1.2 sequential data

A sequential data is the data type with one or more than one value/object in it.

Four major sequential data types

• Strings

• Lists

• Tuple

• Range

Since a sequential data type consists of more than one value, we can check the length of a sequential data using len function.

Strings

In python, a string is a data type which does not have a numeric value and is therefore treated as a text.

s = 'Ali'
print(type(s))

<class 'str'>

The value/data of string need not to be only English language characters. They can be anythin such as numbers unless they are defined as string.

s = '23'
print(type(s))

<class 'str'>

Although, 23 above is a numeric value, but since is enclosed inside quotation marks '', python considers it a string and not a number.

There are four common ways to define a string in python:

• single quotation 'I am a string'

• double quotation "I am a string"

• triple doubel quotations """I am a string"""

• triple quotation '''I am a string'''

s1 = 'I am a string'
s2 = "I am a string"
s3 = """I am a string"""
s4 = '''I am a string'''
print(type(s1), type(s2), type(s3), type(s4))

<class 'str'> <class 'str'> <class 'str'> <class 'str'>

A string can be as long as we wish!

s = 'What is the first question that should come to our mind in this life?'
s2 = "Should Immanuel Kant be condemned for his racist views?"

print(type(s), type(s2))

<class 'str'> <class 'str'>

s3 = 'Why the colonization isn\'t considered a crime?'
print(s3)

Why the colonization isn't considered a crime?

s3 = "Why the colonization isn't considered a crime?"
print(s3)

Why the colonization isn't considered a crime?

If we want to quote something with double strings inside a double quoted string, we can do it as following.

txt = "He said: \"It doesn't matter, if you enclose a string in single or double quotes!\""
print(txt)

He said: "It doesn't matter, if you enclose a string in single or double quotes!"

txt = '''Baqir al sadr was an Iraqi scholar.
He was born in 1935 and wrote his famous book "our philosophy" just at the age of 25.
He was killed at the age of 45 by Saddam Husain.'''
print(txt)

Baqir al sadr was an Iraqi scholar.
He was born in 1935 and wrote his famous book "our philosophy" just at the age of 25.
He was killed at the age of 45 by Saddam Husain.

Indexing

Indexing refers to selecting a value at a certain position from a sequential data. It shhould be noted that in python, the indexing starts from 0 and not from 1. This means, 0 refers to 1st position and 1 refers to second position. %%

s = "Assalam o alaikum"
print(s[0])

A

print(s[7])

len(s)

17

print(s[len(s)-1], s[-1])

m m

Slicing

print(s[-3:], s[5:8], s[8:])

kum am  o alaikum

Concatenation

a = " Assalam" + " o" " alaikum"
print(a)

Assalam o alaikum

Repetition

b = a*3
print(b)

Assalam o alaikum Assalam o alaikum Assalam o alaikum

We can find the length of a sequence object in python using the function len. Since strings are also sequences, their length/size can also be found by len.

len(a), len(b)

(18, 54)

immutability

# uncomment following line
# a[-1] = ". "  # TypeError

a = "Muhammad"
b = "Muhammad"
print(a is b)

True

print(a == b)

True

a = "Muhammad!"
b = "Muhammad!"
print(a is b)

True

print(a == b)

True

a = "Muhammad1"
b = "Muhammad1"
print(a is b)

True

print(a == b)

True

Lists

Lists are array likes, enclosed by square brackets.

a = [1, 2, 3, 4]
print(a)

[1, 2, 3, 4]

we can verify that variable a is of list type

type(a)

But a is not just an array like in Fortran programming language. It is rather, much more than that. It is a collection of objects.

type(a[0]), type(a[1])

(<class 'int'>, <class 'int'>)

All elements in above list were of type int but a list can hold any kind of objects all in same list. The following list contains, int, float, str and a list type in it. Yes a list can have a list inside it as well. That is why it is called a collection of objects.

a = [1, 2.0, '3', [4, 5]]
type(a[0]), type(a[1]), type(a[2]), type(a[3])

(<class 'int'>, <class 'float'>, <class 'str'>, <class 'list'>)

as we have seen above that we can index list elements as well. However, if we try to access for an index in a list which is not present, it will result in error. For example above list contains four elements. If we try to access a[4] (which means element at 4th index or 5th element), it will result in error.

# uncomment following line
# a[4]  # IndexError

We have also seen that list is an ordered collection of objects. If we print list a, it will print its elements in same sequence as they are originally.

print(a)

[1, 2.0, '3', [4, 5]]

We can change contents of lists using indexing.

a[3] = 'a new element'
print(a)

[1, 2.0, '3', 'a new element']

We can replace a sequence of elements in a list with a new sequence and the new sequence does not have to be of same length and type as old sequence.

print(a[0:3])

a[0:3] = [2.0, 2]
print(a)

[1, 2.0, '3']
[2.0, 2, 'a new element']

So we see the size of list is changed automatically/dynamically.

nested lists

A list can contain several lists and every list in side a list further sublists and so on.

pakistan = [[['Nawakali', 'Alamdar Road', 'Killi Ismail', 'Kharotabad'],
             'Kallat', 'Ziarat', 'Gawadar'],
            ['Sukkur', 'Rohri', 'Hayderabad', 'Karachi'],
            ['Peshawar', 'Hangue', 'Mardan', 'Charsadda'],
            ['Lahore', ['ugoki', 'sambrial', 'pasrur', 'daska'], 'Sadiqabad', 'Multan']]

len(pakistan)

4

Finding length of pakistan will give length of outermost list. We can find out lengths of inner lists as well.

len(pakistan[0]), len(pakistan[1]), len(pakistan[2]), len(pakistan[3])

(4, 4, 4, 4)

print(pakistan[0][0][0:])

['Nawakali', 'Alamdar Road', 'Killi Ismail', 'Kharotabad']

print(pakistan[3][0][:])

Lahore

print(pakistan[3][1][-3:])

['sambrial', 'pasrur', 'daska']

print(pakistan[3][1][3][3:])

ka

enigma = 'IRtu diysa rtdo oK icpolnivnegn iweanst  at ow hbiet ea  sluipbeerrmaalc!i s t'
print(enigma[::2])

It is too convenient to be a liberal!

print(enigma[1::2])

Rudyard Kipling was a white supermacist

concatenation

List concatenation works same as that of strings.

provinces = ['sind', 'balochistan', 'kpk', 'punjab'] + ['janubi punjab', 'kashmir', 'potohar']
print(provinces)

['sind', 'balochistan', 'kpk', 'punjab', 'janubi punjab', 'kashmir', 'potohar']

provinces += ['hazara', 'karachi']
print(provinces)

['sind', 'balochistan', 'kpk', 'punjab', 'janubi punjab', 'kashmir', 'potohar', 'hazara', 'karachi']

Tuples

In contrast to lists, tuples are immutables which means, once they are created, we can not change/modify their content.

a = (1, 2, 3)
type(a)

Just like lists, the contents of tuples can also be of different types.

a = (1, 2.0, 'a', pakistan)
print(a)

(1, 2.0, 'a', [[['Nawakali', 'Alamdar Road', 'Killi Ismail', 'Kharotabad'], 'Kallat', 'Ziarat', 'Gawadar'], ['Sukkur', 'Rohri', 'Hayderabad', 'Karachi'], ['Peshawar', 'Hangue', 'Mardan', 'Charsadda'], ['Lahore', ['ugoki', 'sambrial', 'pasrur', 'daska'], 'Sadiqabad', 'Multan']])

We can not change a value in a tuple.

# uncomment following line
# a[2] = 'last'  # TypeError

Tuples are used to store data where we know it will not change. This also makes sure that we don’t change the data accidentally.

in

The in keyword is used to check whether an element is present in a sequence or not.

print("Bahawalpur" in provinces)

False

print("Multan" not in provinces)

True

print("pubjab" in provinces)

False

print('a' in a)

True

print('at' in enigma)

True

Repetition

text = ["Gaza is an open air prison. "]
t = [text] * 3
print(t)

[['Gaza is an open air prison. '], ['Gaza is an open air prison. '], ['Gaza is an open air prison. ']]

Caveat

print(t[0][0])

Gaza is an open air prison.

t[0][0] = "Yemen is an open air prison. "
print(t)

[['Yemen is an open air prison. '], ['Yemen is an open air prison. '], ['Yemen is an open air prison. ']]

Indexing

# a = ['Makran',' coastal', 'highway', 'in', 'Balochistan', 'is', 'stunning']
a = "Lasbela and Loralai!"
# a = ('Makran',' coastal', 'highway', 'in', 'Balochistan', 'is', 'stunning')

start = 2
stop = 7
print(a[start:stop])  # items start through stop-1
print(a[start:])      # items start through the rest of the array
print(a[:stop])       # items from the beginning through stop-1
print(a[:])           # every item in sequence

sbela
sbela and Loralai!
Lasbela
Lasbela and Loralai!

print(a[-1])    # last item in the sequence
print(a[-2:])   # last two items in the sequence
print(a[:-2])   # whole sequence

!
i!
Lasbela and Lorala

print(a[::-1])    # all items in the sequence, reversed
print(a[1::-1])   # the first two items, reversed
print(a[:-3:-1])  # the last two items, reversed
print(a[-3::-1])  # everything ex

!ialaroL dna alebsaL
aL
!i
alaroL dna alebsaL

Range

It gives immutable sequence. We will further study its use later during in 1.10 for loops.

a = range(4)
print(a)

range(0, 4)

print(type(a))

<class 'range'>