Nested for Loop in One Line in Python

1. Nested for Loop in One Line Using List Comprehension in Python
2. Nested for Loop in One Line Using the exec() Function in Python
3. Conclusion

The for loop is one of the most commonly used loops to iterate items from a list. In Python, we write the for loop in one line, but how can we write it in one line when we have to use another loop inside it?

This tutorial will discuss the ways that can be used to write a nested for loop in just one line.

Nested for Loop in One Line Using List Comprehension in Python

One way of writing nested for loop in one line is utilizing the list comprehension. List comprehension is a concise and expressive way to create lists in Python, and it allows us to create a new list by specifying its elements using a single line of code.

Basic Syntax:

result = [
    expression
    for outer_loop_variable in outer_iterable
    for inner_loop_variable in inner_iterable
]

In the syntax, the expression is the value to be included in the new list, and the comprehension iterates over combinations of elements from the outer and inner iterables. You can also include an optional if statement for filtering.

Let’s go through an example using list comprehension. First of all, we will create two lists, listOne and listTwo, and then use them to print the result as shown below.

Example Code 1:

listOne = [1, 3, 5]
listTwo = [2, 4, 6]

[print(a + b) for a in listOne for b in listTwo]

Output:

3
5
7
5
7
9
7
9
11

In the code above, we have two lists, listOne and listTwo, with elements [1, 3, 5] and [2, 4, 6], respectively. We utilize a list comprehension to iterate through all possible combinations of elements from these two lists, denoted by the nested for loops.

For each pair of elements, represented by 'a' from listOne and 'b' from listTwo, we calculate their sum using the expression (a + b) and print the sum of each combination using the print() statement.

Let’s have another example where we nest a for loop with a condition.

Example Code 2:

listOne = [1, 3, 5]
listTwo = [2, 4, 6]

result = [(a + b) for a in listOne for b in listTwo if a != b]
print(result)

Output:

[3, 5, 7, 5, 7, 9, 7, 9, 11]

In the code above, we initialize two lists, listOne and listTwo. We then employ a list comprehension with nested for loops to iterate through all possible combinations of elements from the two lists.

Next, the condition if a != b ensures that we exclude pairs where the elements from both lists are equal. For each valid pair, we calculate the sum using the expression a + b.

The resulting list, named result, stores these sums. Finally, we print the contents of the result list.

The output, [3, 5, 7, 9, 7, 9, 11], reflects the sums of distinct pairs, excluding cases where elements from both lists are the same.

List comprehensions are preferred for writing nested for loops in Python due to their concise and expressive syntax, which simplifies code structure and enhances readability, making it easier to write and understand complex nested iterations in a single line.

Nested for Loop in One Line Using the exec() Function in Python

Now, we will discuss another function that can help us achieve a nested for loop in one line, the exec() function. The exec() function in Python is used for the dynamic execution of Python programs that are stored in strings or code objects, and It allows us to execute dynamically generated Python code.

Basic Syntax:

exec(object)

In the syntax, the object is a string containing a Python program or a code object.

Now, let’s go through an example in which we will add two lists and print the sum as shown below.

Example Code 1:

listOne = [1, 3, 5]
listTwo = [2, 4, 6]

exec("for a in listOne:\n    for b in listTwo:\n      print(a+b)")

Output:

3
5
7
5
7
9
7
9
11

In the code above, we have two lists, listOne and listTwo. We use the exec() function to dynamically execute a nested for loop, iterating through all combinations of elements from listOne and listTwo.

Inside the loop, we print the sum of each pair (a, b) using the print(a + b) statement.

Now, let’s have another example using the .join() string method that concatenates (joins) the elements of an iterable, such as a list(listOne and listTwo), using a specified string as the separator ("\n").

Example Code 2:

listOne = [1, 3, 5]
listTwo = [2, 4, 6]

expression = "\n".join(
    f"a = {a}; b = {b}; print(a + b)" for a in listOne for b in listTwo
)
exec(expression)

Output:

3
5
7
5
7
9
7
9
11

In the above, we start with two lists, listOne and listTwo. Using a list comprehension, we create a string expression by joining together formatted print statements for the sum of each pair (a, b) from the Cartesian product of the two lists.

Next, the expression is built using a generator expression with the f-string syntax to incorporate the values of a and b. The "\n".join() method is then applied to concatenate these formatted print statements, separated by newline characters.

Then, we use the exec() function to dynamically execute the generated string expression. During execution, each formatted print statement inside the string is evaluated, leading to the output displaying the sums of all possible combinations.

However, using exec() in this way is generally not recommended for readability and security reasons. Using a list comprehension is usually a better approach.

Conclusion

In conclusion, the article demonstrates two methods for achieving nested for loops in one line in Python. It first emphasizes the use of list comprehension, highlighting its concise and expressive syntax as the preferred approach for readability.

The second method involves employing the exec() function, showcased with dynamic execution of nested loops. However, the article advises caution in using exec() regularly due to readability and security concerns, reaffirming the preference for list comprehensions in most cases.