In order to write useful programs, we almost always need the ability to
check conditions and change the behavior of the program accordingly. Conditional statements give us this ability. The simplest
way to do is to use a

**if statement.**
In
programming and scripting languages, conditional statements or conditional
constructs are used to perform different computations or actions depending on
whether a condition evaluates to true or false. (Please note that true and
false are always written as True and False in Python.)

The condition usually uses comparisons and arithmetic expressions with variables. These expressions are evaluated to the Boolean values True or False. The statements for the decision taking are called conditional statements, alternatively they are also known as conditional expressions or conditional constructs.

The condition usually uses comparisons and arithmetic expressions with variables. These expressions are evaluated to the Boolean values True or False. The statements for the decision taking are called conditional statements, alternatively they are also known as conditional expressions or conditional constructs.

**if condition:**

**block of statements1**

**[else:**

**block of statements2]**

**OR**

**if condition:**

**block of statements1**

**[elif condition:**

**block of statements2**

**else:**

**block of statements3]**

statements with in [] are optional

the condition after the if statement is evaluated and block of
statements1 will be executed if the condition is true else block of statements2
will be executed.

**conditions are specified with relational and logical operators which will return a Boolean value ( True or False).**

**Equality Operators == != <>**

**Comparison Operators < <= > >=**

**Logical operators and or not**

**Eg:**

**if x%2==0:**

**print “number is even”**

**else:**

**print “number is odd”**

**if x>y:**

**print “ x is greater than y”**

**elif x<y:**

**print “x is smaller than y”**

**else:**

**print “x and y are equal”**

You can also write nested if statements

**if x==y:**

**print “ x and y are equal”**

**else:**

**if x<y:**

**print “x is smaller than y”**

**else:**

**print “x is larger than y”**

Unfortunately it is not as easy in real life as it is in Python
to differentiate between true and false:

The following objects are evaluated by Python as False:

The following objects are evaluated by Python as False:

- numerical zero
values (0, 0L, 0.0, 0.0+0.0j),
- the Boolean
value False,
- empty strings,
- empty lists and
empty tuples,
- empty
dictionaries.
- plus the special
value None.

All other values are considered to be True.

Sometimes,
it is useful to have a body with no statements, in that case you can use

**pass**
statement.
Pass statement does nothing.

**Example**

if
condition:

pass

**Sample Programs ( All university questions)**

**1.Program to find the roots of a quadratic equation**

import math

print "enter a b and c the coefficients"

a=input()

b=input()

c=input()

if a==0:

print "Not a quadratic eqtn..root is ", -c/b

else:

d=b*b-4*a*c

if d==0:

print "only one root",-b/(2*a)

elif d>0:

print "roots are real"

print "root1",-b+math.sqrt(d)/(2*a)

print "root2",-b-math.sqrt(d)/(2*a)

else:

print "roots are imaginary"

**2.Write a program that accepts the length of three sides of a triangle as input and determine whether or not the triangle is a right triangle**print "enter a b and c the three sides ( c is the hypotinous and c > a+b)"

a=input()

b=input()

c=input()

if a+b > c and a+c >b and b+c >a:

if a**2+b**2==c**2:

print "Right angled triangle"

else:

print "Not a right angled triangle"

else:

print "given sides does not form a triangle"

**3.Write a program to get the absolute value of a number without using the abs() function**

n=input("enter a number....")

if n<0:

print "absolute value=",-n

else:

print "absolute value=",n

**Try the following program using if else or if elif statement**

1)Compare
two numbers

2)Check
whether the given number is even or odd.

3)Find
the biggest of 3 numbers

4)Read
internal marks( out of 50) and external mark( out of 100) and print passed /failed in internal or external ( use KTU criteria..45%)

5)Check
whether the given number is zero, positive or negative.

6)Find
the roots of a quadratic equation ( ax^2+bx+c)

7)Read length of 3 sides and check whether it forms a triangle.

(All you have to do is use the Triangle Inequality Theorem, which states that the sum of two side lengths of a triangle is always greater than the third side. If this is true for all three combinations of added side lengths, then you will have a triangle.)

7)Read length of 3 sides and check whether it forms a triangle.

(All you have to do is use the Triangle Inequality Theorem, which states that the sum of two side lengths of a triangle is always greater than the third side. If this is true for all three combinations of added side lengths, then you will have a triangle.)

8)Check
the type of a triangle after reading the three sides ( scalene, isosceles,
equilateral)

9)
Check the type of a triangle after reading the three vertices ( scalene,
isosceles, equilateral)

Note:

From the vertices length of the sides can be computed
using the distance formulae.

A
scalene triangle is a triangle that has three unequal sides.

An
isosceles has two equal sides and two equal angles.

An equilateral triangle is a triangle with
all three sides of equal length and also has three equal 60 degree angles.

10) Read a mark ( out of 100) and print the corresponding
grade.

**Percentage Range Grade**

>=90 O

>=85
and <90 A+

>= 80 and <85 A

> =70 and <80 B+

> =60 and <70 B

> =50 and <60 C

>=45 and <50 P

<45 F

11)
answer = raw_input("Do you like Python? ")

if
the answer is "yes" print
"That is great!" else print "That is disappointing!".

If
the user input something else print “Enter yes/no”
12)Check
whether a given year is Leap Year or not. A year is leap year if following
conditions are satisfied

1) Year is multiple of 400

2) Year is multiple of 4 and not multiple of 100.

13)Write a program to check the quadrant of a given point(x,y)( University question)

14)Write a program to get the absolute value of a number without using the abs() function.(university question)

15)Write a program that accepts the length of three sides of a triangle as input and determine whether or not the triangle is a right triangle.( university question)

16)Given three points (x1,y1 ) ,(x2,y2) and (x3,y3), check whether they form a triangle.

2) Year is multiple of 4 and not multiple of 100.

13)Write a program to check the quadrant of a given point(x,y)( University question)

14)Write a program to get the absolute value of a number without using the abs() function.(university question)

15)Write a program that accepts the length of three sides of a triangle as input and determine whether or not the triangle is a right triangle.( university question)

16)Given three points (x1,y1 ) ,(x2,y2) and (x3,y3), check whether they form a triangle.