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Loops in Python

For and While Loops in Python

There are two types of loops in Python: for loops and while loops.

The for loop is used for iterating over a sequence (that is either a list, a tuple, a set, a dictionary, a string or any other iterable object) and executing a block of code for each element in the sequence. The while loop is used for executing a block of code repeatedly as long as a certain condition is true. In the while loop, the condition is checked at the beginning of each iteration, and the loop is exited when the condition becomes false. It is important to make sure that the condition eventually becomes false, otherwise, the loop will continue running indefinitely, which is called an infinite loop.

for Loop Syntax in Python

An example how to use the for for a looping through a list in Python:

cars = ["bmw", "audi", "renault"]
for car in cars:
    print(car)

In this example, we have a list of cars, and we want to print each car on a new line. We use the for loop to iterate over each element in the list, and for each element, we print it to the console using the print function.

Inline for Loop

In this example, we have a list of numbers, and we want to create a new list that contains the square of each number. Instead of using a traditional for loop, we use a list comprehension, which is a more concise way of achieving the same result.

numbers = [1, 2, 3, 4, 5]
squares = [x**2 for x in numbers]
print(squares)

In this case, the expression is x**2, which is the square of the current number in the iteration, and the iterable is the numbers list. The result of the list comprehension is a new list that contains the square of each number: [1, 4, 9, 16, 25].

for Loop with Index

In this example, the loop iterates over the cars list and prints the index and value of each element:

cars = ["bmw", "audi", "renault"]
for i in range(len(cars)):
    print(i, car[i])

Power Function in Python Using a Loop

def power(a, b):
    pow = 1
    for i in range(b):
        pow *= a
    return pow

a = 2
b = 3
print(power(a, b))

while Loops in Python

Here is an example of the while loop that counts from 1 to 10:

count = 1
while count <= 10:
    print(count)
    count += 1

In this example, the loop starts with count equal to 1. The condition count <= 10 is true, so the code within the loop is executed. This code prints the value of count (which is currently 1) and then increments count by 1 using the += operator. The loop then checks the condition again, which is still true because count is now 2. This process repeats until count reaches 11, at which point the condition becomes false and the loop terminates.

Note: Python does not have a built-in do-while loop like some other programming languages. However, you can achieve similar functionality using a combination of a while loop and an initial check.

break and continue Statements

In Python, break and continue are reserved keywords used to modify the behavior of loops. They allow you to control when a loop should terminate or skip over certain iterations based on a condition.

Example: How to break a for Loop

numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num == 3:
        # end for loop
        break
    print(num)

In this example, the loop iterates over the numbers list and prints each number. However, when the value of num is equal to 3, the break statement end for loop. The output would be: 1 2.

How to Use continue with a for Loop

The continue, on the other hand, is used to skip loop iteration based on a condition. When encountered, it causes the current iteration of the loop to end and proceeds with the next iteration. Here's an example of how to skip an iteration in the for loop in Python:

numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num == 3:
        continue
    print(num)

In this example, the loop iterates over the numbers list and prints each number. However, when the value of num is equal to 3, the continue statement is executed, causing the current iteration of the loop to end and proceeding with the next iteration. The output would be: 1 2 4 5.

Example of continue in a while Loop

Using continue in while loop in Python is a way to skip certain iterations of the loop and proceed to the next iteration. This can be useful when you need to skip over certain values or conditions in your loop.

i = 0
while i < 10:
    i += 1
    if i % 2 == 0:
        continue
    print(i)

In this example, the loop iterates over the numbers 1 through 10. However, when the value of i is even, the continue statement is triggered, causing the loop to skip to the next iteration without executing any more code in the current iteration.

my_list = [1, 2, 3, 4, 5]
while my_list:
    val = my_list.pop()
    if val == 3:
        continue
    print(val)

In this example, the loop iterates over the values in my_list. When the value of val is equal to 3, the continue statement causes the loop to skip to the next iteration without printing the value.

How to break a while Loop

To break a while loop in Python, you can use the break statement. This statement is responsible for stopping the loop from iterating further, as soon as a certain condition gets fulfilled. The syntax for using break in a while loop is as follows:

while <condition>:
    ### statements to be executed inside the loop

    if <break-condition>:
        break

Here, <condition> is the loop's condition that initially decides whether the loop should be executed or not, and <break-condition> is an additional conditional statement that defines the condition(s) for stopping the loop. Once this condition is met, the loop is automatically terminated, and control is transferred to the next statement after the loop.

n = 1
while n <= 10:
    print(n)
    if n == 5:
        break
    n += 1
print("Loop Ended")

# Output:
# 
# 1
# 2
# 3
# 4
# 5
# Loop Ended

In this example, the while loop will print numbers from 1 to 5, and then stop as soon as n == 5. The break statement is used to achieve this, and the output shows that the loop ended after completing the execution of the desired condition.

How to Stop an Infinite Loop in Python

while True:
    value = input("Enter a number: ")
    if value == "stop":
        break
    print(int(value) ** 2)
print("Loop Ended")

# Output:
# Enter a number: 2
# 4
# Enter a number: 5
# 25
# Enter a number: 3
# 9
# Enter a number: stop
# Loop Ended

In this example, the while loop will keep asking the user to enter a number, and compute its square if the input is a number. If the input is 'stop', the loop will break, and control will be transferred to the statement after the loop.

This approach helps to avoid infinite loops in Python, and to control the flow of execution based on certain criteria.

A Count in a for Loop

To count in a for loop in Python, you can use a loop counter variable. This variable keeps track of the number of times the for loop has executed. Here are two examples:

count = 0
for i in range(10):
    count += 1
print("The loop executed", count, "times.")

In this example, we create a variable count and set its initial value to 0. We then use a for loop to iterate 10 times, incrementing the count variable by 1 each time. Finally, we print out the total number of times the loop executed.

fruits = ['apple', 'banana', 'cherry']
for i, fruit in enumerate(fruits, 1):
    print(i, fruit)

In this example, we use the built-in enumerate() function to loop over a list of fruits and their indices. We start the index at 1 by passing the second argument to enumerate(). Inside the loop, we print out the index and the fruit name.

By using a loop counter variable, you can easily keep track of the number of times a for loop has executed in Python. This can be useful for debugging, testing, and analyzing performance.

Nested Loops

Nested loops are loops that are placed inside another loop. In Python, nested loops are primarily used for iterating over arrays, matrices, and multi-dimensional lists. They are useful for solving problems that involve repeating a task for each element of a nested structure.

Nested For Loops in Python

The most common type of nested loop in Python is the nested for loop. This loop is used to iterate over elements in a nested structure using two or more for statements.

Simple Example of Nested for Loops

numbers = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

for row in numbers:
    for num in row:
        print(num)

In this example, we have a 3 by 3 matrix that is represented by a multi-dimensional list. The nested for loop iterates over the elements of the matrix and prints each number on a new line.

Nested while Loops in Python

Nested while loops in Python use one or more inner loops that repeat the same process multiple times. They are used to iterate over elements of a nested data structure until a certain condition is met.

Example of Nested while Loops

x = 1
y = 1

while x <= 5:
    y = 1
    while y <= x:
        print(y, end="")
        y += 1
    print()
    x += 1

In this example, we use two nested while loops to print a triangle of numbers. The outer while loop iterates over each row of the triangle, while the inner while loop iterates over each number in that row.

Overall, nested loops are an important concept in Python programming, and they can help you solve complex problems by iterating over nested data structures. By incorporating nested for and while loops into your code, you can write more efficient, readable and powerful programs.

How to Use a for Loop with a range Statement

The range statement in Python is a built-in function that can be used to generate a sequence of numbers. It is often used in for loops to repeat a task a certain number of times. Here are two examples of how to use the range statement in Python:

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


for i in range(0, 10, 2):
    print(i)

In the first example, the for loop is used to iterate through a range of numbers from 0 to 4. In the second example, the range function is called with three arguments: the starting number (0), the ending number (10), and the step (2). This will generate a sequence of numbers from 0 to 10 (not including 10) with a step of 2. The for loop then iterates through this sequence and prints each number.

A Reverse for Loop

for i in range(5, 0, -1):
    print(i)
# Output:
# 5
# 4
# 3
# 2
# 1
a = ['1', '2', '3', '4', '5']

for i in reversed(a):
    print(i)
# Output:
# 5
# 4
# 3
# 2
# 1

Using the range statement in Python is an efficient way to loop through a sequence of numbers and perform a task. It is a commonly used technique in programming and is easy to implement with the for loop in Python.

One Line for Loop

List comprehension makes it easier to write a for loop in one line in Python. Using a single line loop can help simplify the code and make it more concise.

squares = [x**2 for x in range(10)]
print(squares)

Output:

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
even_numbers = [num for num in range(20) if num % 2 == 0]
print(even_numbers)

# Output:
# 
# [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

In both examples, the for loop is condensed into a single line using list comprehension. The loop iterates over a range of values and applies a condition to the given range. This results in a list with the desired output.

Using a one line for loop in Python is a powerful tool that can optimize your code and make it more efficient.

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Updated: 02/22/2024 - 15:15
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