Python Course: for loop

In this course, we will take a closer look at one of the important control structures in Python, the for loop. The for loop is primarily used to iterate over a specific range of values. In Python, the for loop is useful for traversing sequences (lists, tuples, strings, etc.) and helps to efficiently handle repetitive tasks.

1. Basic Structure of for Loop

The for loop in Python is designed to repeatedly execute a specific block of code for each element in a sequence. The basic structure is as follows:

for variable in sequence:
    code to execute

Here, “variable” will receive one element of the sequence in each iteration, and “code to execute” contains the actual tasks to perform.

2. Example: Iterating Over a List

Let’s take a look at the most basic example of iterating over a list.

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

This code will print each fruit name from the fruits list. The output will be as follows:

apple
banana
cherry

3. Using for Loop with range() Function

The range() function is mainly used with the for loop to repeat code a specified number of times. It has the syntax range(start, stop[, step]), and the parameters are as follows:

  • start: The number where the count begins. The default is 0.
  • stop: The number where the count ends (not included).
  • step: The interval between counts. The default is 1.

Let’s understand this with an example:

for i in range(3):
    print(i)

Output:

0
1
2

4. Nested for Loops

A nested for loop simply means having another for loop inside a for loop. Here is an example of a nested for loop that iterates over a two-dimensional list:

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for row in matrix:
    for num in row:
        print(num)

This code will print all the numbers in the 2D list.

5. for Loop and else

Surprisingly, in Python, the for loop can be used with else. You can write code in the else block that will execute when the for loop has completed normally. Let’s see the following example:

for fruit in fruits:
    if fruit == 'banana':
        print('Banana found!')
        break
else:
    print('No bananas.')

In this case, ‘Banana found!’ will be printed, demonstrating how the for-else construct works.

6. Using with enumerate()

The enumerate() function helps to deal with both index and elements simultaneously when iterating through a sequence. Let’s take a closer look with the following example:

for index, fruit in enumerate(fruits):
    print(index, fruit)

Output:

0 apple
1 banana
2 cherry

7. Using Break to Avoid Infinite Loops

If you want to stop the for loop under certain conditions, you can use the break statement. This helps avoid infinite loops or unnecessary iterations.

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

Output:

0
1
2
3
4

8. Performance Considerations

When using multiple loops, performance should be considered. Especially with large datasets, nested loops can adversely affect performance. Here are some tips to minimize this:

  • Use list comprehensions when possible.
  • Utilize ‘break’ and ‘continue’ statements appropriately to avoid unnecessary iterations.

9. for Loop and List Comprehensions

List comprehensions are a way to create lists more concisely. They can be shorter than loops and sometimes faster:

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

Output:

[1, 4, 9, 16, 25]

Conclusion

The for loop in Python is an essential tool for traversing and processing various data structures and greatly aids in writing efficient code. Understanding the various uses of the for loop and applying them to actual code is important for writing effective code.