Python
Programming Notes and Practical Programs BCA 3rd Semester CSJM
University Kanpur
Python If Statement explained with examples
If statements are control flow statements which
helps us to run a particular code only when a certain condition is satisfied.
For example, you want to print a message on the screen only when a condition is
true then you can use if statement to accomplish this in programming. In this
guide, we will learn how to use if statements in Python programming with the
help of examples.
There are other control flow statements available in
Python such as if..else, if..elif..else,
nested if etc. However in this guide, we will only cover the if statements,
other control statements are covered in this .
Syntax of If statement in Python
The syntax of if statement in Python is pretty simple.
if condition:
block_of_code
If statement flow diagram
Python – If statement Example
flag = True
if flag==True:
print("Welcome")
print("To")
print("csaankitblogspot.com")
Output:
Welcome
To
csaankitblogspot.com
In the above example we are checking the value of
flag variable and if the value is True then we are executing few print
statements. The important point to note here is that even if we do not compare
the value of flag with the ‘True’ and simply put ‘flag’ in place of condition,
the code would run just fine so the better way to write the above code would
be:
flag = True
if flag:
print("Welcome")
print("To")
print("csaankitblogspot.com")
By seeing this we can understand how if statement
works. The output of the condition would either be true or false. If the
outcome of condition is true then the statements inside body of ‘if’ executes,
however if the outcome of condition is false then the statements inside ‘if’
are skipped.
Lets take another example to understand this:
flag = False
if flag:
print("You Guys")
print("are")
print("Awesome")
The output of this code is none, it does not print
anything because the outcome of condition is ‘false’.
Python if example without boolean variables
In the above examples, we have used the boolean variables in place
of conditions. However we can use any variables in our conditions. For example:
num = 100
if num < 200:
print("num is less than 200")
Output:
num is less than 200
num = 10
1. The name of the variable must always start with
either a letter or an underscore (_). For example: _str, str, num, _num are all
valid name for the variables.
2. The name of the variable cannot start with a number. For example: 9num is
not a valid variable name.
3. The name of the variable cannot have special characters such as %, $, # etc,
they can only have alphanumeric characters and underscore (A to Z, a to z, 0-9
or _).
4. Variable name is case sensitive in Python which means num and NUM are two different
variables in python.
Python identifier example
In the following example we have three variables. The name of the
variables num, _x and a_b are the identifiers.
# few examples of identifiers
num = 10
print(num)
_x = 100
print(_x)
a_b = 99
print(a_b)
Output:
10
100
99
Python If else Statement Example
In the last tutorial we learned how to use if
statements in Python. In this guide, we will learn another control statement ‘if..else’.
We use if statements when we need to execute a certain block of
Python code when a particular condition is true. If..else
statements are like extension of ‘if’ statements, with the
help of if..else we can execute certain statements if condition is true and a
different set of statements if condition is false. For example, you want to
print ‘even number’ if the number is even and ‘odd number’ if the number is not
even, we can accomplish this with the help of if..else statement.
Python – Syntax of if..else statement
if condition:
block_of_code_1else:
block_of_code_2
block_of_code_1: This would execute if the given condition is true
block_of_code_2: This would execute if the given condition is false
If..else flow control
If-else example in Python
num = 22
if num % 2 == 0:
print("Even Number")
else:
print("Odd Number")
Output:
Even Number
Python If elif else statement example
In the previous tutorials we have seen if statement and if..else
statement.
In this tutorial, we will learn if elif else statement in Python. The if..elif..else
statement is used when we need to check multiple conditions.
Syntax of if elif else statement in Python
This way we are checking multiple conditions.
if condition:
block_of_code_1elif condition_2:
block_of_code_2elif condition_3:
block_of_code_3..
..
..
else:
block_of_code_n
Notes:
1. There can be multiple ‘elif’ blocks, however there is only ‘else’ block is
allowed.
2. Out of all these blocks only one block_of_code gets executed. If the
condition is true then the code inside ‘if’ gets executed, if condition is
false then the next condition(associated with elif) is evaluated and so on. If
none of the conditions is true then the code inside ‘else’ gets executed.
Python – if..elif..else statement example
In this example, we are checking multiple conditions using
if..elif..else statement.
num = 1122
if 9 < num < 99:
print("Two digit number")
elif 99 < num < 999:
print("Three digit number")
elif 999 < num < 9999:
print("Four digit number")
else:
print("number is <= 9 or >= 9999")
Output:
Python Nested If else statement
In the previous tutorials, we have covered the if statement, if..else
statement and if..elif..else
statement.
In this tutorial, we will learn the nesting of these control statements.
When there is an if statement (or if..else or if..elif..else) is
present inside another if statement (or if..else or if..elif..else) then this
is calling the nesting of control statements.
Nested if..else statement example
Here we have a if statement inside another if..else statement
block. Nesting control statements makes us to check multiple conditions.
num = -99
if num > 0:
print("Positive Number")
else:
print("Negative Number")
#nested if
if -99<=num:
print("Two digit Negative Number")
Output:
Negative Number
Two digit Negative Number
Python for Loop explained with examples
A loop is a used for iterating over a set of
statements repeatedly. In Python we have three types of loops for, while and do-while.
In this guide, we will learn for loop and the other two loops are covered
in the separate tutorials.
Syntax of For loop in Python
for <variable> in <sequence>:
# body_of_loop that has set of statements
# which requires repeated execution
Here <variable> is a variable that is used for
iterating over a <sequence>. On every iteration it takes the next value from
<sequence> until the end of sequence is reached.
Lets take few examples of for loop to understand the
usage.
Python – For loop example
The following example shows the use of for loop to iterate over a
list of numbers. In the body of for loop we are calculating the square of each
number present in list and displaying the same.
# Program to print squares of all numbers present in a list
# List of integer numbers
numbers = [1, 2, 4, 6, 11, 20]
# variable to store the square of each num temporary
sq = 0
# iterating over the given list
for val in numbers:
# calculating square of each number
sq = val * val
# displaying the squares
print(sq)
Output:
1
4
16
36
121
400
Function range()
In the above example, we have iterated over a list
using for loop. However we can also use a range() function in for loop to
iterate over numbers defined by range().
range(n): generates a set of
whole numbers starting from 0 to (n-1).
For example:
range(8) is equivalent to [0, 1, 2, 3, 4, 5, 6, 7]
range(start, stop):
generates a set of whole numbers starting from start to stop-1.
For example:
range(5, 9) is equivalent to [5, 6, 7, 8]
range(start, stop, step_size):
The default step_size is 1 which is why when we didn’t specify the step_size,
the numbers generated are having difference of 1. However by specifying
step_size we can generate numbers having the difference of step_size.
For example:
range(1, 10, 2) is equivalent to [1, 3, 5, 7, 9]
Lets use the range() function in
for loop:
Python for loop example using range() function
Here we are using range() function to calculate and display the
sum of first 5 natural numbers.
# Program to print the sum of first 5 natural numbers
# variable to store the sum
sum = 0
# iterating over natural numbers using range()
for val in range(1, 6):
# calculating sum
sum = sum + val
# displaying sum of first 5 natural numbers
print(sum)
Output:
15
For loop with else block
Unlike Java, In Python
we can have an optional ‘else’ block associated with the loop. The ‘else’ block
executes only when the loop has completed all the iterations. Lets take an
example:
for val in range(5):
print(val)
else:
print("The loop has completed execution")
Output:
0
1
2
3
4
The loop has completed execution
Note: The else block only executes when the loop is finished.
Nested For loop in Python
When a for loop is present inside another for loop then it is
called a nested for loop. Lets take an example of nested for loop.
for num1 in range(3):
for num2 in range(10, 14):
print(num1, ",", num2)
Output:
0 , 10
0 , 11
0 , 12
0 , 13
1 , 10
1 , 11
1 , 12
1 , 13
2 , 10
2 , 11
2 , 12
2 , 13
Python While Loop
While loop is used to iterate over a block of code repeatedly until a
given condition returns false. In the last tutorial, we have seen for loop in
Python,
which is also used for the same purpose. The main difference is that we
use while
loop when we are not certain
of the number of times the loop requires execution, on the other hand when we
exactly know how many times we need to run the loop, we use for loop.
Syntax of while loop
while condition:
#body_of_while
The body_of_while is set of Python statements which requires
repeated execution. These set of statements execute repeatedly until the given
condition returns false.
Flow of while loop
1. First the given condition is checked, if the condition returns
false, the loop is terminated and the control jumps to the next statement in
the program after the loop.
2. If the condition returns true, the set of statements inside loop are
executed and then the control jumps to the beginning of the loop for next
iteration.
These two steps happen repeatedly as long as the condition
specified in while loop remains true.
Python – While loop example
Here is an example of while loop. In this example, we have a
variable num and we are displaying the value of num in a loop, the loop has
a increment operation where we are increasing the value of num. This is very important step,
the while loop must have a increment or decrement operation, else the loop will
run indefinitely, we will cover this later in infinite while loop.
num = 1
# loop will repeat itself as long as
# num < 10 remains true
while num < 10:
print(num)
#incrementing the value of num
num = num + 3
Output:
1
4
7
Infinite while loop
Example 1:
This will print the word ‘hello’ indefinitely because the condition will always
be true.
while True:
print("hello")
Example 2:
num = 1
while num<5:
print(num)
This will print ‘1’ indefinitely because inside loop we are not
updating the value of num, so the value of num will always remain 1 and the
condition num < 5 will always return true.
Nested while loop in Python
When a while loop is present inside another while loop then it is
called nested while loop. Lets take an example to understand this concept.
i = 1
j = 5
while i < 4:
while j < 8:
print(i, ",", j)
j = j + 1
i = i + 1
Output:
1 , 5
2 , 6
3 , 7
Python – while loop with else block
We can have a ‘else’ block associated with while loop. The ‘else’
block is optional. It executes only after the loop finished execution.
num = 10
while num > 6:
print(num)
num = num-1
else:
print("loop is finished")
Output:
10
9
8
7
loop is finished
Python break Statement
The break statement is used to terminate the loop
when a certain condition is met. We already learned in previous tutorials (for loop and while loop) that a loop
is used to iterate a set of statements repeatedly as long as the loop
condition returns true. The break statement is generally
used inside a loop along with a if statement so that
when a particular condition (defined in if statement) returns true, the break
statement is encountered and the loop terminates.
For example, lets say we are searching an element in a list, so
for that we are running a loop starting from the first element of the list to
the last element of the list. Using break statement, we can terminate the loop
as soon as the element is found because why run the loop unnecessary till the
end of list when our element is found. We can achieve this with the help of
break statement (we will see this example programmatically in the example
section below).
Syntax of break statement in Python
The syntax of break statement in Python is similar to what
we have seen in Java.
break
Flow diagram of
break
Example of break statement
In this example, we are searching a number ’88’ in the given list
of numbers. The requirement is to display all the numbers till the number ’88’
is found and when it is found, terminate the loop and do not display the rest
of the numbers.
# program to display all the elements before number 88
for num in [11, 9, 88, 10, 90, 3, 19]:
print(num)
if(num==88):
print("The number 88 is found")
print("Terminating the loop")
break
Output:
11
9
88
The number 88 is found
Terminating the loop
Note: You would always want to use the break statement with a if
statement so that only when the condition associated with ‘if’ is true then
only break is encountered. If you do not use it with ‘if’ statement then the
break statement would be encountered in the first iteration of loop and the
loop would always terminate on the first iteration.
Python Continue Statement
The continue statement is used inside a loop to
skip the rest of the statements in the body of loop for the current iteration
and jump to the beginning of the loop for next iteration. The break and
continue statements are used to alter the flow of loop, break terminates the
loop when a condition is met and continue skip the current iteration.
Syntax of continue statement in Python
The syntax of continue statement in Python is similar to what we
have seen in Java(except the semicolon)
continue
Flow diagram of continue
Example of continue statement
Lets say we have a list of numbers and we want to print only the
odd numbers out of that list. We can do this by using continue statement.
We are skipping the print statement inside loop by using continue statement
when the number is even, this way all the even numbers are skipped and the
print statement executed for all the odd numbers.
# program to display only odd numbers
for num in [20, 11, 9, 66, 4, 89, 44]:
# Skipping the iteration when number is even
if num%2 == 0:
continue
# This statement will be skipped for all even numbers
print(num)
Output:
11
9
89
Python pass Statement
The pass statement acts as a placeholder and
usually used when there is no need of code but a statement is still required to
make a code syntactically correct. For example we want to declare a function in
our code but we want to implement that function in future, which means we are
not yet ready to write the body of the function. In this case we cannot leave
the body of function empty as this would raise error because it is
syntactically incorrect, in such cases we can use pass statement which does nothing but makes the
code syntactically correct.
Pass statement vs comment
You may be wondering that a python
comment works
similar to the pass statement as it does nothing so we can use comment in place
of pass statement. Well, it is not the case, a comment is not a placeholder and
it is completely ignored by the Python interpreter while on the other
hand pass is
not ignored by interpreter, it says the interpreter to do nothing.
Python pass statement example
If the number is even we are doing nothing and if it is odd then
we are displaying the number.
for num in [20, 11, 9, 66, 4, 89, 44]:
if num%2 == 0:
pass
else:
print(num)
Output:
11
9
89
Other examples:
A function that does nothing(yet), may be implemented in future.
def f(arg): pass # a function that does nothing (yet)
A class that does not have any methods(yet), may have methods in
future implementation.
class C: pass # a class with no methods (yet)
