Python is a versatile, high-level programming language known for its readability and ease of execution. As an open-source language, it is freely available to everyone. This article will explore one of Python's core looping constructs: the for loop.
Python for loops
In Python, a for loop is used to iterate over a sequence (such as a list, tuple, string, or range) and execute a block of code multiple times. It's particularly useful for performing repetitive tasks efficiently and can handle both numerical and non-numerical data. Python's for loop is simple and flexible, making it a fundamental tool for programming.
Flowchart of Python for loop
Start
|
[Initialize Sequence]
|
[First Item in Sequence] --> [Process Item] --> [Next Item in Sequence]
| |
+-------------[Last Item?]--------+
|
[End]
Syntax
for item in sequence:
# Code block to be executed
Example
numbers = [1, 2, 3, 4, 5]
for number in numbers:
print(number)
In this example, the loop iterates over each item in the numbers list and prints it to the console. The output will be:
1
2
3
4
5
How to Iterate Over a String With a for loop?
You can treat the string as a sequence of characters to iterate over a string with a for loop in Python. Each iteration of the loop will process one character from the string. Here's an example to illustrate this:
text = "Hello, World!"for char in text: print(char)text = "Hello, World!"
for char in text:
print(char)
Explanation
- Initialization: The string text is initialized with "Hello, World!".
- Iteration: The for loop iterates over each character in the string.
- Processing: The current character (char) is printed during each iteration.
Output
In this example, the loop iterates over each character in the string text and prints it. The output will be:
H
e
l
l
o
,
W
o
r
l
d
!
Elevate your coding skills with Simplilearn's Python Training! Enroll now to unlock your potential and advance your career.
Break Statement in for loop
The break statement in a for loop terminates the loop prematurely when a certain condition is met. Once the break statement is executed, the loop stops iterating, and the program control moves to the next statement after the loop.
Example
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for number in numbers:
if number == 5:
break
print(number)
print("Loop terminated when number reached:", number)
Explanation
- Initialization: The list numbers are initialized with values from 1 to 10.
- Iteration: The for loop iterates over each number in the list.
- Condition: The loop checks if the current number equals 5 during each iteration.
- Break: The break statement is executed when the number 5 is encountered, terminating the loop.
- Output: The loop stops, and the program prints the numbers from 1 to 4, followed by a message indicating that the loop was terminated when the number reached 5.
Output
1
2
3
4
Loop terminated when the number reached: 5
Continue Statement in for loop
The continue statement in a for loop is used to skip the current iteration and move to the next iteration of the loop. When the continue statement is executed, the loop doesn't terminate but rather jumps to the next iteration, bypassing any code that follows the continue statement within the loop.
Example
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for number in numbers:
if number % 2 == 0:
continue
print(number)
print("Loop skipped even numbers")
Explanation
- Initialization: The list numbers are initialized with values from 1 to 10.
- Iteration: The for loop iterates over each number in the list.
- Condition: During each iteration, the loop checks if the current number is even (i.e., if number % 2 == 0).
- Continue: If the number is even, the continue statement is executed, causing the loop to skip the current iteration and move to the next number.
- Processing: If the number is odd, it is printed to the console.
Output
1
3
5
7
9
Loop skipped even numbers
The Range() Function
The range() function in Python generates a sequence of numbers, often used to iterate over a specific set of values in a for loop. The range() function can take one, two, or three arguments: start, stop, and step.
Example
for i in range(1, 10, 2):
print(i)
Explanation
- Initialization: The range() function is called with three arguments: start = 1, stop = 10, and step = 2.
- Iteration: The for loop iterates over the sequence generated by range(1, 10, 2).
- Processing: During each iteration, the current value of i is printed.
Output
1
3
5
7
9
In this example, the range() function generates the sequence [1, 3, 5, 7, 9]. The for loop then iterates over this sequence, printing each value. The loop starts at 1 and increments by 2 each time, stopping before it reaches 10.
Python for loop Enumerate
The enumerate function in Python is used in conjunction with a for loop to iterate over a list (or other iterable) and have access to both the index and the item at that index during each iteration. This can be very useful when you need to know the position of items in a list as you process them.
Example
# Sample list
fruits = ["apple", "banana", "cherry", "date"]
# Using enumerate in a for loop
for index, fruit in enumerate(fruits):
print(f"Index {index}: {fruit}")
Explanation
In this example, enumerate(fruits) returns pairs of an index and an item from the fruits list. The for loop then iterates over these pairs, unpacking them into the variables index and fruit. The print statement outputs the index and the corresponding fruit for each iteration.
Output
Index 0: apple
Index 1: banana
Index 2: cherry
Index 3: date
Elevate your coding skills with Simplilearn's Python Training! Enroll now to unlock your potential and advance your career.
Else in for loop
In Python, the else block after a for loop is executed when the loop completes normally (i.e., it is not terminated by a break statement). This can be useful when distinguishing between a loop that completed all iterations, and one exited early.
Example
# Sample list
numbers = [1, 2, 3, 4, 5]
# Using else in a for loop
for number in numbers:
if number % 7 == 0:
print(f"Found a multiple of 7: {number}")
break
else:
print("No multiples of 7 found in the list")
Explanation
In this example, the for loop iterates over the numbers list, looking for a number that is a multiple of 7. If it finds such a number, it prints a message and breaks out of the loop. If the loop completes all iterations without finding a multiple of 7, the else block is executed.
Output
Since there are no multiples of 7 in the numbers list, the output will be:
No multiples of 7 found in the list
Nested Loops
Nested loops are loops within loops. In Python, you can nest any type of loop inside another loop.
Example
# Outer loop for rows
for i in range(1, 6):
# Inner loop for columns
for j in range(1, 6):
print(f"{i * j:2}", end=" ")
print() # Newline after each row
Explanation
In this example, the outer loop iterates over the range from 1 to 5, representing the rows of the multiplication table. The inner loop also iterates over the range from 1 to 5, representing the columns. The product of the current row and column indices (i * j) is printed, with a space (end=" ") to separate the numbers on the same row. After each inner loop completes, a new line is printed to move to the next row.
Output
The output of this code will be:
1 2 3 4 5
2 4 6 8 10
3 6 9 12 15
4 8 12 16 20
5 10 15 20 25
Pass Statement
The pass statement in Python is a null operation; it does nothing when executed. It's used as a placeholder for code that hasn't been written yet or to create minimalistic function bodies that do nothing.
Example
# Example of pass statement in a function
def placeholder_function():
pass
# Example of pass statement in a loop
for i in range(5):
if i % 2 == 0:
pass # Placeholder for future code
else:
print(f"Odd number: {i}")
Explanation
In this example:
- placeholder_function is defined but does nothing because of the pass statement.
- In the for loop, the pass statement is used as a placeholder for future code in the if block that checks if i is an even number. For odd numbers, it prints the number.
Output
Odd number: 1
Odd number: 3
Odd number: 5
Python for loop with Dictionary
In Python, you can use a for loop to iterate over the keys and values of a dictionary. The items method of a dictionary returns a view object that displays a list of a dictionary's key-value tuple pairs, which can be used in a for loop.
Example
# Sample dictionary
student_grades = {
"Alice": 85,
"Bob": 90,
"Charlie": 78,
"Diana": 92
}
# Using a for loop to iterate over dictionary items
for student, grade in student_grades.items():
print(f"{student}: {grade}")
Explanation
In this example, student_grades.items() returns pairs of keys (student names) and values (grades). The for loop iterates over these pairs, unpacking them into the variables student and grade. The print statement outputs the student's name and their grade for each iteration.
Output
Alice: 85
Bob: 90
Charlie: 78
Diana: 92
Python for loop with Tuple
You can use a for loop to iterate over a list of tuples in Python. During each iteration, you can unpack the tuple into individual variables.
Example
# List of tuples
students = [("Alice", 90), ("Bob", 85), ("Charlie", 92), ("David", 88)]
# Using a for loop to iterate over the list of tuples
for name, grade in students:
print(f"{name} scored {grade}")
Explanation
In this example, students is a list of tuples, each containing a student's name and grade. The for loop iterates over each tuple in the list, unpacking it into the variables name and grade. The print statement then outputs the student's name and corresponding grade for each iteration.
Output
Alice scored 90
Bob scored 85
Charlie scored 92
David scored 88
Python for loop with Zip()
The zip function in Python allows you to iterate over multiple iterables (like lists, tuples, etc.) in parallel, combining their elements into tuples. This can be very useful when you need to process multiple sequences in tandem.
Example
# Sample lists
names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]
# Using zip in a for loop
for name, age in zip(names, ages):
print(f"{name} is {age} years old.")
Explanation
In this example, zip(names, ages) pairs up elements from the names and ages lists, creating tuples. The for loop then iterates over these tuples, unpacking them into the variables name and age. The print statement outputs the name and corresponding age for each iteration.
Output
Alice is 25 years old.
Bob is 30 years old.
Charlie is 35 years old.
Conclusion
In this tutorial, we've explored Python's powerful and versatile `for` loop, learning how to use it effectively with functions like `enumerate` and `zip` to iterate over lists and other iterables. By mastering these techniques, you can write more efficient and readable code, making your programming tasks easier and more enjoyable.
Consider enrolling in our Python Training Course to enhance your Python skills further and dive deeper into advanced topics. It is designed to provide you with a solid foundation in Python programming, with expert-led sessions and hands-on projects to reinforce your learning.
FAQs
1. How are while loops and for loops different in python?
In Python, for loops are used to iterate over a sequence (like a list, tuple, or string) directly, accessing each element in the sequence. while loops, on the other hand, repeatedly execute a block of code as long as a specified condition is True. for loops are generally preferred for iterating over known sequences, while while loops are better for situations where the number of iterations isn't predetermined and depends on a condition being met.
2. How to run two for loops simultaneously in python?
To run two for loops simultaneously, you can use the zip function to pair elements from two iterables. Here's an example:
list1 = [1, 2, 3]
list2 = ['a', 'b', 'c']
for num, char in zip(list1, list2):
print(num, char)
This will output:
1 a
2 b
3 c
3. How to exit out of multiple for loops in python?
To exit out of multiple nested for loops, you can use a flag variable or raise an exception. Here's an example using a flag:
flag = False
for i in range(5):
for j in range(5):
if some_condition:
flag = True
break
if flag:
break
Alternatively, you can use a custom exception:
class ExitLoop(Exception): pass
try:
for i in range(5):
for j in range(5):
if some_condition:
raise ExitLoop
except ExitLoop:
pass
4. How to avoid nested for loops in python?
To avoid nested for loops, you can use built-in functions like itertools.product for Cartesian products, list comprehensions, or map and filter functions for better readability and performance. Here's an example using itertools.product:
import itertools
list1 = [1, 2, 3]
list2 = ['a', 'b', 'c']
for num, char in itertools.product(list1, list2):
print(num, char)
