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CSC 241 NOTES
Y o s e f M e n de l s o h n
W i t h ma ny t h a n ks t o D r . A mb e r S e t t l e f o r t h e or i g i n al v e r s i o n of t he s e no t e s .
P r of S e t t l e ' s n ot e s i n t u r n a r e b a s e d o n t he P e r ko vi c t e x t .
Getting started with Python
The rest of this week will be spent learning some basic Python syntax.
The Python interpreter
One way you can use Python is by using the command-line window. This will
look something like:
You can then execute the classic first program (hello world) as follows:
>>> print("Hello world!")
Hello world!
The function print() is not like a function in mathematics.
It takes input, for example the phrase Hello world!, and does
something with it, specifically prints it to the screen.
Alternatively you can use the IDLE environment to run Python programs. The
screen shot below shows an execution of the hello world program:
We will mostly use IDLE for developing, running, and debugging Python
programs. It provides more helpful features than the command-line window.
Expressions
One of the most fundamental things in any programming language is an
expression.
The easiest types of expressions are mathematical expressions, which will
have us using the Python interpreter like a calculator. To do this, we type
an ordinary algebraic expression at the prompt and press enter to evaluate
it:
>>> print(3+7)
10
Note: Simply typing an expression, will not only cause that
expression to be executed, but IDLE (and related Python shell
environments) will also typically output the result of that
expression to the console:
>>> 3+7
10
Let's look at some more expressions:
>>> 12/5
2.4
>>> 12 // 5
2
>>> 12 % 5
2
2
>>> 3*2+1
7
>>> (3-1)*(4+1)
10
>>> 4.321/3+10
11.440333333333333
>>> 4.321/(3+10)
0.3323846153846154
>>> 2.75
2.75
>>> 2**3
8
Variables and Identifiers
Like in algebra, values may be assigned to variables. For example, we can create
a variable and then assign it a number.
Let's create a variable called 'age' and assign it a value of 65:
>>> age = 65
>>> print(age)
65
Identifiers: An identifier is the name we give to something. Any time we name
something (such as a variable) we must choose an identifier. Choose your
identifiers wisely! Good code is clear code.
Suppose we wanted to have a variable in which we recorded the age at which
people are allowed to consume alcohol. Which of the following identifiers do you
think is best?
>>> x = 21
>>> age = 21
>>> legal_drinking_age = 21
I hope you agree that the last one is far more clear.
That being said, when you are just practicing, you don't need to go overboard on
this. When we are learning Python, I don't mind if you use identifiers such as 'x'
or 'n1' and things like that. However, when writing code such as for your
homework, you should always try to use clear identifiers!
Getting back to variables…
>>> y = 21
>>> print(y)
21
>>> x
>>> print(x)
3
Traceback (most recent call last):
File "<pyshell#10>", line 1, in <module>
x
NameError: name 'x' is not defined
Note the initial error message above. The variable name x is undefined until it is
assigned. Any attempt to use the variable before it is defined will result in an
error. However, the code below will execute just fine:
>>> x = 3
>>> x
3
Note: Again, we do not necessarily have to type 'print(x)' above. In a live
interpreting environment such as IDLE, Python will automatically output the
result However, when you are executing a complete program, you should use the
'print' function if you wish to output information to the console.
Assignment Statements
The statement x = 4 is called an assignment statement.
An assignment statement has the following syntax:
<variable name> = <expression>
Important: Note that the assignment operator (=) here is not checking
whether one value is equal to another. Instead, the '=' operator is assigning
a value to the variable 'x'. (If you want to compare two things to see if
they are identical, we use a different operator. We will discuss this
shortly.)
In an assignment statement, the expression to the right of the assignment
operator is evaluated and then the value of the expression is placed into
the variable on the left side of the assignment operator.
Once a value has been assigned to a variable, the variable can be used in
expressions.
When Python evaluates an expression containing a variable, it will
evaluate the variable to its currently assigned value.
Consider the following example:
>>> x = 4
>>> 4*x
16
>>> y = 4*x
>>> y
4
16
It is also possible to modify the value of an existing variable:
>>> score = 83
>>> score = score + 3
>>> print(score)
86
>>> salary = 50000
>>> salary = salary + (0.1*salary)
>>> print(salary)
55000.0
Let's say we have exam scores for 3 students and wish to calculate the
average of those scores:
>>> score1 = 87
>>> score2 = 93
>>> score3 = 62
>>> mean = (score1+score2+score3)/3.0
>>> print(mean)
80.66666666667
>>> print(score4)
Traceback (most recent call last):
File "<pyshell#10>", line 1, in <module>
score4
NameError: name 'score4' is not defined
Note: In our example for 'mean' it was necessary to divide by 3.0 rather
than 3. We will discuss the reason for this down the road.
CodeLab
Accessing CodeLab: see this document
Note: Without a full license, CodeLab will limit you to 10 exercises. This will be
enough for today, but by the next session, there will be many more than 10. So it
is vital that you take care of setting up your CodeLab today.
CodeLab Terminology:
CodeLab is very specific about terminology. When answering questions, you
should be very careful about recognizing exactly what CodeLab is asking for.
Therefore, as preparation for your first assignment, we need to familiarize
ourselves with some CodeLab terminology.
A literal is a value. You can think of it as the simplest possible value that can be
stored into a variable.
Examples:
 12
 -5
 3.1415
5

‘z’
An expression is a piece of programming code that evaluates to a literal. For
example, an arithmetic expression such as 4*x + 2 is an expression, assuming that
x is defined to hold an integer value.
A statement is a complete Python statement. For example, the assignment x = 5
would be a statement.
Note: If CodeLab asks you for a literal and you give it a statement, it will evaluate
your answer is incorrect – even if the correct answer is present somewhere inside
your statement! So be sure to give CodeLab exactly what it is asking for.
Exercises
For the remainder of the class session you will do some exercises to familiarize yourself
with CodeLab and with basic expressions in Python.
When you submit, if the code turns green, it is telling you that it is correct. If it turns red,
there is a problem. This will be accompanied by one or more error messages. Sometimes
these are helpful, sometimes they are not.
Do the following exercises from the Week 1, Lecture 1 section of the CodeLab
site:
 51001
 51013
 51014
 51043
 51032
 51030
 51033
 51037
Note: If you haven’t already signed up for CodeLab, you should now. Instructions
are found on the 'Resources' page.
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