Week 1: Python Basics

Question 1

WHAT DOES A COMPUTER DO

Answer 1

Fundamentally:

◦performs calculations a billion calculations per second!
two operations in same time light travels 1 foot

◦remember results
100s of gigabytes of storage!
typical machine could hold 1.5M books of standard size

What kinds of calculations?
◦built-into the language
◦ones that you define as the programmer

Question 2

TYPES OF KNOWLEDGE / Declarative knowledge

Answer 2

Is statements of fact.

◦there is candy taped to the underside of one chair

Question 3

TYPES OF KNOWLEDGE / Imperative knowledge

Answer 3

Is a recipe or “how-to”knowledge:

It uses a sequence of steps to find a solution:

1) face the students at the front of the room
2) count up 3 rows
3) start from the middle section’s left side
4) count to the right 1 chair
5) reach under chair and find it

Question 4

WHAT IS A RECIPE?

Answer 4

1) sequence of simple steps
2) flow of controlprocess that specifies when each step is executed
3) a means of determining when to stop

Steps 1+2+3 = an algorithm!

Question 5

How to capture a recipe in a mechanical process?

Answer 5

Fixed program computer:
◦calculator
◦Alan Turing’s Bombe

Stored program computer:
◦machine stores and executes instructions

Question 6

What is a Stored program computer?

Answer 6

Sequence of instructions stored inside computer

◦built from predefined set of primitive instructions:

1) arithmetic and logic
2) simple tests
3) moving data

Special program (interpreter) executes each instruction in order:

◦use tests to change flow of control through sequence
◦stop when done

Question 7

In computability theory what is the Turing completeness?

Answer 7

Turing showed you can compute anything using 6 primitives

Anything computable in one language is computable in any other programming language

Question 8

What is an Expression?

Answer 8

Expressions are complex but legal combinations of primitives in a programming language

Question 9

Syntax

Answer 9

Particular way that you need to write and structure your code in a language so that the computer understands.

English:
“cat dog boy” not syntactically valid
“cat hugs boy” syntactically valid

Programming language: “hi”5 not syntactically valid
3.2*5 syntactically valid

Question 10

Static semantics

Answer 10

Static semantics is which syntactically valid strings have meaning

◦English: “I are hungry”
syntactically valid but static semantic error

◦programming language:
3.2*5 syntactically valid
3+”hi” static semantic error

Question 11

Semantics

Answer 11

Semantics is the meaning associated with a syntactically correct string of symbols with no static semantic errors.

English: can have many meanings_
◦“Flying planes can be dangerous”
◦“This reading lamp hasn’t uttered a word since I bought it?”

◦programming languages: have only one meaning but may not be what programmer intended

Question 12

WHERE THINGS GO WRONG

Answer 12

syntactic errors:
◦common and easily caught

static semantic errors:
◦some languages check for these before running program
◦can cause unpredictable behavior

no semantic errors but different meaning than what programmer intended:
◦program crashes, stops running
◦program runs forever
◦program gives an answer but different than expected

Question 13

What is a program?

Answer 13

A program is a sequence of definitions and commands

◦definitions evaluated
◦commands executed by Python interpreter in a shell

Question 14

What is a command?

Answer 14

Commands (statements) instruct interpreter to do something

Can be typed directly in a shell or stored in a file that is read into the shell and evaluated

Question 15

What are the characteristics a an object?

Answer 15

Programs manipulate data objects

Objects have a type that defines the kinds of things programs can do to them

objects are
◦scalar (cannot be subdivided)
◦non-scalar (have internal structure that can be accessed)

Question 16

What are the scalar objects?

Answer 16

int
represent integers, ex. 5 or -5
int, or integer: a number without a fractional part. savings, with the value 100, is an example of an integer.

float
or floating point: a number that has both an integer and fractional part, separated by a point. factor, with the value 1.10, is an example of a float.

str
String: a type to represent text. You can use single or double quotes to build a string.

bool
Represent Booleanvalues Trueand False
bool, or boolean: a type to represent logical values. Can only be True or False (the capitalization is important!).

NoneType–specialand has one value, None

You can use type() in Python to see the type of an object:

In [1]: type(5)
Out[1]: int

In [2]: type(3.0)
Out[2]: float

Question 17

What does the conversion (CAST) do?

Answer 17

Can convert object of one type to another:

float(3)converts integer 3 to float 3.0
int(3.9)truncates float 3.9to integer 3

Question 18

What forms an expression?

Answer 18

Combine objects and operators to form expressions

syntax for a simple expression

Question 19

How basic arithmetic operations work?

Answer 19

i+j  = the sum
i-j   = the difference
i*j   = the product

i/j = division
10/2=5.0
10/3=3.3333333333333335

i//j = int division
10/2=5
10/3=3

Modulo: %. This operator returns the remainder of the division of the number to the left by the number on its right.

i%j = the remainder when i is divided by j
10/2=0
10/3=1

i**j = i to the power of j

Question 20

What is the arithmetic operators precedence?

Answer 20

Operator precedence without parentheses:

**
*
/
+ and –executed left to right, as appear in expression

Question 21

What are the comparison operators on INT and FLOAT?

Answer 21

i and j are any variable names:
i > j
i > = j
i < j
i < = j
i = = j equality test, True if i equals j
i ! = j inequality test, True if i not equal to j

Question 22

What are the Logic operators in boolean?

Answer 22

a and b are any variable names:

not a True if a is False
False if a is True

a and b True if both are True

a or b True if either or both are True

Question 23

What are branching programs?

Answer 23

The simplest branching statement is a conditional

◦A test (expression that evaluates to True or False)
◦A block of code to execute if the test is True
◦An optional block of code to execute if the test is False

Question 24

What is indentation?

Answer 24

Leading white space (spaces and tabs) at the beginning of a logical line is used to compute the indentation level of the line, which in turn is used to determine the grouping of statements.

Question 25

What is a String?

Answer 25

Letters, special characters, spaces, digits

Enclose in quotation marks or single quotes
hi = “hello there”
greetings = ‘hello’

concatenate strings
name = “eric”
greet = hi + name
greeting = hi + “ “+ name

Question 26

Operations on strings

Answer 26

concatenation
»>”a” + “bc”
‘abc’

successive concatenation
»> 3 * “bc”
‘bcbcbc’

the length
»> len(“eric”)
4

indexing

• Begins with index 0
• Attempting to index beyond length –1 is an error

> > > “abcd”[2]
‘c’

slicing

• Extracts sequence starting at first index, and ending before second index
• If no value before :, start at 0
• If no value after :, end at length
• If just :, make a copy of entire sequence

> > > “abcd”[0:2]
‘ab’

> > > “abcd”[:2]
‘ab’

> > > “abcd”[2:]
‘cd’

Question 27

What does the Length operation do?

Answer 27

The length of a string can be found using the len function. For example, the value of

len(‘ abc’) is 3.

Question 28

What does the Indexing operation do?

Answer 28

Indexing can be used to extract individual characters from a string. For example:

Typing ‘abc’ [0] into the interpreter will cause it to display the string
‘a’.

Typing ‘abc’ [3] will produce the error message IndexError: string index out of range. Since Python uses 0 to indicate the first element of a string, the
last element of a string of length 3 is accessed using the index 2.

Negative numbers are used to index from the end of a string. For example, the value of
‘abc’ [ -1] is ‘ c’

Question 29

What does the Slicing operation do?

Answer 29

Slicing is used to extract substrings of arbitrary length. If s is a string, the expression s [start: end] denotes the substring of s that starts at index start and ends at index end-1.

When constructing a slice, as in [6:11], the first index number is where the slice starts (inclusive), and the second index number is where the slice ends (exclusive), which is why in our example above the range has to be the index number that would occur just after the string ends.

S a m m y S h a r k !
0 1 2 3 4 5 6 7 8 9 10 11

ss = “Sammy Shark!”
print(ss[6:11])
Output: Shark

print(ss[7:])
Output
hark!

print(ss[:5])
Output
Sammy

S a m m y S h a r k !
-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1

print(ss[-4:-1])
Output
ark

Why does it end at index end-1 rather than end? So that expressions such as ‘abc’ [0: len(‘ abc’)] have the value one might expect. If the value before the colon is omitted, it defaults to 0.

If the value after the colon is omitted, it defaults to the length of the string. Consequently, the expression ‘abc’[:] is semantically equivalent to
the more verbose ‘abc’ [0: len (‘abc’)].

Question 30

What is Object-oriented programming?

Answer 30

Organizes the computation around a set of objects and the things that can be done to those objects.

Question 31

What does IDE mean?

Answer 31

Integrated Development Environment

Question 32

What does IDLE mean?

Answer 32

Integrated Development and Learning Environment

Interactive shell

Question 33

How do you add comments in Python?

Answer 33

You can add comments to your Python scripts. Comments are important to make sure that you and others can understand what your code is about.

To add comments to your Python script, you can use the # tag. These comments are not run as Python code, so they will not influence your result.

Question 34

type(a)

Answer 34

To find out the type of a value or a variable that refers to that value, you can use the type() function. Suppose you’ve defined a variable a, but you forgot the type of this variable. To determine the type of a, simply execute:

type(a)