Introduction to python programming ( part-3 )

INTRODUCTION
TO
PYTHON
PROGRAMMING
-By
Ziyauddin Shaik
Btech (CSE)
(PART - 3)

Contents
• Reassignment
• loops ( for,while)
• break & continue statements
• string
• string slices
• string operations
• string methods
• lists
• list operations
• list methods

Reassignment
it is to make more than one assignment to the same variable.
 A new assignment makes an existing variable refer to a new value (and stop
referring to the old value).
Ex:
x = 5
print( x)
x = 7
print( x )
Output :
5
7
The first time we display x, its value is 5; the second time, its value is 7.
Update :
A common kind of reassignment is an update,
where the new value of the variable depends
on the old.
EX:
x = x + 1
This means “get the current value of x, add
one, and then update x with the new value.”

Loops :
when you need to execute a block of code several number of times.
A loop statement allows us to execute a statement or group of statements
multiple times
The python provide two types of loops for iterative statements execution:
1. for loop
2. While loop
1. for loop :
A for loop is used for iterating over a sequence (that is a list, a tuple, a
dictionary, a set, or a string).
In Python, there is no C style for loop, i.e., for (i=0; i<n; i++). There is “for
in” loop which is similar to “for each” loop in other languages.

Syntax:
for iterator_variable in sequence:
statements(s)
Example :
fruits = ["apple", "banana",
"cherry"]
for x in fruits:
print(x)
Example :
s = "python"
for i in s :
print(i)
OUTPUT:
apple
banana
cherry
OUTPUT:
p
y
t
h
o
n

For loop Using range() function:
The range() function is used to generate the sequence of the numbers.
If we pass the range(10), it will generate the numbers from 0 to 9.
Syntax: range(start, stop, step size)
optional
optional
beginning of the iteration. the loop will iterate till stop-1 used to skip the specific
numbers from the iteration.
Example :
for i in range(10):
print(i,end = ' ')
Output:
0 1 2 3 4 5 6 7 8 9

Nested for loop :
Python allows us to nest any number of for loops inside a for loop.
The inner loop is executed n number of times for every iteration of the outer
loop. The syntax is given below.
Syntax:
for var1 in sequence:
for var2 in sequence:
#block of statements
#Other statements
Example :
rows = int(input("Enter the rows:"))
for i in range(0,rows+1):
for j in range(i):
print("*",end = '')
print()
Output:
Enter the rows:5
*
**
***
****
*****

2. while loop
The Python while loop allows a part of the code to be executed until
the given condition returns false.
Syntax:
while expression:
statements Program :
i=1;
while i<=10:
print(i);
i=i+1;
Output:
1
2
3
4
5
6
7
8
9
10

break statement:
The break statement is used to
terminate the loop intermediately.
 “break” is keyword in python
Syntax:
#loop statements
break;
Example :
n=10
for i in range(n):
if i==5:
break;
print(i)
OUTPUT:
0
1
2
3
4
continue Statement:
With the continue statement we can
stop the current iteration of the loop,
and continue with the next.
Syntax:
#loop statements
continue;
Example :
n=10
for i in range(n):
if i==5:
continue;
print(i)
OUTPUT:
0
1
2
3
4
6
7
8
9

String:
string is the collection of the characters enclosed by single /double /triple quotes.
Ex: str=‘Hello’
str= “Hello”
str=‘’’Hello’’’
1. Length using len()
Example :
str=“hello”
print(len(str))
String length : 5
Output :
5
2. Traversal with a for loop:
Example :
str=“HELLO”
for i in str:
print(i)
OUTPUT:
H
E
L
L
O

String slices:
We can access the portion(sub part ) of the string using slice operator–[:]”.
we use slice operator for accessing range elements.
Syntax: Variable[begin:end:step]
The step may be +ve( or ) –ve
If step is +ve it is forward direction (left to right)(begin to end-1)
If step is -ve it is backward direction(right to left)(begin to end+1)
In forward direction The default value of “begin” is 0
In forward direction The default value of “end” is length of sequence
The default value of “step” is 1
In backward direction The default value of “begin” is -1
In forward direction The default value of “end” is –(length of sequence)

Introduction to python programming ( part-3 )

Example :
str=“welcome”
print(str[2:5]) output: lco
print(str[2:5:2]) output: lo
print(str[-1:-5:-1]) output: emoc
print(str[-1:5:1]) output:
default step size is 1,in forword
direction
end position is:5-1=4
step size is 2,in forword direction end
position is:5-1=4
step size is -1,in back word direction end
position is:-5+1=-4
because in forward
direction there is no -1 index

Strings are immutable:
String are immutable in python that mean we can’t change the string
elements, but we replace entire string.
Example :
str=“HELLO”
str[0]=‘M’ output: error
print(str)
Example :
str=“HELLO”
str=“welcome”
print(str) output: welcome

Looping and Counting:
While iterating the string we can count the number of characters(or) number of
words present in the string.
Ex:
Str=“welcome to python programming”
count=0
for c in str:
if c==‘e’:
count=count+1
print(“the letter e is present:”,count,”times”)
OUTPUT:
The letter e is present 2 times

String Operations:
Operator Description
+
It is known as concatenation operator used to join the strings
given either side of the operator.
*
It is known as repetition operator. It concatenates the multiple
copies of the same string.
[] It is known as slice operator. It is used to access the sub-strings
of a particular string.
[:] It is known as range slice operator. It is used to access the
characters from the specified range.
in It is known as membership operator. It returns if a particular
sub-string is present in the specified string.
not in It is also a membership operator and does the exact reverse of
in. It returns true if a particular substring is not present in the
specified string.
Example :
str = "Hello"
str1 = " world"
print(str*3)
print(str+str1)
print(str[4])
print(str[2:4])
print('w' in str)
print('wo' not in str1)
Output:
HelloHelloHello
Hello world
o
ll
False
False

String Methods:
3.Find() : The find() method finds the first
occurrence of the specified value.
Ex :
txt = "Hello, welcome to my world.“
x = txt.find("e")
print(x)
4.index():
It is almost the same as the find() method, the only
difference is that the find() method returns -1 if
the value is not found
Ex:
txt = "Hello, welcome to my world.“
print(txt.find("q"))
print(txt.index("q"))
Output
1
Output :
-1
valueError
1.capitalize():Converts the first character to
upper case
Ex : txt = "hello, and welcome to my world.“
x = txt.capitalize()
print (x)
Output : Hello, and welcome to my world.
2.count() :Returns the number of times a
specified value occurs in a string
Ex : txt = "I love apple, apple is my favorite fruit"
x = txt.count("apple")
print(x)
Output
2

5. isalnum(): returns True if all the
characters are alphanumeric
Ex:
txt = "Company 12“
x = txt.isalnum()
print(x)
6.isalpha(): Returns True if all
characters in the string are in the
alphabet
Ex:
txt = "Company10“
x = txt.isalpha()
print(x)
Output :
False
Output :
False
7.islower(): Returns True if all characters in the
string are lower case
Ex:
a = "Hello world!“
b = "hello 123“
c = "mynameisPeter“
print(a.islower())
print(b.islower())
print(c.islower())
Output :
False
True
False
8. isupper(): Returns True if all characters in the
string are upper cases
Ex:
a = "Hello World!"
b = "hello 123"
c = "HELLO“
print(a.isupper())
print(b.isupper())
print(c.isupper())
Output :
False
False
True

9.split(): It splits a string into a list.
Ex:
txt = "welcome to the jungle“
x = txt.split()
print(x)
Output:
['welcome', 'to', 'the', 'jungle']
13.replace():It replaces the old sequence of
characters with the new sequence.
Ex :
str1 = ”python is a programming language"
str2 = str.replace(“python","C")
print(str2)
Output:
C is a programming language

Reading word lists:
Ex:
file= open(‘jk.txt','r‘)
for line in file:
for word in line.split():
print(word)
input file consisting the following
text file name is ‘jk.txt’
Welcome To Python Programming
Hello How Are You
Output:
Welcome
To
Python
Programming
Hello
How
Are
You

Lists:
 A list is a collection of different elements. The items in the list are
separated with the comma (,) and enclosed with the square brackets [].
Creating Empty list
Syntax: List variable=[ ]
Ex: l=[ ]
Creating List with elements
Syntax: List variable=[element1,element2,--------,element]
Ex1: l=[10,20,30,40]
Ex2: l1=[10,20.5,”Selena”]
Program:
l=[10,20,30]
l1=[10,20.5,”Selena”]
print(l)
print(l1)
Output :
[10,20,30]
[10, 20.5, Selena]

Accessing List elements (or)items:
 We Access list elements by using “index” value like Array in C language. We
use index vale for access individual element.
 We can also Access list elements by using “Slice Operator –[:]”. we use slice
operator for accessing range elements.

Loop Through a List:
You can apply loops on the list items. by using a for loop:
Program:
list = [10, "banana", 20.5]
for x in list:
print(x)
Output:
10
banana
20.5
Program:
List = ["Justin", "Charlie", "Rose", "Selena"]
for i in List:
print(i)
Output:
Justin
Charlie
Rose
Selena

Operations on the list:
 The concatenation (+) and repetition (*) operator work in the same way as
they were working with the strings.

The + operator concatenates lists:
>>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> c = a + b
>>> print c
[1, 2, 3, 4, 5, 6]
The * operator repeats a list a given number of times:
>>> [0] * 4
[0, 0, 0, 0]
>>> [1, 2, 3] * 3
[1, 2, 3, 1, 2, 3, 1, 2, 3]
The first example repeats [0] four times.
The second example repeats the list [1, 2, 3] threetimes.

List Built-in functions:
Program
L1=[10,20,4,’a’]
print(len(L1))
Program:
L1=[10,20,4,2]
print(max(L1))
Program:
L1=[10,20,4,2]
print(min(L1))
Output :
4
Output :
20
Output :
2
len()
max()
min()

List built-in methods:
Program:
l=[ ]
l.append(10)
l.append(20)
l.append(30)
print(“The list elements are:”)
print(l)
Output:
The list elements are:
[10,20,30]
Program:
l=[10,20,30,15,20]
print(“Before insert list elements:”,l)
l.insert(1,50)
print(“After insert list elements:”,l)
Output:
Before insert list elements:[10, 20, 30, 15, 20]
After insert list elements:[10, 50,20, 30, 15, 20]
append() insert() extend()
reverse() index() clear()
sort() pop() copy() count()

Ex:
l1=[10,20,30]
l2=[100,200,300,400]
print(“Before extend list1 elements are:”,l1)
l1.extend(l2).
print(“After extend list1 elements are:”,l1)
Output:
Before extend list1 elements are:[10,20,30]
After extend list1 elements
are:[10,20,30,100,200,300,400]
Ex:
l=[10,20,30,40]
x=l.index(20)
print(“The index of 20 is:”,x)
Output:
The index of 20 is:1
Program:
L=[10,20,30,15]
print(“Before reverse the list elements:”,L)
L.reverse()
print(“After revers the list elements:”,L)
Output:
Before reverse the list elements:[10,20,30,15]
After reverse the list elements:[15,30,20,10]
Program:
l=[10,20,30,40]
print(“Before clear list elements are:”, l)
l.clear()
print(“After clear list elements are:”, l)
Output:
Before clear list elements are: [10,20,30,40]
After clear list elements:

Program:
L=[10,20,5,14,30,15]
print(“Before sorting list elements are:”,L)
L.sort()
print(“After sort list elements are:”,L)
Output:
Before sorting list elements are:[10,20,5,14.30,15]
After sorting list elements are:[5,10,14,15,20,30]
Program:
l=[10,20,30,40]
print(“Before pop list elements are:”, l)
#here pop last element in the list
x=l.pop()
print(“The pop element is:”, x)
print(“After pop list elements are:”, l)
Output:
Before pop list elements are:[10, 20, 30, 40]
The pop element is: 40
After pop list elements are:[10, 20, 30]
Program :
l1=[10,20,30,40]
l2=[]
l2=l1.copy()
print(“Original list element L1:”,l1)
print(“Copy list elements l2:”,l2)
Output:
Original list element L1:[10,20,30,40]
Copy list element L2:[10,20,30,40]
Ex:
l1=[10,20,30,40,20,50,20]
x=l1.count(20)
print(“The count of 20 is:”,x)
Output:
The count of 20 is:3

Introduction to python programming ( part-3 )

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