for Loop

What is for loop in Python?

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:
    loop body

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.

Flowchart of for Loop

Example

# Program to find the sum of all numbers stored in a list
                    
# List of numbers
numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]
                    
# variable to store the sum
sum = 0
                    
# iterate over the list
for val in numbers:
    sum = sum+val
                    
print("The sum is", sum)

When you run the program, the output will be:

The sum is 48

The range() function

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

The range object is "lazy" in a sense because it doesn't generate every number that it "contains" when we create it. However, it is not an iterator since it supports in, len and __getitem__ operations.

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().

The following example will clarify this.

print(range(10))
                    
print(list(range(10)))
                    
print(list(range(2, 8)))
                    
print(list(range(2, 20, 3)))

Output

range(0, 10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[2, 3, 4, 5, 6, 7]
[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 through a sequence using indexing. Here is an 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.

The break keyword can be used to stop a for loop. In such cases, the else part is ignored.

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

Here is an example to illustrate this.

digits = [0, 1, 5]
                    
for i in digits:
    print(i)
else:
    print("No items left.")

When you run the program, the output will be:

0
1
5
No items left.

Here, the for loop prints items of the list until the loop exhausts. When the for loop exhausts, it executes the block of code in the else and prints No items left.

This for...else statement can be used with the break keyword to run the else block only when the break keyword was not executed. Let's take an example: 

# program to display student's marks from record
student_name = 'Soyuj'
                            
marks = {'James': 90, 'Jules': 55, 'Arthur': 77}
                            
for student in marks:
    if student == student_name:
        print(marks[student])
        break
else:
    print('No entry with that name found.')

Output

No entry with that name found.
programiz.com

while Loop

What is while loop in Python?

The while loop in Python is used to iterate over a block of code as long as the test expression (condition) is true.

We generally use this loop when we don't know the number of times to iterate beforehand.

Syntax of while Loop in Python

while test_expression:
    Body of while

In the while loop, test expression is checked first. The body of the loop is entered only if the test_expression evaluates to True. After one iteration, the test expression is checked again. This process continues until the test_expression evaluates to False.

In Python, the body of the while loop is determined through indentation.

The body starts with indentation and the first unindented line marks the end.

Flowchart

Example

# Program to add natural
# numbers up to
# sum = 1+2+3+...+n
                            
# To take input from the user,
# n = int(input("Enter n: "))
                            
n = 10
                            
# initialize sum and counter
sum = 0
i = 1
                            
while i <= n: 
    sum = sum + i
    i = i + 1 # update counter
# print the sum 
print("The sum is", sum)

When you run the program, the output will be:

Enter n: 10
The sum is 55

In the above program, the test expression will be as long as our counter variable is less than or equal to (10 in our program).Truein

We need to increase the value of the counter variable in the body of the loop. This is very important (and mostly forgotten). Failing to do so will result in an infinite loop (never-ending loop).

Finally, the result is displayed.

While loop with else

Same as with for loops, while loops can also have an optional block.else

The part is executed if the condition in the while loop evaluates to .elseFalse

The while loop can be terminated with a break statement. In such cases, the part is ignored. Hence, a while loop's part runs if no break occurs and the condition is false. elseelse

Here is an example to illustrate this.

'''Example to illustrate
the use of else statement
with the while loop'''
                                            
counter = 0
                                            
while counter < 3:
    print("Inside loop")
    counter=counter + 1
else:
    print("Inside else")

Output

Inside loop
Inside loop
Inside loop
Inside else

Here, we use a counter variable to print the string three times.Inside loop

On the fourth iteration, the condition in becomes . Hence, the part is executed. whileFalseelse

programiz.com

break and continue

What is the use of break and continue in Python?

In Python, break and continue statements can alter the flow of a normal loop.

Loops iterate over a block of code until the test expression is false, but sometimes we wish to terminate the current iteration or even the whole loop without checking test expression.

The break and continue statements are used in these cases.

programiz.com

break statement

Python break statement

The break statement terminates the loop containing it. Control of the program flows to the statement immediately after the body of the loop.

If the break statement is inside a nested loop (loop inside another loop), the break statement will terminate the innermost loop.

Syntax of break

break

Flowchart of break

The working of break statement in for loop and while loop is shown below.

Example

# Use of break statement inside the loop
                    
for val in "string":
    if val == "i":
          break
    print(val)
                    
print("The end")

Output

s
t
r
The end

In this program, we iterate through the "string" sequence. We check if the letter is i, upon which we break from the loop. Hence, we see in our output that all the letters up till i gets printed. After that, the loop terminates.

programiz.com

continue statement

Python continue statement

The statement is used to skip the rest of the code inside a loop for the current iteration only. Loop does not terminate but continues on with the next iteration.continue

Syntax of Continue

continue

Flowchart of continue

The working of the statement in for and while loop is shown below. continue

Example

# Program to show the use of continue statement inside loops
                    
for val in "string":
    if val == "i":
        continue
    print(val)
                    
print("The end")

Output

s
t
r
n
g
The end

This program is same as the above example except the statement has been replaced with .breakcontinue

We continue with the loop, if the string is i, not executing the rest of the block. Hence, we see in our output that all the letters except i gets printed.

programiz.com

pass statement

What is pass statement in Python?

In Python programming, the pass statement is a null statement. The difference between a comment and a pass statement in Python is that while the interpreter ignores a comment entirely, pass is not ignored.

However, nothing happens when the pass is executed. It results in no operation (NOP).

Syntax of pass

pass

We generally use it as a placeholder.

Suppose we have a loop or a function that is not implemented yet, but we want to implement it in the future. They cannot have an empty body. The interpreter would give an error. So, we use the pass statement to construct a body that does nothing.

Example

'''pass is just a placeholder for
functionality to be added later.'''
sequence = {'p', 'a', 's', 's'}
for val in sequence:
    pass

We can do the same thing in an empty function or class as well.

def function(args):
    pass
class Example:
    pass
programiz.com