Q and A for Sections 1 – 2.2.4
CS 106
© 2015 Victor Norman
Difference between data and
information
Q: What is the difference between data and
information?
A: It is a difference of focus. Data is more
interested in how something is represented in
bits in a computer. Information is more about
what the data represents. (This is not an
important concept.)
High- vs. low-level language
Q: What is the difference between a high-level and
low-level language? If a computer cannot run a
high-level language, then why have it?
A: (not in book). A low-level language is a series of
bits – the only thing a machine can truly run. No
one wants to program in bits:
0010000100101111010101000101110001110
A high-level language is easier to program in, easier
to debug, and (perhaps) portable to different
computers.
Source code
Q: What is “source code”?
A: Source code is the program you are creating
(and typically storing) in a file. For us, it will be
written in python. It is either translated by a
compiler into bits to be executed by the
computer hardware, or, interpreted and
executed by an interpreter (as in python).
Aka “software”.
Compiled vs. Interpreted Languages
Q: Why would there be interpreters and compilers and what
would the advantages be to using one over the other?
A: Compiler: creates object/executable code specific to the
computer – Windows or Mac OS or Linux or Android. Code
runs fast. But, you have to recompile each time you make a
change before you can try your change.
Interpreter: runs your source code directly (converts it to
machine-specific instructions internally). No recompiling!
But, usually much slower.
Picture:http://interactivepython.org/runestone/static/thinkcs
py/GeneralIntro/ThePythonProgrammingLanguage.html
More about the python interpreter
• You write your code and store it in a file, say computeIQ.py.
• You run it in the interpreter. In PyCharm, you click the run button.
What that really does is execute this: python.exe computeIQ.py
• The interpreter does this:
– Set the current line pointer to the first line of the
file.
– While we have a line to execute:
• Read in the line from the file.
• Execute that line:
– Could be an assignment, function call, function
definition, class definition, if statement,
etc., etc.
– Store results in memory: like variable
assignments, etc.
– Move the pointer to the next line to execute.
Syntax vs. semantic errors
Q: I did not understand the difference between
syntax and semantic errors. Can you explain?
A: A syntax error is illegal code. It is like not
English saying correct.
A semantic error is wrong logic. The code is
fine, but computes the wrong thing.
Syntax vs. semantic errors quiz
Q: Which errors are these?
3.14159 = piApprox
piSquared = piApprox * 2
(assume a, b, c, and x defined)
discrim = ax^2 + bx + c
A: syntax, semantic, syntax and semantic
(discriminant is b**2 – 4 * a * c)
What is an object?
Q: What is an object?
A: It is a variable or literal of a certain type (and
thus can hold certain values, and can do certain
operations).
Q: How do we know what operations are allowed
on certain objects?
A: You can see them online, or use help(), or see the
info in the book.
Literals
Q: What is a literal?
A: A value you type in.
(Give me an integer literal. How about a string?
Float? bool?)
Literals
In python, you have to specify a literal such that
the interpreter can figure out what type it is. So:
“something”: the quotes means “this is a string
of characters”.
1 : because it is only digits (and optionally + / -),
it is an integer.
-10.314: because it has digits and a ., it is a
floating point number.
[ ‘stuff’ ] : the [ means “here is a list literal”
Variables
Q: What is a variable?
A: A variable is a name referring to a value of a
certain type stored in memory. A variable is one
kind of identifier – a name for something.
Terminology!
Student: I am FREAKING OUT because there is so
much terminology here! What is an object?
What is the difference between a method and a
function? What is a type? Ahhhhh!!!!
Professor: OK, deep breath! In… out… in…
out…
Identifiers
Q: In
groceries = list()
what is the identifier?
A: groceries
(It is a variable that “labels” the object created
on the right hand side of the =.)
Assignment Operator
• Syntax: identifier = value or expression
– age = 3
– ageIn30Years = age + 30
– cs106 = [ ‘Adam’, ‘Joshua’, ‘Gabrielle’ ]
• What the interpreter does:
– Look first at the right-hand side (rhs) and evaluate it
(“evaluate” means “turn into a value”).
– Look up the identifier on the left-hand side (lhs).
– If the identifier is new, make it refer to the new value.
– Else: change the existing identifier to make it refer to
the new value.
Legal identifier names
Q: Select the legal identifiers (i.e., labels for objects
(in this case)):
miles
_
per_hour
1_
Miles
1_ab1
_1
1ab
_gallons
__1abc
A: 1_, 1_ab1, 1ab are illegal.
List creation
Q: How do you make a new list? I.e., how do
you construct a list? I.e., how do you call the
constructor of the list class?
A: You call list().
E.g., groceries = list()
This calls the list constructor to make a new list
instance which is referred to by variable
groceries
list() vs. []
Q: Does typing list() and just using brackets
accomplish the same thing?
A: Yes.
The latter is called the “literal method” of
constructing a list. It is also a way to put stuff in
the list when you create it.
List append method
Q: append() is a method of a list object that
takes one parameter -- the object to be
appended to the end of a list. Write the code
to append the name ‘Rachel’ to the end of the
list knights_who_say_ni.
A: knights_who_say_ni.append(‘Rachel’)
append() vs. add()
Q: It seems that it's just the 'append' method,
but if so, why isn't it just called the 'add'
method? Is there something else that can be
done with 'append'?
A: Someone decided it should be called
‘append’… Perhaps ‘append’ is a little more
descriptive than ‘add’? (There is no ‘add’
method defined.)
More list practice
Q: What does this code result in?
cs106 = list()
cs106.append(‘Karissa’)
cs106.append(‘Kenton’)
cs106.append(‘Isabelle’)
A: Result is a list object containing 3 strings:
[ ‘Karissa’, ‘Kenton’, ‘Isabelle’ ]
Append() is a ….
Q: append() is a method that changes the object
upon which it is called. Thus, it is called a
_____________ method. (p. 34)
A: mutator
Inserting in a list
Q: Write the code to insert the string ‘Moses’
between the strings ‘Cassidy’ and ‘Levi’ in the
following list, called cs106:
>>> cs106
[ ‘Benjamin’, ‘Cassidy’, ‘Levi’, ‘Ari’ ]
A: cs106.insert(2, ‘Moses’)
Inserting into a list
Q: Given this list:
>>> cs106
[ ‘Dorthea’ ]
create this list using only insert statements:
[ ‘Justine’, ‘Justin’, ‘Jeremy’, ‘Dorthea’ ]
A: cs106.insert(0, ‘Jeremy’)
cs106.insert(0, ‘Justin’)
cs106.insert(0, ‘Justine’)
Using remove()
Q: What is the result of this code?:
>>> cs106
[ ’Joshua', ’Seth', ’Allison', ’Seth' ]
>>> cs106.remove(’Seth')
A: [ ’Joshua', ’Allison', ’Seth' ]
Remove second one?
Q: If there are two identical names on a list and I
want to remove the latter one, how do I do
that?
A: We don’t know how to do it, but let’s figure
out the algorithm to do it. How would you like
to do it?
Legal code?
Q: Is this code legal?:
>>> cs106 = list()
>>> cs106.append(’Elianna')
>>> cs106.append(3.1415926)
>>> cs108 = [’Jacob', ’Rachel']
>>> cs106.append(cs108)
A: Yes!
Q: what does cs106 look like now?
Return value
Q: Define return value.
A: A return value is a value returned to the caller
by a method call. (Some method calls do not
return anything, like list’s append().)
Combining lists
Q: Given two lists rock and roll, create a
new list called rock_n_roll that contains all
the elements of rock followed by all the
elements of roll.
A:
rock_n_roll = rock + roll
The range function
range() is very useful function.
• Creates a list of integers.
• range(start, stop, step)
– start and step are optional: if omitted, start is 0,
step is 1.
– list contains numbers up to but not including stop.
Extra slides
• The following slides are from previous years
and might still be helpful for you.
What is an object?
Q: What is an object?
A: An object is an instance (or example) of a
certain type. (Thus, the object has legal values
and operations associated with it.)
We often say that the variable is an object, but
really the value in memory is the object.
Trick question… bwa ha ha…
Q: What does this statement do?
list()
(i.e., not a_list = list())
A: It creates a list object but doesn’t label it, so it
can’t be used. I.e., it throws it away
immediately.
Literal list
Q: In section 2.2.4, what does it mean when
they say “Square brackets delimit the list”? Like
in a statement like this:
groceries = [‘bread’, ‘cheese’, ‘eggs’]
A: It means the square brackets tell the
interpreter that 1) it is a list, and 2) where the
list definition is starting and ending.