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Flow control

Looping through a List

Looping through a list means iterating over each element in the list one by one. This is commonly done using a for loop, which allows you to access and manipulate each item in the list during each iteration.

1. Using a for loop to iterate over a List:

You can loop through a list directly using a for loop:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

  • The list fruits contains three items: apple, banana, and cherry.
  • The for loop iterates over the list, and for each iteration, the variable fruit takes the value of the current item in the list.
  • The print(fruit) statement outputs the current item during each iteration.

Output:

apple
banana
cherry


2. Looping through a list using range() and index:

You can also loop through the list by using range() and the length of the list to access items by their index.

Example:

fruits = ["apple", "banana", "cherry"]
for i in range(len(fruits)):
    print(fruits[I])

  • range(len(fruits)) generates a sequence of numbers from 0 to len(fruits) - 1 (i.e., the indices of the list).
  • The loop variable i takes these index values, and fruits[i] accesses the corresponding item in the list.

Output:

apple
banana
cherry


3. Using a while loop to iterate over a List:

You can use a while loop to iterate through a list by keeping track of the index manually.

Example:

fruits = ["apple", "banana", "cherry"]
i = 0
while i < len(fruits):
    print(fruits[i])
    i += 1

  • The loop continues as long as i is less than the length of the list.
  • Each iteration prints the element at index i and increments i by 1 to move to the next item.

Output:

apple
banana
cherry


4. break and continue in a List loop:

You can use the break and continue statements to control the flow of the loop:

- break: Exits the loop early.
- continue: Skips the current iteration and moves to the next one.

Example using break:

fruits = ["apple", "banana", "cherry", "orange"]
for fruit in fruits:
    if fruit == "cherry":
        break  # Stop the loop when fruit is "cherry"
    print(fruit)

Output:

apple
banana


Example using continue:

fruits = ["apple", "banana", "cherry", "orange"]
for fruit in fruits:
    if fruit == "banana":
        continue  # Skip the current iteration when fruit is "banana"
    print(fruit)

Output:

apple
cherry
orange


5. Looping through a list with else:

An optional else clause can be added to a loop. The else block will execute once the loop finishes, unless it is stopped by a break statement.

Example:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
else:
    print("Finished looping through the list.")

Output:

apple
banana
cherry
Finished looping through the list.

However, if the loop is terminated with a break, the else block will not be executed.

Example with break:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    if fruit == "banana":
        break
    print(fruit)
else:
    print("Finished looping through the list.")

Output:

apple


6. Use cases:

- You can use a loop to calculate the sum of all items in a list of numbers:

numbers = [1, 2, 3, 4, 5]
total = 0
for num in numbers:
    total += num
print("Sum:", total)

Output:

Sum: 15


- You can also modify items in a list while looping through it by using the index:

numbers = [1, 2, 3, 4, 5]
for i in range(len(numbers)):
    numbers[i] = numbers[i] * 2
print(numbers)

Output:

[2, 4, 6, 8, 10]


7. When to use looping through Lists:

- Accessing each item in a list.
- Searching for a specific value in a list.
- Modifying or updating items in a list.
- Aggregating data (e.g., finding the sum, product, or average).
- Filtering items (e.g., removing items that meet a certain condition).

It's time to take a quiz!

Test your knowledge and see what you've just learned.

What will be the output of the following code? fruits = ["apple", "banana", "cherry"] for fruit in fruits: print(fruit)

apple banana cherry
apple, banana, cherry
Apple Banana Cherry
Apple Banana Cherry
Check Answer

What will be the output of the following code? fruits = ["apple", "banana", "cherry"] for i in range(len(fruits)): print(fruits[i])

Apple Banana Cherry
apple banana cherry
apple, banana, cherry
Apple Banana Cherry
Check Answer

What will be the output of the following code? fruits = ["apple", "banana", "cherry"] i = 0 while i < len(fruits): print(fruits[i]) i += 1

apple banana cherry
apple, banana, cherry
Apple Banana Cherry
Apple Banana Cherry
Check Answer

What will be the output of the following code? fruits = ["apple", "banana", "cherry", "orange"] for fruit in fruits: if fruit == "cherry": break print(fruit)

banana cherry
apple banana
apple, banana
apple banana cherry
Check Answer

What will be the output of the following code? fruits = ["apple", "banana", "cherry", "orange"] for fruit in fruits: if fruit == "banana": continue print(fruit)

apple cherry orange
apple banana cherry orange
apple cherry
banana orange
Check Answer

What will be the output of the following code? fruits = ["apple", "banana", "cherry"] for fruit in fruits: print(fruit) else: print("Finished looping through the list.")

Finished looping through the list.
apple banana cherry Stopped early
apple banana cherry Finished looping through the list.
apple banana cherry
Check Answer

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