Chapter 4

Question 1

T or F : An integer can’t be factored into more primitive parts

Answer 1

True

Question 2

What is a string

Answer 2

A data structure which consists of smaller pieces of data

Question 3

What is the form of a subscript operator

Answer 3

“String”[integer operator- position of a particular character in the string]

Ie:
»> name = “Alan Turing”
»> name[0] # Examine the first character
‘A’

Question 4

subscript operator in loops: count- controlled loop

Answer 4

IN: data = “Hi there!”
IN: for index in range(len(data)):
IN: print(index, data[index])
OUT:
0 H
1 i
2
3 t
4 h
5 e
6 r
7 e
8 !

Question 5

How to use a subscript operator to obtain a substring

Answer 5

Slicing- place colon in the subscript; an integer value can appear on either side of the colon
»> name = “myfile.txt” # The entire string
»> name[0:]
‘myfile.txt’
»> name[0:1] # The first character
‘m’
»> name[0:2] # The first two characters
‘my’
»> name[:len(name)] # The entire string
‘myfile.txt’
»> name[−3:] # The last three characters
‘txt’
»> name[2:6] # Drill to extract ‘file’
‘file’

Question 6

Testing for a substring with the in operator

Answer 6

When used with strings, the left operand of in is a target substring and the right
operand is the string to be searched
* Returns True if target string is somewhere in search string, or False otherwise
* This code segment traverses a list of filenames and prints just the filenames
that have a .txt extension:
»> fileList = [“myfile.txt”, “myprogram.exe”, “yourfile.txt”]
»> for fileName in fileList:
if “.txt” in fileName:
print(fileName)
myfile.txt
yourfile.txt

Question 7

Caesar cipher

Answer 7

replaces each character in plain text with a character a given distance away

Question 8

How to decrypt Caesar cipher

Answer 8

Apply a method that uses the same distance value but looks to the left of each character for replacement value

Question 9

ord and chr

Answer 9

ord - returns the ordinal position in the ASCII sequence
chr- is the inverse function

Question 10

Block cipher

Answer 10

• uses plaintext character to compute 2+ encrypted characters
• uses an invertible matrix

Question 11

What number system do the arithmetic operations use

Answer 11

Decimal number system- base 10 number system

Question 12

binary number system

Answer 12

• used to represent info in a digital computer
• its called the base 2 number system (0 and 1)

Question 13

T or F: The digits use in each system are counted from 1 to n - 1, where n is the system’s base

Answer 13

FALSE: The digits used in each system are counted from 0 to n − 1, where n is the system’s base

Question 14

Positional notation

Answer 14

In positional notation, a digit has a positional value, determined by raising the base to the power specified by the position
(base^position)

Question 15

converting binary to decimal

Answer 15

• Each digit or bit in binary number has positional value that is power of 2
• We occasionally refer to a binary number as a string of bits or a bit string
  -to determine the integer quantity that a string of bits represents: multiply the value of each bit (0 or 1) by its positional value and add the results.

Question 16

code of converting binary to decimal

Answer 16

• A positional value is computed by using the ** operator
  “““
  File: binarytodecimal.py
  Converts a string of bits to a decimal integer.
  ”””
  bitString = input(“Enter a string of bits: ”)
  decimal = 0
  exponent = len(bitString) − 1
  for digit in bitString:
  decimal = decimal + int(digit) * 2 ** exponent
  exponent = exponent − 1
  print(“The integer value is”, decimal)
  Enter a string of bits: 1111
  The integer value is 15
  Enter a string of bits: 101
  The integer value is 5

Question 17

How are integers converted from decimal to binary

Answer 17

• One algorithm uses division and subtraction instead of multiplication and
  addition
• Repeatedly divides the decimal number by 2
• After each division, the remainder (either a 0 or 1) is placed at the beginning of a
  string of bits
• Quotient becomes the next dividend in the process
• Process continues while the decimal number is greater than 0

Question 18

Convert from octal to binary

Answer 18

Start by assuming that each digit in the octal number represents 3 digits in the corresponding binary number

Question 19

Convert binary to octal

Answer 19

Begin at the right and factor the bits into groups of 3 bits each

Question 20

convert hex to binary

Answer 20

replace each hex digit with the corresponding 4 bit binary number

Question 21

convert binary to hex

Answer 21

factor the bits into groups of 4 and look up the corresponding hex digits

Question 22

What is a method

Answer 22

• behaves like a function
• always called with a given data value called an object
• syntax:

<an>.<method>(<argument‐1>,..., <argument‐n>)
</method></an>

Question 23

T or F: In Python, there are a variety of different data values

Answer 23

False: In Python, all data values are objects

Question 24

How do you view a complete list/documentation of string methods

Answer 24

enter dir(str) at a shell prompt
and enter
help(str.<method‐name>) to receive documentation on an individual method

Question 25

What is a text file?

Answer 25

• software object that stores data on permanent medium such as disk or CD

Question 26

Advantages of a text file

Answer 26

• data set can be much larger
• data can be input much more quickly and less chance of error
• data can be used repeatedly with the same program or with different programs

Question 27

T or F: All data output to or input from a text file must be strings

Answer 27

True: number must be converted to string before output

Question 28

Data can be output to a text file using a ___ object

Answer 28

file
* To open a file for output:
»> f = open(“myfile.txt”, ‘w’)
* If file does not exist, it is created
* If it already exists, Python opens it; when data are written to the file and the file is
closed, any data previously existing in the file are erased

Question 29

How do you open a file for input

Answer 29

method read

Question 30

open(filename, mode)

Answer 30

Opens a file at the given filename and returns a file
object. The mode can be ‘r’, ‘w’, ‘rw’, or ‘a’. The last
two values mean read/write and append

Question 31

f.close()

Answer 31

Closes an output file. Not needed for input files.

Question 32

f.write(aString)

Answer 32

outputs aString to a file.

Question 33

f.read()

Answer 33

Inputs the contents of a file and returns them as a
single string. Returns “” if the end of file is reached.

Question 34

f.readline()

Answer 34

Inputs a line of text and returns it as a string,
including the newline. Returns “” if the end of file is
reached.

Question 35

Absolute pathname

Answer 35

when the chain starts with the root directory

Question 36

relative pathname

Answer 36

when chain starts from the current working directory

Question 37

myfile.txt

Answer 37

current target directory

Question 38

child/myfile.txt

Answer 38

child target directory

Question 39

../myfile.txt

Answer 39

parent target directory

Question 40

../sibling/myfile.txt

Answer 40

sibling target directory

Question 41

chdir(path)

Answer 41

Changes the current working directory to path

Question 42

getcwd()

Answer 42

Returns the path of the current working directory

Question 43

list dir(path)

Answer 43

Returns a list of the names in directory named
path

Question 44

mkdir(path)

Answer 44

Creates a new directory named path and places it
in the current working directory

Question 45

remove(path)

Answer 45

Removes the file named path from the current
working directory.

Question 46

rename(old, new)

Answer 46

Renames the file or directory named old to new.

Question 47

rmdir(path)

Answer 47

Removes the directory named path from the
current working directory

Question 48

sep

Answer 48

A variable that holds the separator character (‘/’ or
‘\’) of the current file system

Question 49

exists(path)

Answer 49

Returns True if path exists and False otherwise

Question 50

isdir(path)

Answer 50

Returns True if path names a directory and False
otherwise

Question 51

isfile(path)

Answer 51

Returns True if path names a file and False
otherwise.

Question 52

getsize(path)

Answer 52

Returns the size of the object names by path in
bytes.

Question 53

normcase(path)

Answer 53

Converts path to a pathname appropriate for the
current file system; for example, converts forward
slashes to backslashes and letters to lowercase on
a Windows system.