For Loop in Python

  flowchart TD
  A["for x in items:"] --> B{"how did the loop end?"}
  B -->|"ran out of items"| C["else runs"]
  B -->|"items was empty"| C
  B -->|"break"| D["else is SKIPPED"]
  B -->|"return / exception"| E["leaves the function entirely"]
  C --> F["the search found nothing -- handle the miss here"]
  D --> G["the search found something -- handled at the break"]
  H["the same rule applies to while ... else"] -.-> B

Unveiling the Power of for Loops in Python

In Python, the for loop is a versatile and fundamental construct that facilitates the iteration over a sequence of elements. Whether you're working with lists, strings, or other iterable objects, the for loop simplifies the process of executing a set of statements for each item in the sequence. In this comprehensive guide, we'll explore the syntax, functionality, and best practices associated with for loops in Python.

What Is a for Loop?

A for loop is a Python statement which repeats a group of statements a specified number of times. You can iterate over any object that is iterable, such as a list, tuple, string, and so on.

In Python, you can use the for loop in two ways:

• Iterating over a sequence: We can use a for loop to iterate over a sequence. The sequence can be a list, tuple, string, or any other iterable object.
• Iterating over a range: In Python, we can also use a for loop to iterate over a range. Using a for loop with a range() function, we can execute a set of statements for each item in a range.

Syntax of for Loop

The syntax of the for loop in Python is:

for val in sequence:
    Body of for

Here, val is the variable that takes the value of the item inside the sequence on each iteration.

Loop continues until we reach the last item in the sequence. The body of for loop is separated from the rest of the code using indentation.

Structure of for Loop

• Initialization: In this step, we initialize the variable val with the first item in the sequence. This is the first item in the sequence.

  PythonCopy

  for val in sequence:

  Here, val is the variable that takes the value of the item inside the sequence on each iteration.

• Condition: In this step, the val variable is checked to be less than or equal to the last item in the sequence. If it is true, the body of the for loop is executed. Otherwise, the loop terminates.

  PythonCopy

  for val in sequence:

  Here, val is the variable that takes the value of the item inside the sequence on each iteration.

• Body: The body of the for loop is separated from the rest of the code using indentation.

  PythonCopy

  for val in sequence:
      Body of for

  Here, Body of for is the body of the loop. It can contain one or more statements. It is executed once for each item in the sequence.

• Increment/Decrement: In this step, the variable val is incremented or decremented. The next item in the sequence is assigned to the variable val. This continues until the last item in the sequence is reached.

  PythonCopy

  for val in sequence:
      Body of for

  Here, Loop continues until we reach the last item in the sequence. The body of for loop is separated from the rest of the code using indentation.

Example: Python for Loop

# for loop
numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]
sum = 0
for val in numbers:
    sum = sum+val
print("The sum is", sum)

Output:

C:\Users\Your Name> python for_loop.py
The sum is 48

In this program, we have used the for loop to iterate over a list and calculate the sum of numbers. We initialize the sum variable to zero and iterate over each element of the list using a for loop and add it to the sum variable. Finally, we print the sum variable which contains the sum of numbers in the given list.

range function

range() function

The range() function is used to generate a sequence of numbers. It returns an object that produces a sequence of integers from start (inclusive) to stop (exclusive) by step. range(i, j) produces i, i+1, i+2, ..., j-1. Start defaults to 0, and stop is omitted! range(4) produces 0, 1, 2, 3. These are exactly the valid indices for a list of 4 elements. When step is given, it specifies the increment (or decrement). For example, range(1, 5) produces 1, 2, 3, 4. range(6, 1, -1) produces 6, 5, 4, 3, 2.

Syntax of range()

The syntax of the range() function is:

range(start, stop[, step])

• range() takes three arguments.
• start is the starting number of the sequence. It defaults to 0 if not specified.
• stop is the last number of the sequence. It does not include this number. The range() function goes up to stop - 1.
• step is the difference between each number in the sequence. It defaults to 1 if not specified.
• The range() function only works with the integers. Float is not allowed. If you need a range of floats, you can use the numpy library.
• It returns a range object. You need to convert it to a list to see the sequence of numbers.

Example: range() function

# range() function
print(range(10))
print(list(range(10)))
print(list(range(2, 8)))
print(list(range(2, 20, 3)))

Output:

C:\Users\Your Name> python range_function.py
range(0, 10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[2, 3, 4, 5, 6, 7]
[2, 5, 8, 11, 14, 17]

In this program, we have used the range() function to generate a sequence of numbers. We have passed one, two, and three arguments to the range() function. When we pass only one argument, it starts generating numbers from 0 and stops before the specified number. When we pass two arguments, it starts from the first argument and stops before the second argument. The third argument is the step size. The range() function increments the number by the step size until it reaches the second argument. The third argument is optional. If we do not specify the step size, it increments the number by 1.

In the above example, we have converted the range object to a list using the list() function. The list() function converts the range object to a list.

For Loop with else

For loops also have an else clause which most of us are unfamiliar with. The else clause executes after the loop completes normally. This means that the loop did not encounter any break statement. The else clause won't execute if the loop breaks prematurely.

Syntax of for loop with else

The syntax of the for loop with the else clause is:

for val in sequence:
    Body of for
else:
    Body of else

The else clause is only executed when your for loop ends normally. This means that the loop didn't encounter any break or return statement.

Example of for loop with else

# for loop with else
digits = [0, 1, 5]
for i in digits:
    print(i)
else:
    print("No items left.")

Output:

C:\Users\Your Name> python for_loop_else.py
0
1
5
No items left.

In this program, we have used the for loop with the else clause. The else clause executes after the completion of the for loop. The else clause will be executed even if there is a continue statement inside the for loop (but not if break is encountered).

Iterating by Sequence Index

In Python, we can also access the sequence items using the index. The index starts from 0 in Python. We can use the index to iterate over a sequence. Consider the following example:

# iterating by index
genre = ['pop', 'rock', 'jazz']
for i in range(len(genre)):
    print("I like", genre[i])

Output:

C:\Users\Your Name> python for_loop_index.py
I like pop
I like rock
I like jazz

In this program, we have used the len() function to get the size of the list. The range() function then generates a sequence of numbers which is used to iterate over the list using the for loop. We use the len() function in the range() function so that the for loop runs the same number of times as the number of items in the list. We use the i index to access each element in the list genre.

Iterating by Range Function

We can use the Python range() function to iterate over a sequence of numbers. It can be combined with the len() function to iterate over a sequence using indexing. Here is an example:

# iterating by range()
for i in range(0, 10):
    print(i)

Output:

C:\Users\Your Name> python for_loop_range.py
0
1
2
3
4
5
6
7
8
9

In this program, we have used the range() function to iterate over a sequence of numbers. We can also specify the start, stop, and step size as range(start, stop, step size). We have used 0 as the start, 10 as the stop, and 1 as the step size in the range() function. It returns a sequence of numbers starting from 0 to 9 (10 numbers).

For Loop Through a String

Even strings are iterable objects, they contain a sequence of characters:

# for loop through a string
genre = 'Rock'
for i in range(len(genre)):
    print("I like", genre[i])

Output:

C:\Users\Your Name> python for_loop_string.py
I like R
I like o
I like c
I like k

In this program, we have used the len() function to get the size of the string. The range() function then generates a sequence of numbers which is used to iterate over the string using the for loop. We use the len() function in the range() function so that the for loop runs the same number of times as the number of characters in the string. We use the i index to access each character in the string genre.

You can also iterate over a string using a for loop without the range() function. For example:

# for loop through a string
genre = 'Rock'
for i in genre:
    print("I like", i)

Output:

C:\Users\Your Name> python for_loop_string.py
I like R
I like o
I like c
I like k

In this program, we have used the for loop to iterate over the string genre. We have not used the range() function. We have directly accessed each character using the i index in the for loop.

You can also use the for loop to iterate over a sequence of lists. Consider the following example:

# for loop with else
digits = [0, 1, 5]
for i in digits:
    print(i)

Output:

C:\Users\Your Name> python for_loop_list.py
0
1
5

In this program, we have used the for loop to iterate over a list digits. We have not used the range() function. We have directly accessed each element using the i index in the for loop.

Nested for Loop

In Python, a nested for loop is a loop inside a loop. The "inner loop" will be executed one time for each iteration of the "outer loop":

Syntax of Nested for Loop

for [first iterating variable] in [outer loop]: # Outer loop
    [do something]  # Optional
    for [second iterating variable] in [nested loop]:   # Nested loop
        [do something] # Optional

Example of Nested for Loop

# nested for loop
for i in range(1, 6):
    for j in range(i):
        print(i, end=' ')
    print()

Output:

C:\Users\Your Name> python nested_for_loop.py
1
2 2
3 3 3
4 4 4 4
5 5 5 5 5

In this program, we have used the nested for loop to iterate over a sequence of numbers. We have used the range() function to generate a sequence of numbers. We have used the i index to print the number of stars in each row. We have used the j index to print the stars in each row. The j index depends on the i index. The value of the i index is used to determine the number of stars in each row. The value of the j index is used to print the stars in each row. The value of the j index is incremented by 1 in each iteration of the inner for loop. The inner for loop is executed i times for each iteration of the outer for loop. The outer for loop is executed 5 times. The inner for loop is executed 1 time in the first iteration of the outer for loop. The inner for loop is executed 2 times in the second iteration of the outer for loop. The inner for loop is executed 3 times in the third iteration of the outer for loop. The inner for loop is executed 4 times in the fourth iteration of the outer for loop. The inner for loop is executed 5 times in the fifth iteration of the outer for loop. The inner for loop is executed 1 + 2 + 3 + 4 + 5 = 15 times.

Reverse Iteration/Loop

In Python, we can also iterate over a sequence in reverse, i.e., from the last item to the first item. We can do this by using the negative index. Consider the following example:

# reverse iteration
for val in range(10, 0, -1):
    print(val)

Output:

C:\Users\Your Name> python for_loop_reverse.py
10
9
8
7
6
5
4
3
2
1

In this program, we have used the range() function to generate a sequence of numbers. We have used the negative index to iterate over the sequence in reverse. We have used -1 as the step size in the range() function. It returns a sequence of numbers starting from 10 to 1 (10 numbers).

Visualize it

A for loop is really a conversation with an iterator: Python keeps asking the sequence for its next value until there are none left, and only then runs the optional else block.

  flowchart TD
  A([Start]) --> B["Get iterator from the sequence"]
  B --> C{"More items?"}
  C -- yes --> D["item = next value"]
  D --> E["Run the loop body"]
  E --> C
  C -- no --> F["Run else block (if present)"]
  F --> G([Done])

And here is the loop variable actually moving — one visit per iteration, exactly what for item in seq does under the hood:

Conclusion

In this tutorial, we have learned about the for loop in Python. We have learned how to use the for loop to iterate over a sequence of elements. We have also learned how to use the range() function to iterate over a sequence of numbers. We have also learned how to use the for loop to iterate over a string and a list. We have also learned how to use the nested for loop to iterate over a sequence of numbers. We have also learned how to use the else clause with the for loop.

As you delve deeper into Python programming, experiment with different for loops, explore their applications in real-world scenarios, and use them to enhance the efficiency and clarity of your code. For more hands-on examples and in-depth tutorials, explore our resources on Python Central Hub!

Check yourself

Try it: For Loop Exercises

Exercise 1 – Loop over a List

Press Run to see what your code prints.

Exercise 2 – Using range()

Press Run to see what your code prints.

Exercise 3 – Nested For Loop

Press Run to see what your code prints.