Conditional Statement In Python
§ If for some reason you want to interrupt this flow and
execute certain statements, only, if certain condition is met, then we use
conditional statements.
§ Conditional
expressions, involving keywords such as if, elif, and else, provide Python
programs with the ability to perform different actions depending on a Boolean
condition: True or False.
§ When you want to justify one condition while
the other condition is not true, then you use "if statement".
if statement
§ This is the simplest decision-making statement
in Python. It is used to decide if a particular block of code needs to
be executed or not. Whenever the given condition is True, then the block of code inside it gets
executed, else it does not.
The syntax for if statement:
if condition:
body
§ The if statements check
the condition. If it evaluates to True, it executes the body of the if
statement. If it evaluates to False, it skips the body.
if else statement
§ The statements written within the else
block get executed whenever the if condition is False. You can imagine the flow of execution
this way,
Syntax:
if condition:
body
else:
body
Example
if True:
print "It
is true!"
else:
print "This
won't get printed.."
# Output: It is true!
if False:
print "This
won't get printed.."
else:
print "It
is false!"
# Output: It is false!
In Python you can define a series of conditionals using if for the first one, elif for the rest, up until the final (optional) else for anything not caught by the other conditionals.
Syntax:
if expression-1
Statement-1
elif expression-2
Statement-2
elif expression-3
Statement-3
v Write
a python program to enter number from 1 to 9 and print in words. If user enter
number apart from 1 to 9 then it will print 'Number is allowed only between 0-9
n=int(input('enter
number'))
if n==1:
print("One")
elif n==2:
print("Two")
elif n==3:
print("Three")
elif n==4:
print("Four")
elif n==5:
print("Five")
elif n==6:
print("Six")
elif n==7:
print("Seven")
elif n==8:
print("Eight")
elif n==9:
print("Nine")
else:
print('Number is allowed only between 0-9')
Boolean Logic Expressions
§ Boolean
logic expressions, in addition to evaluating to True or False, return the value
that was interpreted as True or False. It is Pythonic way to represent logic
that might otherwise require an if-else test.
And operator
§ The
and operator evaluates all expressions and returns the last expression if all
expressions evaluate to True. Otherwise it returns the first value that
evaluates to False:
print(10 and 20) #output
will be 20
print(10 and 0) #output
will be 0
print(0 and 10) #output
will be 0
print(10 and 20 and 30) #output will be 30
print(10 and "Python") #output will be python
print(0 and "python") #output will be 0
print("python" and 10) #output will be 10
print("" and "python") #output will be “ ”
print(" " and"python") #output will be python
Or operator
§ The
or operator evaluates the expressions left to right and returns the first value
that evaluates to True or the last value (if none are True).
print(10 or 20)#output will be 10
print(10 or 0)#output will be 10
print(0 or 10)#output will be 10
print(10 or 20 or 30)#output will be 10
print(10 or "Python")#output
will be 10
print(0 or "python")#output
will be python
print("python" or 10)#output
will be python
print("" or "python")#output
will be
print(" " or "python")#output
will be
print(None or 1)#output will be 1
print(0 or [])#output will be []
Testing for multiple conditions
§ A
common mistake when checking for multiple conditions is to apply the logic
incorrectly.
§ trying
to check if two variables are each greater than 2.
§ If
you write the statement in such way
if (a) and (b > 2)
§ this
gives an unexpected result because bool(a) evaluates as True when a is not
zero.
a=int(input('enter
first number'))
b=int(input('enter first number'))
if a and b>2:
print("Yes")
else:
print("No")
§ Each
variable needs to be compared separately.
a=int(input('enter first number'))
b=int(input('enter first number'))
if a>2 and b>2:
print("Yes")
else:
print("No")
§ Another,
similar, mistake is made when checking if a variable is one of multiple values.
The statement in this example is evaluated as - if (a == 3) or (4) or (6). This
produces an unexpected result because bool (4) and bool(6) each evaluate to
True
a=1
if a==3 or 4 or 5 or 6:
print('Yes')
else:
print('No')
#output will
be Yes
§ Again
each comparison must be made separately
a=1
if a==3 or a==4 or a==5 or a==6:
print('Yes')
else:
print('No')
#output will
be No
§ Using the in operator is the canonical way to write this.
n=int(input('enter number'))
if n in(10,20,30):
print('Yes')
else:
print('No')
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