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Python For loop

In Python, the for loop is used to iterate over a sequence of elements. This sequence can be a list, tuple, string, dictionary, or any other iterable object. The basic syntax of a for loop in Python is as follows:

for item in iterable:
    # code block to be executed for each item

Here’s a breakdown of the components:

• item: A variable that takes on the value of each element in the iterable, one at a time, during each iteration of the loop.
• iterable: A collection of items to iterate.

Example 1: Looping over a List

days_list = ["Sun", "Mon", "Tue", "Wed", "Thur", "Fri", "Sat"]
print("Iterating over a List.")
for day in days_list:
    print(day, end=" ")

# Output
Iterating over a List.
Sun Mon Tue Wed Thur Fri Sat 

Example 2: Looping over a String

message = "Hello Proedu.co!!!"
print("Iterating over a String.")
for char in message:
    print(char, end=", ")

# Output
Iterating over a String.
H, e, l, l, o,  , P, r, o, e, d, u, ., c, o, !, !, !, 

Example 3: Looping over a Range

for num in range(5):
    print(num)

# Output
0
1
2
3
4

Example 4: Using enumerate() to Access Index and Value

days_list = ["Sun", "Mon", "Tue", "Wed", "Thur", "Fri", "Sat"]
print("Using enumerate()")
for index, day in enumerate(days_list):
    print("Index: ", index, " Day: ", day)

# Output
Using enumerate()
Index:  0  Day:  Sun
Index:  1  Day:  Mon
Index:  2  Day:  Tue
Index:  3  Day:  Wed
Index:  4  Day:  Thur
Index:  5  Day:  Fri
Index:  6  Day:  Sat

In this example, enumerate() is used to iterate over both the index and value of each element in the days list. Inside the loop, index contains the index of each element, and day contains the value of each element.

For loop control statements

In Python, for loops support the same loop control statements as while loops: break, continue, and pass. These statements provide flexibility and control over the flow of execution within a for loop.

break Statement:

The break statement is used to exit the loop prematurely. When encountered, it causes the program to immediately exit the loop and resume execution at the next statement following the loop.

days_list = ["Sun", "Mon", "Tue", "Wed", "Thur", "Fri", "Sat"]
print("break for loop example.")
for day in days_list:
    if day == "Wed":
        break # breaking loop on "Wed"
    print("Day: ", day)

# Output
break for loop example.
Day:  Sun
Day:  Mon
Day:  Tue

continue Statement

The continue statement is used to skip the rest of the code block within the loop for the current iteration, and the loop continues with the next iteration.

days_list = ["Sun", "Mon", "Tue", "Wed", "Thur", "Fri", "Sat"]
print("continue for loop example.")
for day in days_list:
    if day == "Wed":
        continue # "Wed" is skipped here.
    print("Day: ", day)

# Output
continue for loop example.
Day:  Sun
Day:  Mon
Day:  Tue
Day:  Thur
Day:  Fri
Day:  Sat

pass Statement

The pass statement is a null operation; nothing happens when it executes. You can use it as a placeholder when you require a statement syntactically but you don’t want any action.

for i in range(5):
    if i == 2:
        pass  # Do nothing when i equals 2
    else:
        print(i)

# output
0
1
3
4

These loop control statements provide similar functionality in both for and while loops, allowing you to control the flow of execution within the loop based on certain conditions.

Nested For Loops

Nested for loops are used when you need to perform iterations within iterations. This means one or more for loops are placed inside another for loop. Nested for loops are handy for tasks like working with 2D arrays, generating combinations, or traversing through multi-dimensional data structures.

Here’s a simple example of a nested for loop:

for i in range(3):
    for j in range(2):
        print(f"({i},{j})")

# output
(0,0)
(0,1)
(1,0)
(1,1)
(2,0)
(2,1)

In this example, we have two nested for loops. The outer loop iterates over the values 0, 1, and 2 generated by range(3). For each iteration of the outer loop, the inner loop iterates over the values 0 and 1 generated by range(2). So, for each value of i, the inner loop runs twice, generating all combinations of (i, j).

Nested loops can go deeper with more levels of nesting, allowing you to work with higher-dimensional data structures or perform more complex iterations. However, be cautious with deep nesting as it can make code harder to read and understand.

Here’s another example that uses nested loops to print a pattern:

for i in range(5):
    for j in range(i + 1):
        print("*", end=" ")
    print()

# Output
* 
* * 
* * * 
* * * * 
* * * * * 

In this example, the outer loop controls the number of rows, and the inner loop controls the number of stars printed in each row. As the outer loop iterates from 0 to 4, the inner loop iterates from 0 to i, where i is the current value of the outer loop. This creates a pattern of increasing stars in each row.

Lets take another example to print another triangle pattern.

size = 5
for i in range(size):
    for j in range(size - i):
        print("*", end=" ")
    print()

# output
* * * * * 
* * * * 
* * * 
* * 
* 

Else in for loop

In Python, you can also use the else statement in conjunction with a for loop. The else block associated with a for loop is executed when the loop completes all its iterations without encountering a break statement. Here’s how you can use it:

for item in iterable:
    # code block to be executed for each item
else:
    # code block to be executed once after the loop ends.

Here is an example

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
else:
    print("All fruits have been printed")

# Output
apple
banana
cherry
All fruits have been printed

In this example, after iterating over all the elements in the fruits list, the loop completes its iterations without encountering a break statement. Therefore, the else block is executed, printing “All fruits have been printed”.

The else block associated with a for loop won’t be executed if the loop is terminated prematurely using a break statement. It only executes when the loop completes all its iterations normally.