Chapters 7&8: Files and Lists

Question 1

“Thank you/nYou’re welcome”

Answer 1

When printed, returns:
Thank you
You’re welcome.
/n = 1 charactcer, new line. If simply entered, will show initial version.

Question 2

fhand=open(‘blah.txt’)

Answer 2

Opens the given text file if it exists in the python directory (in quick access). Value fhand is assigned with is only the file handle. But some subsequent functions, but not print, will be done on entire text file.

Question 3

x=fhand.read()

Answer 3

assigns actual text of file to x as a string, not just handle

Question 4

x.rstrip()

Answer 4

strips white space from right of the string, but NOT left. as opposed to lstrip or just strip

Question 5

lift only lines starting with ‘From:’ from file, getting rid of double spacing.

Answer 5

fhand=open('givenfile.txt')
for line in fhand:
   line=line.rstrip()
   if line.startswith('From:'):
      print(line)

Question 6

text=banana

text.find(‘c’)

Answer 6

returns -1, as no c in banana.

Question 7

lift only lines containing given substr, getting rid of double spacing

Answer 7

fhand=open('givenfile.txt')
for line in fhand:
   line=line.rstrip()
   if line.find('given_substr')==-1:
      continue
   print(line)

Question 8

if blah blah: continue

Answer 8

same effect as:
if blah blah:
continue

Question 9

newtext=open(‘output.txt’, ‘w’)

Answer 9

Write mode (second parameter, w). This creates new file, or overwrites existing file with then entered name if it exists.

Question 10

writing lines

Answer 10

line1=’This is the first line\n’
newtext.write(line1)
returns 24, number of characters entered.

Question 11

finish writing

Answer 11

newtext.close()

Question 12

list vs string

Answer 12

both sequences of values: string values are characters, list values cacn be anything

Question 13

values of a list

Answer 13

called elements or items. comma and space between them, enclosed in square brackets, strings have speech marks numbers and variables dont (as usual). Lists within lists work as you’d think - lists, within square brackets, just become another element.

Question 14

print 1st item in a list

Answer 14

print(list[0])

Question 15

reassign 3rd item in a list to x

Answer 15

list[2]=’x’

Question 16

print all of list of oxen individually

Answer 16

for ox in oxen:

print(ox)

Question 17

concocenating lists 1 and 2, to make {1, 2, 3] and [4, 5, 6] into one list

Answer 17

list1+list 2

Question 18

list1*2 gives…

Answer 18

[1, 2, 3, 1, 2, 3]

Question 19

slice operator to extract just [1, 2] from list1:

Answer 19

list1[0:2], or just list1[:2}

Question 20

append method: adds onto end of list, e.g. add 4 to list 1 to make [1,2,3,4]

Answer 20

list1.append(4)

Question 21

extend method,

Answer 21

same, but can take a list as argument and append each element onto the list to be appended. using append method appends the whole list as a single new element

Question 22

Warning: list methods are not like functions or string methods - they permanently alter, x=x.blah isnt needed, and may delete content. they’re ‘void’.

Answer 22

Whereas with string methods this is fine, and will set the new value to a new variable, leaving previous variable unaltered. eg line=line.rstrip; for lists it would just be eg list.split(), no list= before.
while list1=list1[:2] won’t work, return list1[:2] will if eg defining a function

Question 23

sort list reverse alphabetically

Answer 23

list1. sort()

list1. reverse()

Question 24

pop method - remove an element from list, but assign it a variable

Answer 24

x=list1.pop(1)

x now equals 2 (element 1, the 2nd element), list1 is now [1,3,4]

Question 25

element not needed? del operator

Answer 25

del list1[1]. can use a slice index, eg [2:6]

Question 26

know the name of element but not the index?

Answer 26

list1.remove(2) or (‘x’), this removed the actual element 2, not the element with [2] as its index

Question 27

to do to list1:

• count elements
• find highest number element
• lowest number
• total
• average

Answer 27

(all in print() )
len(list1)
max(list1)
min(list1)
sum(list1)
sum(list1)/len(list1)
sum is numerical list only. Others can be used "on lists of strings etc where elements are comparable" - not sure what that means, doesnt count or find one with most chars.

Question 28

allow user to input list:

Answer 28

list=list() #tells python it'll be list type
While True:
   inp= blah
   list.append(inp)
#then eg average=sum(list)/len(list)

Question 29

convert str to list of characters:

and to list of words:

Answer 29

list1=list(str)

list1=str.split()

Question 30

convert str to list of words, but something other than space seperating words (space is default, leave brackets empty)

Answer 30

list1=str.split(delimiter)

where delimiter is the object seperating them, eg - for str ‘qw-ty-fd’

Question 31

join - the inverse of split.

convert list to string with chosen delimiter e.g. ‘ ‘

Answer 31

’ ‘.join(list1) will make [1,3,4] into ‘1 3 4’. want no delimiter? just use ‘’, ie. empty string

Question 32

using split to parse lines of a file - print all the days of week from lines starting ‘From ‘ in mbox-short.txt

Answer 32

fhand=open('mbox-short.txt')
for line in fhand:
   line=line.rstrip()
   if not line.startswith('From '):
      continue
   words=line.split()
   print(words[2]) #the day of week is 3rd word on line

Question 33

equivalent vs identical

Answer 33

equivalent strings are identical, the same object. a=x, b=x, a is b will return True. But lists which are the same are NOT the same object, a is b will return False. They are equivalent, NOT identical

Question 34

aliasing

Answer 34

using one variable as a reference for another. eg, list3=list1, list1 is list3 returns True. as the variable is the value of a new variable, they are identical. Best avoided if possible though with lists - altering one of them may cause an error.

Question 35

method to avoid aliasing list1

Answer 35

list3=list1[:]

Question 36

can use and or or logic operators to combine guardian code lines. e.g. with and:
if len(words)<=2: continue
if words[0]!='From': continue
   print(words[2])
#cacn be turned into:

Answer 36

if len(words)>2 and words[0]=='From':
    print(words[2])

Question 37

elem

Answer 37

short for element of a list. eg for elem in list1: