11. PYTHON IF....ELSE STATEMENT AND PYTHON FOR LOOP

Decision making is required when we want to execute a code only if a certain condition is satisfied.

The if…elif…else statement is used in Python for decision making.

Python if Statement Syntax

if test expression:
    statement(s)

Here, the program evaluates the test expression and will execute statement(s) only if the text expression is True.

If the text expression is False, the statement(s) is not executed.

In Python, the body of the if statement is indicated by the indentation. Body starts with an indentation and the first unindented line marks the end.

Python interprets non-zero values as True. None and 0 are interpreted as False.

Example: Python if Statement:

# If the number is positive, we print an appropriate message

num = 3

if num > 0:

    print(num, "is a positive number.")    # indented

print("This is always printed.")              # Unindented

num = -1

if num > 0:

    print(num, "is a positive number.")    # indented

print("This is also always printed.")      # Unindented

Output:

3, is a positive number.

This is always printed.

This is also always printed.

Python if...else Statement:

Syntax of if...else

if test expression:
    Body of if
else:
    Body of else

The if..else statement evaluates test expression and will execute body of if only when test condition is True.

If the condition is False, body of else is executed. Indentation is used to separate the blocks.

Example of if...else

# Program checks if the number is positive or negative

# And displays an appropriate message

num = -3

if num >= 0:

    print("Positive or Zero")

else:

    print("Negative number")

Output:

Negative Numer

Python if...elif...else Statement:

Syntax of if...elif...else

if test expression:
    Body of if
elif test expression:
    Body of elif
else: 
    Body of else

The elif is short for else if. It allows us to check for multiple expressions.

If the condition for if is False, it checks the condition of the next elif block and so on.

If all the conditions are False, body of else is executed.

Only one block among the several if...elif...else blocks is executed according to the condition.

The if block can have only one else block. But it can have multiple elif blocks.

Example of if...elif...else

# In this program, we check if the number is positive or negative or zero and 

# display an appropriate message

num = 3.4

if num > 0:

    print("Positive number")

elif num == 0:

    print("Zero")

else:

    print("Negative number")

Output:

Positive Number

Python Nested if statements:

We can have a if...elif...else statement inside another if...elif...else statement. This is called Nesting in computer programming.

Any number of these statements can be nested inside one another. Indentation is the only way to figure out the level of nesting. This can get confusing, so must be avoided if we can.

Python Nested if Example:

# In this program, we input a number
# check if the number is positive or
# negative or zero and display
# an appropriate message
# This time we use nested if

num = float(input("Enter a number: "))
if num >= 0:
    if num == 0:
        print("Zero")
    else:
        print("Positive number")
else:
    print("Negative number")

Output 1

Enter a number: 5
Positive number

Output 2

Enter a number: -1
Negative number

Output 3

Enter a number: 0
Zero

Python for Loop:

The for loop in Python is used to iterate over a sequence (list, tuple, string) or other iterable objects. Iterating over a sequence is called traversal.

Syntax of for Loop:

for val in sequence:
 Body of for

Here, val is the variable that takes the value of the item inside the sequence on each iteration.

Loop continues until we reach the last item in the sequence. The body of for loop is separated from the rest of the code using indentation.

Example: Python for Loop

# Program to find the sum of all numbers stored in a list

numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]   # List of numbers

sum = 0   # variable to store the sum

# iterate over the list

for val in numbers:

sum = sum+val

print("The sum is", sum)    # Output: The sum is 48

The range() function:

We can generate a sequence of numbers using range() function. 

Examaple:

range(10) will generate numbers from 0 to 9 (10 numbers).

We can also define the start, stop and step size as range(start,stop,step size). step size defaults to 1 if not provided.

This function does not store all the values in memory, it would be inefficient. So it remembers the start, stop, step size and generates the next number on the go.

To force this function to output all the items, we can use the function list().

For Example:

1. print(range(10)) # Output: range(0, 10)

2. print(list(range(10))) # Output: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

3. print(list(range(2, 8))) # Output: [2, 3, 4, 5, 6, 7]

4. print(list(range(2, 20, 3)) # Output: [2, 5, 8, 11, 14, 17]

We can use the range() function in for loops to iterate through a sequence of numbers. 

It can be combined with the len() function to iterate though a sequence using indexing.

For Example:

# Program to iterate through a list using indexing

genre = ['pop', 'rock', 'jazz']

# iterate over the list using index

for i in range(len(genre)):

print("I like", genre[i])

Output:

I like pop
I like rock
​I like jazz

for loop with else

A for loop can have an optional else block as well. The else part is executed if the items in the sequence used in for loop exhausts.

Break statement can be used to stop a for loop. In such case, the else part is ignored.

Hence, a for loop's else part runs if no break occurs.

For Example:

digits = [0, 1, 5]

for i in digits:

    print(i)

else:

    print("No items left.")

Output:

0

1

5

No items left.