How To Combine Two For Loops In Python

Combining two for loops in Python is a common scenario that one can encounter when working on a programming project.

It can make the code more efficient and cleaner. In this tutorial, we will learn how to combine two for loops in Python, which can be helpful in various situations such as nested iterations, Cartesian products, or even flattening lists.

Step 1: Understand the basic syntax of a for loop

Before we dive into combining two for loops, let’s quickly understand the basic syntax of a for loop. A for loop in Python is used to iterate over a sequence (like a list or a tuple) and run a block of code for each element in the sequence.

Here is a basic example of a for loop:

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

This will print:

apple
banana
cherry

Step 2: Combining two for loops using nested loops

One way of combining two for loops is by using nested loops. Nested loops are loops within loops. In this approach, the inner loop will be executed for each iteration of the outer loop. Let’s take a look at an example:

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fruits = ["apple", "banana", "cherry"]
colors = ["red", "yellow", "purple"]
 
for fruit in fruits:
    for color in colors:
        print(fruit, color)

This will print:

apple red
apple yellow
apple purple
banana red
banana yellow
banana cherry
cherry red
cherry yellow
cherry purple

In this example, we have two lists, one containing fruits and the other containing colors. We use nested loops to iterate through all possible combinations of fruit and colors. This is called the Cartesian product.

Step 3: Combining two for loops using a single loop and itertools.product()

Another way to combine two for loops is using a single loop and the product() function from the itertools module. This function computes the Cartesian product of input iterables. Let’s take a look at an example using the same fruit and color lists as before:

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import itertools
 
fruits = ["apple", "banana", "cherry"]
colors = ["red", "yellow", "purple"]
 
for fruit, color in itertools.product(fruits, colors):
    print(fruit, color)

This will produce the same output as the nested loop example:

apple red
apple yellow
apple purple
banana red
banana yellow
banana cherry
cherry red
cherry yellow
cherry purple

In this version, we import the itertools module and use the product() function to iterate through all possible combinations of fruit and colors. This results in a simplification of our code, as we only need one for loop.

Step 4: Combining two for loops to flatten a list

In some cases, you may want to combine two for loops to flatten a list. For example, let’s imagine that we have a list of lists, where each sublist contains fruit names. We want to create a new list with all fruit names. Here’s one way to do it using nested loops:

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nested_list = [["apple", "banana"], ["cherry", "grape"], ["pineapple", "watermelon"]]
flat_list = []
 
for sublist in nested_list:
    for item in sublist:
        flat_list.append(item)
 
print(flat_list)

This will print:

['apple', 'banana', 'cherry', 'grape', 'pineapple', 'watermelon']

In this example, we use two nested for loops to iterate through the nested list and append the fruit names into a new flat list.

Full code:

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import itertools
 
# Example 1: Nested loops
fruits = ["apple", "banana", "cherry"]
colors = ["red", "yellow", "purple"]
 
for fruit in fruits:
    for color in colors:
        print(fruit, color)
 
# Example 2: itertools.product()
for fruit, color in itertools.product(fruits, colors):
    print(fruit, color)
 
# Example 3: Nested loops to flatten a list
nested_list = [["apple", "banana"], ["cherry", "grape"], ["pineapple", "watermelon"]]
flat_list = []
 
for sublist in nested_list:
    for item in sublist:
        flat_list.append(item)
 
print(flat_list)

Conclusion

In this tutorial, we have learned three different ways to combine two for loops in Python: using nested loops, using a single loop with the itertools, product() function, and using nested loops to flatten a list.

Each of these methods has its merits and can be useful in different scenarios. Now you are equipped to choose the best method depending on the specific requirements of your project.