Q and A for Sections 1 – 2.2.4

CS 106© 2014 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.)

Bits

Q: Why are bits only 0s or 1s? Why not 2s or 3s?

A: A bit in a computer is stored in a kind of switch. Like a light switch, it can be only on or off. On = 1, off = 0.

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

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: Picture of each.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.

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…

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 = piApproxpiSquared = piApprox * 2(assume a, b, c, and x defined)discrim = ax^2 + bx + cA: syntax, semantic, syntax and semantic (discriminant is b**2 – 4 * a * c)

How a program runs

Q: I am not confident that I could give a complete description of how a program runs in sequential statements.A: I’ll draw a picture of python running code, line by line.

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).

Literals

Q: What is a literal?

A: A value you type in.(Give me an integer literal. How about a string? Float? bool?)

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.

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 =.)

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 ‘Zach’ to the end of the list knights_who_say_ni.

A: knights_who_say_ni.append(‘Zach’)

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(‘Morgan’)cs106.append(‘Arielle’)cs106.append(‘Jesse’)

A: Result is a list object containing 3 strings:[ ‘Morgan’, ‘Arielle’, ‘Jesse’ ]

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 ‘Kristen’ between the strings ‘Isaac’ and ‘Jen Ai’ in the following list, called cs106:>>> cs106[ ‘Amber’, ‘Isaac’, ‘Jen Ai’, ‘Amy’ ]

A: cs106.insert(2, ‘Kristen’)

Inserting into a list

Q: Given this list:>>> cs106[ ‘Shiki’ ]create this list using only insert statements:[ ‘Andrew’, ‘Christina’, ‘Ben’, ‘Shiki’ ]

A: cs106.insert(0, ‘Ben’)cs106.insert(0, ‘Christina’)cs106.insert(0, ‘Andrew’)

Using remove()

Q: What is the result of this code?:>>> cs106[ ’Jacob', ’Paul', ’Elizabeth', ’Paul' ]>>> cs106.remove(’Paul')

A: [ ’Jacob', ’Elizabeth', ’Paul' ]

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(’Julie')>>> cs106.append(3.1415926)>>> cs108 = [’Peter', ’Matt']>>> 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.