Objective

In this unit, we dive into the concept of loops in Python: "while" and "for" loops. The objective of this unit is to understand how these loops function, learn about their control statements such as "break" and "continue", and apply them effectively in a project.

while Loops

while loops in Python allow for repetitive execution of a block of code as long as the provided condition holds true. This form of loop is frequently used when the number of iterations is unknown or dependent on a specific condition.

count = 0
while count < 5:
    print(count)
    count += 1 

In the above example, the code inside the loop executes as long as the count variable is less than 5. Once count becomes 5, the loop terminates.

Note: In this code we used the += the operator. It lets you add a value to a variable and assign the result back to the variable in a single step. It is essentially a shorthand way of writing:

count = count + 1

for Loop

for loops in Python are primarily used to iterate over a sequence, such as a list, tuple, dictionary, set, or string, or other iterable objects. Unlike while loops, for loops are typically used when the number of iterations is known.

Here's the syntax of a for loop:

for variable in sequence:
    # loop body

• variable is the variable that the loop will assign each element in the sequence to, one at a time, as it iterates over the sequence.
• sequence is the object that the loop will iterate over.

Let's look at a simple example:

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

In this example, range(5) creates a sequence of numbers from 0 to 4. The for loop then iterates over this sequence. During each iteration, the loop assigns the current number to the variable i and then executes the code within the loop (which in this case, is simply print(i)). So, it will print each number from 0 to 4, one at a time.

Here's what happens in each iteration:

1. In the first iteration, i is 0.
2. In the second iteration, i is 1.
3. In the third iteration, i is 2.
4. In the fourth iteration, i is 3.
5. In the fifth and final iteration, i is 4.

The loop stops after it has iterated over all items in the sequence (or if a break statement is encountered).

The variable name i is a convention, especially for loops that deal with counting or indexing, but any variable name can be used.

Loop Control: break and continue

Python provides two powerful keywords: break and continue, which control the flow of while and for loops:

The break statement terminates the current loop and resumes execution at the next statement outside the loop. It's typically used when you need to exit the loop prematurely when a certain condition is met. Here's an example:

for i in range(10):
    if i == 5:
        break
    print(i)

In the above code, the for loop is designed to print numbers from 0 to 9. However, when i equals 5, the break statement is encountered, and the loop terminates, so the numbers printed are only from 0 to 4.

The continue statement rejects all the remaining statements in the current iteration and moves the control back to the top of the loop. It's typically used when you want to skip the current iteration and proceed with the next without terminating the loop. Here's an example:

for i in range(10):
    if i == 5:
        continue
    print(i)

In this example, the for loop is also set to print numbers from 0 to 9. However, when i equals 5, the continue statement is encountered, so it skips the print(i) statement for i equals 5. Hence, all the numbers from 0 to 9 are printed except for 5.

These control statements, break and continue, can be effectively used to add advanced control and logic within loops.

Project: Draw a Pattern of Shapes Using For and While Loops

In this project, we will be utilizing the power of while and for loops along with the Turtle graphics library to draw a pattern of shapes. The challenge here is to apply both types of loops creatively.

import turtle

# Create a new turtle screen and set its background color
screen = turtle.Screen()
screen.bgcolor("white")

# Initialize a turtle object
t = turtle.Turtle()

num_sides = 5
angle = 360 / num_sides
distance = 100

# For loop to draw multiple shapes
for _ in range(num_sides):
    # While loop to draw one shape
    count = 0
    while count < num_sides:
        t.forward(distance)
        t.right(angle)
        count += 1
    t.right(angle)
    
# Wait until the window is closed
turtle.done()

In the project code above, we use a for loop to draw multiple shapes, and within each iteration, we use a while loop to draw each individual shape. The result is a captivating pattern of 5 pentagons.

Try changing the value of num_sides to a different value to see what other shapes can be drawn.

In the next unit, we'll look at functions and explore how we can define reusable blocks of code.