UNIT – 3.pptx for first year engineering

UNIT – 3
CONTROL FLOW,
FUNCTIONS

Conditionals: Boolean values and operators, conditional (if),
alternative (if-else), chained conditional (if-elif-else); Iteration:
state, while, for, break, continue, pass; Fruitful functions:
return values, parameters, scope: local and global, composition,
recursion; Strings: string slices, immutability, string functions
and methods, string module; Lists as arrays. Illustrative
programs: square root, gcd, exponentiation, sum the array of
numbers, linear search, binary search

BOOLEAN VALUES
 Any object can be tested for truth value, for use in an if or while condition or
as operand of the Boolean operations below.
 The following values are considered false:
 None False Zero of any numeric type, for example, 0, 0L, 0.0, 0j.
 Any empty sequence, for example, ‘’, (), [].
 Any empty mapping, for example, {}.

Boolean Operations (and, or, not)

OPERATORS
 Operators are the constructs (operator) which can manipulate the value of
operands.
 Consider the expression 4 + 5 = 9.
 Here, 4 and 5 are called operands and + is called operator.

Types of Operators
 Arithmetic Operators
 Comparison (Relational) Operators
 Assignment Operators
 Logical Operators
 Bitwise Operators
 Membership Operators
 Identity Operators

Comparison (Relational) Operators

DECISION MAKING
 The execution of the program taking action according to the conditions.
 This concept is also known as Conditional statements or Branching
statements.
 Python programming language provides following types of decision making
statements.

Types of Conditional statements
 if statements (conditional)
 if-else statements (alternative)
 if-elif-else (chained conditional)
 Nested Conditional

Example
a=10
if ( a>9):
print(“A is Greater than 9”)
Output
A is Greater than 9

If... Else Statement
Flow chart

© Copyright 2012 by Pearson Education, Inc. All Rights Reserved.
if...else Example
20
Example:
a=10
b=20
if(a>b):
print(“A is Greater than B”)
else:
print(“B is Greater than A”)
Output:
B is Greater than A

The Elif Else Statement
Flow chart

Example
num = -3
if( num > 0):
print(“Positive number”)
elif (num == 0):
print(“Zero”)
else:
print(“Negative number”)
Output:
Negative number

Nested Conditionals
Syntax:
if (condition1) :
if (condition2):
statements
else :
statements
else:
statements

Example:
num = float(input(“Enter a number: “))
if (num >= 0):
if (num == 0):
print(“Zero”)
else:
print(“Positive number”)
else:
print(“Negative number”)
Output :
Enter a number: 5
Positive number

ITERATION
 A loop statement allows us to execute a statement or group of
statements multiple times.
 Repeated execution of a set of statements is called iteration.
 Types
 while loop
 for loop
 nested loop

While Loop
 A while loop statement executes a block of statement again and again until the
condition will occur false .
 It tests the condition before executing the loop body so this technique is
known as Entry controlled loop.

Example:
count = 0
while (count < 5):
print (“The count is:”%count)
count = count + 1
print “Good bye!”
Output
The count is: 0
The count is: 1
The count is: 2
The count is: 3
The count is: 4
Good bye!

For Loop :
Executes a sequence of statements multiple times and abbreviates the code that manages the
loop variable.

UNIT – 3.pptx for first year engineering

Example:
#List of numbers
number=[3,2,5,7,9,1,4,6,8]
#variable to store the sum
total=0
for item in number:
total=total+item
print(“total = %d” %total)
Output:
total= 45

Nested Loops - Python programming language allows
using one loop inside another loop.

Example program to print range values
print(“first loop values”)
for x in range(5):
print(x)
print(“second loop values”)
for x in range(3,6):
print(x)
print(“third loop values”)
for x in range(3,8,2):
print(x)
Output:
first loop values
0
1
2
3
4
second loop values
3
4
5
Third loop values
3
5
7

Iterating by Sequence Index - An alternative way of
iterating through each item is by index offset into the
sequence itself.
Example
fruits = [‘banana’, ‘apple’, ‘mango’]
for index in range(len(fruits)):
print (“Current fruit :”%
fruits[index] )
print “Good bye!”
Output
Current fruit : banana
Current fruit : apple
Current fruit : mango
Good bye!

Unconditional Looping Statements
 i) break statement
 ii) continue statement
 iii) pass statement

Break Statement
 Terminates the loop statement and transfers execution to the statement
immediately following the loop.
 The break statement can be used in both while and for loops.

Example
for letter in ‘Python’:
if ( letter == ‘h’):
break
print (“Current Letter : %s” %letter)
Output
Current Letter : P
Current Letter : y
Current Letter : t

Continue Statement
 It returns the control to the beginning of the while loop.
 The continue statement rejects all the remaining statements in the
current iteration of the loop and moves the control back to the top of the
loop.
 The continue statement can be used in both while and for loops.

Example:
for val in “string” :
if(val==‘i’):
continue
print(val)
print(“the end”)
Output:
s
t
r
n
g
the end

Pass Statement
 The pass statement in Python is used when a statement is required
syntactically but you do not want any command or code to execute.
 It execute nothing.
 It is a null statement.
 Difference between a comment and pass statement in python is that,
interpreter ignores a comment entirely but not the pass statement.
 Syntax:
pass

Example
for letter in ‘Python’:
if letter == ‘h’:
pass
print ‘This is pass block’
print ‘Current Letter :’, letter
Output
Current Letter : P
Current Letter : y
Current Letter : t
This is pass block
Current Letter : h
Current Letter : o
Current Letter : n

FRUITFUL FUNCTIONS:
 Functions that return values are sometimes called fruitful
functions.
 In many other languages, a function that doesn’t return a value is
called a procedure, but we will stick here with the Python way of
also calling it a function, or if we want to stress it, a non-fruitful
function.

Return Values
 The built-in functions we have used, such as abs, pow, int, max, and
range, have produced results.
 Calling each of these functions generates a value, which we usually
assign to a variable or use as part of an expression.
 Example
 biggest = max(3, 7, 2, 5)x = abs(3 - 11) + 10

Parameters
 Parameter -- Data sent to one function from another.
 Types of parameter:
 Formal parameter -- The parameter defined as part of the function
definition.
 Example: def add(x1,x2): here x1,x2 are formal parameter
 Actual Parameter -- The actual data sent to a function. It’s found in the
function call.
 Example:add(a,b) here a,b are actual parameter

Parameter Passing Method
 Pass-by-value parameter passing
 Pass-by-Reference parameter passing

Pass-by-value parameter passing
 Pass-by-value parameter passing-- the value of the actual parameter is
copied to the formal parameter.
 In Python, scalar values are sent by-value. Lists and other objects are sent by
reference.
def processNumber(x):
x=72
return x+3
# main
y=54
res = processNumber(y)

Pass-by-Reference parameter passing
 It provide reference to the actual parameter is sent to the function.
 When we trace a program, and a list is sent, we don’t copy the list to the
actual parameter box, we draw an arrow from formal to actual.
 Example:
def processList(list):
list[1]=99
# main
aList = [5,2,9]
processList(aList)

Scope of the Variable –
Variable scope and lifetime
 Scope of a variable specifies the part of a program where a variable is
accessible .
 Lifetime of a variable specifies the time period in which the variable has valid
memory.
 Python uses LEGB Rule for Scope resolution.
Local -> Enclosed -> Global -> Built-in

LEGB Rule:
 Local can be inside a function or class method.
 Enclosed can be its enclosing function.e.g. if a function is wrapped inside
another function.
 Global refers to the uppermost level of the executing script itself and
 Built-in are special names that python reserves for itself

Variable:
 A variable can be either of global or local scope.
 A global variable is a variable declared in the main body of the source
code ,outside all functions. It will be visible through out the program.
 A local variable is declared within the body of a function or a block. It is
accessible only inside the function and gets deleted at the end of function.

Example:Variables in different scopes:
# This is a global variable
a = 0
if a == 0:
# This is still a global variable
b = 1
def my_function(c):
# this is a local variable
d = 3
print(c)
print(d)
# Now we call the function, passing the value 7 as the first and only parameter
my_function(7)
# a and b still exist
print(a)
print(b)
# c and d don’t exist anymore -- these statements will give us name errors!
print(c)
print(d)

Scope rules in function
# global scope
a=10
def func(b):
#local scope
c=a+b
return( c )
z=func(1)
print(z)

Global and local Variables in Functions
 Nonlocal Variables:
 Python3 introduced nonlocal variables as a new kind of variables.
 nonlocal variables have a lot in common with global variables.
 One difference to global variables lies in the fact that it is not possible to
change variables from the module scope, i.e. variables which are not defined
inside of a function, by using the nonlocal statement.
 Example
def f():
global x
print(x)
x = 3
f()
Output: 3

 We will change “global” to “nonlocal” in the following program:
def f():
nonlocal x
print(x)
x = 3
f()
Output:
File “example1.py”, line 2
nonlocal x
SyntaxError: no binding for nonlocal ‘x’ found

Function Composition
 It is the ability to call one function from within another function.
 It is a way of combining functions such that the result of each function is
passed as the argument of the next function.
 Example:
Distance between two points of a circle
Sqrt((x2-x1)**2 +(y2-y1)**2)

Distance between two points of a circle
import math
def distance(x1,y1,x2,y2):
result=math.sqrt(((x2-x1)**2)+((y2-y1)**2))
return result
xc=input(“enter x1 value:”))
yc=input(“enter y1 value:”))
xp=input(“enter x2 value:”))
yp=input(“enter y2 value:”))
print(distance(xc,yc,xp,yp))

Boolean functions
 Functions can return booleans, which is often convenient for hiding complicated
tests inside functions.
 They are often used in conditional statements.
 Example
def is_divisible(x, y):
if x % y == 0:
return True
else:
return False
is_divisible(6, 4)
Output: False

Recursion
 Recursion is a way of programming in which a function calls itself again
and again until a condition is true.
 A Recursion function calls itself and has a termination condition.
Advantage:
 Recursive functions make the code look clean and elegant.
 A complex task can be broken down into simpler sub-problems using
recursion.
Disadvantage:
 Sometime it is difficult to understand the logic
 It is expensive
 Hard to debug

Example:
def factorial(n):
if (n == 0):
return (1 )
else:
return (n* factorial(n-1))
n=input(“Enter a number:”)
print(“factorial= %d”%n)
Output:
Enter a number :5
Factorial=120

STRINGS
 String are sequence of character enclosing with quotes.
Python treats single quotes the same as double quotes.
 Creating strings is as simple as assigning a value to a
variable.
 Example
var1 = ‘Hello World!’
var2 = “Python Programming”

Accessing values in strings
 To access substrings, use the square brackets for slicing
along with the index or indices to obtain your substring.
 Example
var2 = “Python Programming”
print (“var1[0]: “, var1[0])
print (“var2[1:5]: “, var2[1:5])
Output
var1[0]: H
var2[1:5]: ytho

Updating Strings
Print( “Updated String :- “, var1[:6] + ‘Python’)
Output:
Updated String :- Hello Python

String formatting operator
 Example
print (“My name is %s and weight is %d kg!” % (‘Zara’, 21))
 Output
My name is Zara and weight is 21 kg!

Triple Quotes
 The syntax for triple quotes consists of three consecutive single or double
quotes.
 Example:
para_str = “””this is a long string that is made up of several lines and non-
printable characters such as TAB ( t ) and they will show up that way when
displayed.NEWLINEs within the string, whether explicitly given like this
within the brackets [ n ], or just a NEWLINE withinthe variable assignment
will also show up.“””
print( para_str)
 Output:
this is a long string that is made up ofseveral lines and non-printable
characters such asTAB ( ) and they will show up that way when
displayed.NEWLINEs within the string, whether explicitly given likethis
within the brackets [
], or just a NEWLINE withinthe variable assignment will also show up.

String Slices
 The “slice” refer to sub-parts of sequences, typically
strings and lists.
 The slice s[start:end] is the elements beginning at start
and extending up to but not including end
 Example : s = “Hello”
 s[1:4] is ‘ell’
 s[1:] is ‘ello’
 s[:] is ‘Hello’
 s[1:100] is ‘ello’
 s[-1] is ‘o’
 s[-4] is ‘e’
 s[:-3] is ‘He’
 s[-3:] is ‘llo’

String are Immutable
 Strings are immutable, which means you cannot change
an existing string.
 Greeting = “Hello, world!”
 newGreeting = ‘J’ + greeting[1:]
 print(newGreeting)
 print(greeting) # same as it was
 Output
 Jello, world!
 Hello, world!

String Functions and Methods
S.NO Methods Description
1 capitalize() Capitalizes first letter of string
2 isalnum() Returns true if string has at least 1 character
and all characters are alphanumeric and false
otherwise.
3 isalpha() Returns true if string has at least 1 character
and all characters are alphabetic and false
otherwise.
4 isdigit() Returns true if string contains only digits
and false otherwise.
5 islower() Returns true if string has at least 1 cased
character and all cased characters are in
lowercase and false otherwise.
6 isupper() Returns true if string has at least one cased
character and all cased characters are in
uppercase and false otherwise.
7 len(string) Returns the length of the string

String Module
 import string
 text = “Monty Python’s Flying Circus”
 Print( “upper”, “=>”, string.upper(text) )
 print (“lower”, “=>”, string.lower(text) )
 print (“split”, “=>”, string.split(text) )
 print (“join”, “=>”, string.join(string.split(text), “+”) )
 print (“replace”, “=>”, string.replace(text, “Python”, “Java”) )
 print (“find”, “=>”, string.find(text, “Python”), string.find(text, “Java”) )
 print (“count”, “=>”, string.count(text, “n”))

LISTS AS ARRAY
 List basics
 A list in Python is just an ordered collection of items which can be of any
type.
 myList=[1,2,3,4,5,6]

Comprehensions
 A comprehension is roughly speaking just an expression that specifies a sequence
of values think of it as a compact for loop. In Python a comprehension can be used
to generate a list.
 The simplest form of a list comprehension is
 [expression for variable in list]
 Example
To create the list equivalent of a ten-element array you could write:
myList=[0 for i in range(10)]
myList[i]=something

Example:
myList=[i for i in range(10)]
sets myList to [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] and
myList=[i*i for i in range(10)]
sets myList to [0, 1, 4, 9, 16, 25, 36, 49, 64, 81].

Two dimensions
for i in range(len(myArray)):
for j in range(len(myArray[i])):
print (myArray[i][j])

Illustrative Program:
GCD of a number
a=int(input(“enter the value for a:”))
b=int(input(“enter the value for b:”))
result = a%b
while result !=0:
a=b
b=result
result= a%b
print(“n GCD of given no. is %d” % (b))
 Output:
enter the value for a:12
enter the value for b:36
GCD of given no. is 12

SQUARE ROOT OF A NUMBER
print(“Square Root Program”)
num = input(“Enter a number: “)
number = float(num)
number_sqrt = number ** 0.5
Print(“Square Root of %d is %0.2f” %(number, number_sqrt))
Output:
Square Root Program
Enter a number:25
Square Root of 25 is 5.00

Sum the array of numbers
def listsum(numList):
if len(numList) == 1:
return numList[0]
else:
return numList[0] + listsum(numList[1:])
print(listsum([1,3,5,7,9]))
Output:
25

Linear search
i=0
j=0
count=0
list=[]
N=int(input("Enter the no. of elements"))
while i<N:
item=int(input("Enter your %d elements to the list:"%(i)))
list. append(item)
i=i+1
search=int(input("Enter the element to search:"))
while j<N:
if(search==list[j]):
count=1
j=j+1
if(count==1):
print("The element %d is present in the list"%search)
else:
print("The element %d is not present in the list"%search)

Binary search
def binary_search(sortedlist,n,x):
start = 0
end = n - 1
for i in sortedlist:
while(start <= end):
mid = int((start + end)/2)
if (x == int(sortedlist[mid])):
return mid
elif(x < int(sortedlist[mid])):
end = mid - 1
else:
start = mid + 1
return -1
n = int(input("Enter the size of the list: "))
sortedlist = []
for i in range(n):
sortedlist.append(input())
print(sortedlist)
x = int(input("Enter the number to search: "))
position = binary_search(sortedlist,n,x)
if(position != -1):
print("Entered number %d is present at
position: %d"%(x,position))
else:
print("Entered number %d is not present in
the list"%x)

Exponentiation Program
 base=int(input('Enter the base value:'))
 exp=int(input('Enter the exponent value:'))
 res=1
 for _ in range(exp):
 res=res*base
 print("n Exponentiation using loop:",res)


 # Direct method to find Exponentiation
 factor=base ** exp
 print("nFactor is:",factor)



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