is True, the statements indented under the if statement are executed. If the expression is False the program jumps to the next statement after the indented block.

It is important to use consistent indentation. Python uses indentation to signify its block structure.

The else structure allows us to execute one set of statements if the expression is true, and another if the expression is false. It has the following form:

if expression:

true_suite

else:

false_suite

Example:

if age >= 18:

print('You can vote!')

else:

print('Sorry, you can't vote yet!')

The above example will display You can vote! if the variable, age, is equal to or greater than 18, else, it will display Sorry, you can't vote yet!.

The elif structure is used to check multiple criteria while keeping your code easy to read.

if mark >= 85:
    print('HD')
elif mark >= 75:
    print('D')
elif mark >= 65:
    print('C')
elif mark >= 55:
    print('P1')
elif mark >= 50:
    print('P2')
elif mark >= 40:
    print('F1')
else:
    print('F2')

Looking back on the previous exercise to create a program that generates a random number, we are going to modify it to ask the user to guess the random number, and if the user guesses it correctly, display "Well done - you guessed it!". If the guess is too low, display 'Too Low', or if it is too high, display 'Too High'.

The solution to this program can be found below, but try to do the work yourself first before looking.

Okay, so once you have that covered and are confident with the structures we will look at combining comparisons using a couple of examples.

and operator - true only if temperature is greater than zero and less than 40:

if temperature > 0 and temperature < 40:
print('Normal Temperature')
else:
    print('Extreme Temperature')

or operator - true if either of the conditions are true, eg. if it is raining or highUV

raining = True
highUV = True
if raining or highUV:
    print('Take an umbrella!')

If you are having any problems so far, read through again, look at the code, and just play around with different variables and values until you understand how the structures and output works. You can always post a comment and ask for help if you need it, I'm happy to help!

Continue on to: Repetition Structures

-Nathan (everythingPython)

Comparison and Boolean Operators

2011-05-25T11:12:00.002+09:30

In this next lesson we will discuss Comparison operators and Boolean operators.

Comparison Operators are used to compare the values of two objects.
Boolean Operators allow us to combine comparisons.

Operator	Description
==	is equal to
!=	is not equal to
<	is less than
<=	is less than or equal to
>	is greater than
>=	is greater than or equal to

Using the above operators we are able to make decisions based on the outcome of the comparison.

Example

>>> a = 5

>>> b = 2

>>> a < b

False

The result of the comparison will either be True or False, depending on the values being compared.

Boolean Operators (and, or, not) are used to combine comparisons. These can be used when either one, both, or two or more conditions need to be met.

Example

IF (today is monday and payday is monday) THEN

cash check

The above example will only return a result if both the current day is Monday, and the payday variable is Monday.

Example

IF (x >= 0 and x <= 5) THEN

PRINT x is between 0 and 5 (inclusive)

IF (x <= 2 or x >= 4) THEN

PRINT x is less than or equal to 2, or greater than or equal to 4

not a == b and c == d

The first example will only execute if x is both greater than or equal to 0 and less than or equal to 5. Meaning that x must be either the value 0, 1, 2, 3, 4 or 5.

The second example will execute if either x is equal to or less than 2 or if x is greater than or equal to 4. This means that if x is any value except 3, it will execute as the value 3 is neither <= 2 or >= 4.

The third example uses the not operator which negates the value of the operand, converting True to False and False to True. This means that if a is not equal to b and c is equal to d then it will return True. A more expanded example is shown below:

>>> a = 5
>>> b = 3
>>> c = 6
>>> d = 6
>>> a == b
False
>>> c == d
True
>>> not a == b
True
>>> not c == d
False
>>> not a == b and c == d
True

As you can see from above:
a and b are not equal, thus when compared it returns False.
c and d are equal, and thus when compared they return True.
using not a == b returns True when compared because it is evaluating it as: a is not equal to b, which is true.
Hence when using not a == b and c == d, the return is True because a is not equal to be and c is equal to d.

This can be a bit hard to get a grasp of at first, and if you need to, play around with variables and values and see how it works when you use the not operator. You will get the hang of it soon.

Once you have completed this section, move on to the next: Selection Structures

-Nathan (everythingPython)

Exercises Part 2

2011-05-24T10:59:00.002+09:30

Going back to your files from Exercises Part 1, we are going to be modifying them to allow user input.

Modify the first program to allow the user to enter a width and height. The area should be displayed on the screen as: The area is: x
Modify the second program to allow the user to enter a length. The result should be displayed on the screen as: The length is: x
Write a program to generate a random number between 1-10 and display the number on the screen as: Random number is: x
Once done with the above program, modify it to ask the user to guess the number, and display their guess on the screen.
*REMEMBER* You need to import the appropriate libraries when needed.

You can view the above random guessing program below. It is best to try it yourself first, and use this to check your code.

Once you have completed the above exercises you are ready to move on to the next lesson: Comparison and Boolean Operators

-Nathan (everythingPython)

User-Controlled Input and Output

2011-05-24T10:50:00.002+09:30

Continuing from the last lesson we will look further into creating more general programs that allow the user to input values while the program is running, using the input() function.

value = input('Enter a value: ')

The above code displays the following string and waits for the user to provide input.

Enter a value:

The value entered is then referenced by the variable value. Eg.

>>> value = input('Enter a value: ')
Enter a value: 13
>>> value
'13'

In this case, the input function returns a string. This may need to be converted to an integer or floating-point value. We can do this using either of the following methods.

>>> value = int(input('Enter a value: '))
Enter a value: 13
>>> value
13

>>> value = float(input('Enter a value: '))
Enter a value: 13
>>> value
13.0

The int() and float() functions take the user input, and convert it to the appropriate type.

Below is another example, this time with strings. It take the user input for the first and last name, adds them together in the variable name and places a space between them. The name is then output directly and via the print() function.

>>> first = input('Enter your first name: ')
Enter your first name: Peter
>>> last = input('Enter your last name: ')
Enter your last name: Griffin
>>> name = first + ' ' + last
>>> name
'Peter Griffin'
>>> print(name)
Peter Griffin

Once you feel confident working with these functions, continue on to: Exercises Part 2

-Nathan (everythingPython)

Built-in Functions

2011-05-24T10:13:00.002+09:30

The Python interpreter has a number of functions that are always available for use, meaning you do not have to import any libraries. This lesson will cover a few of the built-in functions.

A function consists of three components: Name, Input Argument(s) and Output.
An example function would be a = abs(x), which is shown (exploded) below.

Output Name Input Argument
a = abs ( x )

The input argument is placed inside of the parentheses, and the output is the value that is returned by the function. There are functions that will require multiple inputs to work, such as the pow function which returns x to the power of y. Eg:

pow(x, y)

pow(5, 2)

5 to the power of 2 = 25

Using the example above, you can assign a variable (name) to the output:

result = power(5, 2)

Below is a list of some of the Built-in functions that you may find useful when starting out:

Function	Description	Example
abs(x)	Returns the absolute value of x.	ans = abs(-3) ans = 3
pow(x, y)	Returns x to the power of y.	ans = pow(5, 2) ans = 25
round(x[, n])	Returns the floating point value x rounded to n digits after the decimal point. If n is omitted, it defaults to zero.	ans = round(1.6) ans = 2
int([number\|string[, base]])	Converts a number or string to an integer. If no arguments are given, returns 0.	ans = int('7') ans = 7
float([x])	Converts a string or number to a floating point.	ans = float('1.5') ans = 1.5
range([start], stop[, step])	Constructs progression of integer values. The arguments must be integers. The step value defaults to 1 if it is omitted and if the start value is omitted it defaults to 0.	range(0, 5)
str(x)	Converts object x to a string representation	str(ans)
print([object, ...], sep=' ', end='\n', file=sys.stdout)	Print object(s) to the stream file, separated by sep and followed by end. sep, end and file, if present must be given as keyword arguments.	See below
input([prompt])	If the prompt argument is present, it is written to the standard output without a trailing newline. The function then reads a line from input, converts it to a string and returns that.	See below

Print() Example
ans = 15
print("This is an example!")
print("This is another example:", 777)
print(ans)
print("The answer is:", ans)
print("I", "like", "pizza!")
print("I", "like", "pizza!", sep="-")
print("There are", ans, "pizza slices left...")

Output from above:
This is an example!
This is another example: 777
15
The answer is: 15
I like pizza!
I-like-pizza!
There are 15 pizza slices left...

Input() Example
guess = input('Please enter your guess: ')
name = input('Please enter your name: ')
number = int(input('Enter number: '))
print("Hello", name, "you guessed", guess, "and your number is", number)

Output from above:
Please enter your guess: 10
Please enter your name: nathan
Enter number: 20
Hello nathan you guessed 10 and your number is 20

Python has an extensive library of functions and as such, I can't possibly teach all of them. There is a help feature available to learn more about Pythons functions which can be accessed from the Python Shell.

Help -> Python Docs

You can also view the documentation on the Python website. Two great libraries to look at are math and random, which will be upcoming in later exercises and examples.

Once you have a grasp on this lesson, continue on to User-Controlled Input and Output.

-Nathan (everythingPython)

Exercises Part 1

2011-05-24T10:03:00.001+09:30

Here are a couple exercises to test your current knowledge. These should be completed in a new Python file.

Assign the value 20 to a variable called width.
Assign the value 10 to a variable called height.
Define a variable called area to store the result of width * height.
Display the area to the screen using the print() function. *HINT* See the screenshot in the last lesson.

Assign the value 10 to a variable called length.
Define a variable called area to store the result of length². *HINT* See Python Arithmetic Operators.
Display the area to the screen using the print() function.

Once you have completed both of the exercises above, continue on to the next lesson: Built-in Functions

-Nathan (everythingPython)

Naming Conventions and More

2011-05-24T09:30:00.001+09:30

Okay, so you've made it this far. Great work! Following the last couple of lessons, we will be going a bit more in-depth into the Python Naming Rules and Conventions, Keywords, Constants, Variables and Comments.

To start off, we will look at the rules for naming your variables.

All names must start with a letter
They may contain: letters, numbers, and the underscore character ( _ )
The names are case sensitive. count is different from Count
There are certain keywords that cannot be used (seen later on)

There are also some conventions that are recommended:

Using lowercase letters for variable names
Using UPPERCASE letters for constants (seen later on)
The names should be short enough to remember and must be self descriptive

If you keep these things in mind, you will have a cleaner looking program, as well as one that can be easily read by other programmers.

As discussed above, there are certain keywords that cannot be used for variable names. They are as follows:

False	None	True	and
as	assert	break	class
continue	def	del	elif
else	except	finally	for
from	global	if	import
in	is	lambda	nonlocal
not	or	pass	raise
return	try	while	with
yield

As long as you stay away from these variable names you will have no problems. The next thing to cover is Variables and Constants.

These are defined as:

Variable:

adjective. able or liable to change
noun. a name given to a memory location designed to store a data value (Comp.Sci.)
Called ‘variable’ as the value stored in it is likely to change during program execution

Constant:

noun. a quantity that has a fixed value throughout a set of calculations
Called ‘constant’ as once set, the value stored does not/cannot change during program execution

When using either of these, you have to take into account what the variable will be used to store, and whether or not you will need to change the value of it later on.

Comments are a great way to describe what each part of code is doing in your program. It's beneficial to both you and anyone else who reads your code. To add comments to your code its as simple as adding the hash sign. The comment will span the entire line, when writing longer comments, it is best to split it amongst multiple lines, each starting with the hash sign.

*NOTE* When using the print function, you must use quotations when outputting a string directly. If you are outputting a variable, they will not be needed (eg. print(myVariable))

When running your program, the comments will be ignored, they are only there for your benefit and the benefit of those who read the code.

That's it for this lesson. Play around with the comments and variables and see how you go. If you have any trouble, go back through the lessons, or post a comment, I'm happy to answer any questions you may have!

Continue on to: Exercises Part 1

-Nathan (everythingPython)

Python Files

2011-05-24T09:21:00.000+09:30

This is just a quick lesson to learn about opening and creating new Python files.

To begin, open the Python Shell (IDLE). Go to File -> New Window, or press Ctrl + N if you are on Windows. You should see the following window.

This is where you will write your programs code. It doesn't work like the Shell. The interpreter doesn't wait for your input like with the Shell, here you write your code, and then you can run the program. To do so, go to:

Run -> Run Module

To save your files, you just need to go to File -> Save As... and chose your file location and name. The file extension should be .py (eg. helloWorld.py).

That's it for this lesson. Continue on to the next lesson: Naming Conventions and More

-Nathan (everythingPython)

Getting Started

2011-05-24T00:10:00.002+09:30

So you've decided to start programming and Python is the language you want to learn. The first place you want to go is the Python website to download the latest installation. For these posts, I will be using Python 3.2 which can be downloaded from the following link:

Download Python

You will want to select the appropriate Python 3.2 installation for your operating system. I used the Python 3.2 Windows x86 MSI Installer for my system.

Once you have it downloaded to your computer, you can go ahead and install it. This is a fairly straightforward process and as such I will not cover it unless any users request for a guide.

Okay, so you have it installed, now it's time to open it up and get started! You can access the IDLE from either the desktop (if chosen during install) or from the start menu (windows users).

Start > All Programs > Python 3.2 > IDLE (Python GUI)

You should see the following screen (or similar) once it has finished loading.

As you will see, the blinking cursor will indicate that the interpreter is ready for you to enter a command/expression. This shell (window) can be used to perform calculations, as you would on a calculator.

That's it! You're up and running, and now you can begin getting into the fun stuff..okay, well it may not be too much fun, but it will help you out in the long run when you start making more advanced programs.

If everything is running as above you can now continue to the next lesson: Python Arithmetic Operators

-Nathan (everythingPython)

Python Arithmetic Operators

2011-05-24T00:10:00.001+09:30

Okay, so we're up and running and we're ready to get into it! But don't get ahead of yourself just yet, we need to cover some basics first. It won't take long, but it's good to know what we're using and what it does.

The first thing we are going to cover is the Python Arithmetic Operators. The following table will show the different operators used with Python and what they do:

Operator	Use
+	Addition - Used to add values. If using a floating point and an integer, the result will be a floating point. (Discussed Later)
-	Subtraction - Same as + (addition)
*	Multiplication Same as + (addition)
/	Division - Integer and floating point division will always result in a floating point.
//	'Floor' Operator - Returns the largest integer value less than the result if you use the Division Operator (Above).
%	Modulo - Returns the remainder after division.
**	'Power' Operator - Works for integer and floating point values, returning the initial value to the power of the second value.

Hopefully that will explain what everything does clearly enough. Play around with the different operators and see what kind of answers you get. You can see some examples below of what to expect:

One thing to keep in mind when doing problems as seen above, is the order of operations. Python follows the standard algebraic rules for the order of operation:

Parentheses (innermost to outermost)
Exponentiation operations
Multiplication and division operations (left to right)
Addition and subtraction operations (left to right)

Following are some examples to try out to help you get used to the order of operations:
5 * (2+2) = ?
5 * 2 + 2 = ?
4 * 2 / 2 * 4 = ?
4 * 2 / (2 * 4) = ?

Now that we have that covered, you should have a grasp on the basic arithmetic operators and we can move on to the Assignment Operator.

The Assignment Operator, is used to assign a value to a variable. This is particularly useful when we need to store and access values from memory. The assignment operator is a single equals sign ( = ). In the following example you will see one of the many ways it can be used.

In the above example, we use 3 variables (num1, num2, num3) to hold the values using the assignment operator. To use this, you type the name of the variable (using the appropriate naming conventions), the equal sign, the value, and press [Enter]. The value can then be accessed by typing in the variable name again.

Using the above information, what would the following produce:

num = 6

num = num + 1

num = ??

The same method can be used to store string values (as below):

As you can see, outputting the value is different to the numerical value. When using strings, it is best to output the value using the print function which will be covered later on.

Once you have gotten used to using arithmetic operators and assignment operators, you can move on to the next (short) lesson: Python Files

-Nathan (everythingPython)

An Introduction!

2011-05-24T00:07:00.000+09:30

Hi!

If you've found your way here you are probably interesting in getting started with, or expanding your current knowledge of Python. If you've arrived here accidentally, feel free to have a look around, you may have just found yourself a new hobby.

Before we get into anything, I'd like to give users a look into what will be covered in the upcoming posts. There are obviously many topics that will be covered in the learning process, and each one is just as important as the last. You may find some sections easy, and as such, you may not spend more than a few minutes working through them, while others you may get stuck on for hours. But don't let that get you down. Take a break. Come back later. A relaxed mind is much easier to work with than one that is stressed and over worked.

The following topics are what you can look forward to learning about, and using in the upcoming exercises:

Language concepts; keywords, types, constants, variables,
Expressions, Boolean expressions, precedence and associativity,
Sequence, choice, loops,
Functions, parameters,
Strings,
Lists (arrays).

If you find yourself struggling with any of them, as I said before, take a break and come back later with a clear mind, you may find that it is a lot easier once you're more relaxed. If you are still having trouble getting your mind around it, post a comment, don't be afraid to ask a question, not everyone will learn straight away. It's all part of the process.

I hope everyone finds the information useful and that it will get you started with what will hopefully be a fun programming experience!

-Nathan (everythingPython)