Operations on sequence types

Question 1

What is sequence?

Answer 1

It is an object which consists of sequential elements or other objects through which you can iterate.

Question 2

Name all sequence data types in python.

Answer 2

• strings;
• Unicode strings;
• lists;
• tuples;
• bytearrays;
• buffers;
• xrange objects.

Question 3

What are bytearrays?

Answer 3

It is a mutable sequence of bytes.

A bytearray is very similar to a regular python string (str in python2.x, bytes in python3) but with an important difference, whereas strings are immutable, bytearrays are mutable, a bit like a list of single character strings.

Question 4

What are buffers?

Answer 4

buffer(object[, offset[, size]])

A new buffer object will be created which references the object argument.

An example usage:

> > > s = ‘Hello world’
t = buffer(s, 6, 5)
t

> > > print t
world

The buffer in this case is a sub-string, starting at position 6 with length 5, and it doesn’t take extra storage space - it references a slice of the string.

This isn’t very useful for short strings like this, but it can be necessary when using large amounts of data. This example uses a mutable bytearray:

> > > s = bytearray(1000000) # a million zeroed bytes
t = buffer(s, 1) # slice cuts off the first byte
s[1] = 5 # set the second element in s
t[0] # which is now also the first element in t!
‘\x05’

Question 5

How to check if an item is equal or not to x?

Answer 5

x in s # true if x the items of x

x not in s

Question 6

How to perform concatenation (addition) of two sequences of the same type?

Answer 6

s + t

Question 7

What are next operations?
»> s * n
»> n * s

Answer 7

They are equivalent to adding s to itself n times.

Question 8

How to get ith item of s by it’s index?

Answer 8

s[i] # indexation starts from zero, 0 is the first element

Question 9

How to slice s from i to j? (excluding jth element)

Answer 9

s[i:j]

Question 10

How to slice s from i to j with step k (selecting every kth item)? (excluding jth element)

Answer 10

s[i:j:k]

> > > s = ‘0123456789’
s[0:7:3]
‘036’

Question 11

Get every second element in
s = ‘0123456789’
starting from zero.

Answer 11

s[::2]

Question 12

What can happen in case you do next:
»> l = [‘0’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’, ‘7’, ‘8’, ‘9’]
»> del l[4:9]

Answer 12

Those elements gonna be removed from the object:
»> l
[‘0’, ‘1’, ‘2’, ‘3’, ‘9’]

Question 13

Get length of the sequence s.

Answer 13

len(s)

Question 14

Get smallest item of s.

Answer 14

min(s)

Question 15

Get largest item of s.

Answer 15

max(s)

Question 16

Get index of the first x item occurrence in s

Answer 16

s.index(x)

Question 17

Get total number of occurences of x in s.

Answer 17

s.count(x)

Question 18

Make full copy of the sequence.

Answer 18

s[:]

Question 19

What could happen if you get negative n value in s*n expression?
like
»> ‘text’ * -1

Answer 19

It yields empty sequence of the same type as s:
»> ‘text’ * -1
‘’

Question 20

Are items in the sequence s in s*n copied?

Answer 20

No, they're referenced multiple times:
>>> lists = [[]] * 3
>>> lists 
[[], [], []]
>>> lists[0].append('hi')
>>> lists
[['hi'], ['hi'], ['hi']]

You can create list of different lists this way:
lists = [[] for i in range(3)]