List comprehension two lists python

python list comprehension two lists

How do you use two variables in a list comprehension?

You can do this with two for clauses in your list comprehension. The first iterates over the items in the list. The second iterates over a single-item list containing the list derived from splitting the string (which is needed so we can unpack this into three separate variables).

What is an example of list comprehension in Python?

Example 6: if. else With List Comprehension. Here, list comprehension will check the 10 numbers from 0 to 9. If i is divisible by 2, then Even is appended to the obj list. If not, Odd is appended.

How do I make a list comprehension in Python?

In this tutorial, you’ll learn how to:

Rewrite loops and map() calls as a list comprehension in Python. Choose between comprehensions, loops, and map() calls. Supercharge your comprehensions with conditional logic. Use comprehensions to replace filter()

How do you compare two lists in Python?

1. list1 = [11, 12, 13, 14, 15]
2. list2 = [12, 13, 11, 15, 14]
3. a = set(list1)
4. b = set(list2)
5. if a == b:
6. print(«The list1 and list2 are equal»)
7. else:
8. print(«The list1 and list2 are not equal»)

How do you add multiple values to a list in Python?

extend to extend the list by multiple values from any kind of iterable, being it another list or any other thing that provides a sequence of values. So you can use list. append() to append a single value, and list. extend() to append multiple values.

What kind of operations can a list comprehension do?

List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.

What are the major differences of a set comprehension versus a list comprehension?

A set comprehension is similar to a list comprehension but returns a set instead of a list. The syntax is slightly different in the sense that we use curly brackets instead of square brackets to create a set. The list includes a lot of duplicates, and there are names with only a single letter.

How do you break in list comprehension?

1. a one-liner.
2. no other fancy libraries like itertools, «pure python» if possible (read: the solution should not use any import statement or similar)

What is list comprehension write an example?

List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list. Example: Based on a list of fruits, you want a new list, containing only the fruits with the letter «a» in the name.

Does list comprehension create a new list?

List comprehensions are used for creating new lists from other iterables. As list comprehensions return lists, they consist of brackets containing the expression, which is executed for each element along with the for loop to iterate over each element.

What is list comprehension explain?

A list comprehension is a syntactic construct available in some programming languages for creating a list based on existing lists. It follows the form of the mathematical set-builder notation (set comprehension) as distinct from the use of map and filter functions.

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Python List Comprehension with Multiple Lists

In this tutorial we will show you how to use python list comprehension with two or more lists.

We can use the zip function. See the below example:

listA = [1,2,3,4] listB = [5,6,7,8] listC = [ A * B for A,B in zip(listA,listB)] print(listC) #Output #[5, 12, 21, 32]

Let’s explain what is going on here:

1. We define our two lists listA and listB. Both contain numbers only.
2. Our list comprehension has the job of finding the product of the nth elements of listA and listB lists.
3. The zip() function in Python accepts iterables (lists, dataframes, dictionaries etc.) and returns a zip object which is itself an iterator. This iterator generates a series of tuples containing elements from each iterable supplied in the original function, in the order they were supplied. The first tuple is the first element of each iterable, the second tuple is the second element of each iterable and so on. The zip() function is evaluated left-to-right.
4. Our list comprehension returns listC which is a list of the products of each element of the two lists. The product of the 1st element of each list, the product of the 2nd element and so on. A is the iterator variable for listA, B is the iterator variable for listB. Because we want the product of the nth element in each list our output expression must be A*B, which gives us the product that we are looking for.

Let’s do another example with a pandas dataframe.

import pandas as pd data = df = pd.DataFrame(data) listD = [ B - A for A,B in zip(df["Year 1 Income"],df["Year 2 Income"])] print(listD) #Output #[-555, 10000, 0, -66000, 1000000]

So using zip() works on dataframes also because dataframes are an iterable object. In the above example we used the list comprehension to calculate the difference in income between two years for 5 people. We simply found the difference between Year 1 Income and Year 2 Income for each person using our list comprehension.

zip() can work on more than two lists also. See below:

listE = [1,2,3,4] listF = [5,6,7,8] listG = [9,10,11,12] listH = [ E+F+G for E,F,G in zip(listE,listF,listG)] print(listH) #Output #[15, 18, 21, 24] 

In the above we used 3 lists in our zip() function: ListE, ListF and ListG. Our list comprehension used the iterator variables E, F and G in the expression E+F+G to give us the sum of nth elements of 3 different lists. Our output is a list of 4 elements, as expected.

The above example used lists with the same number of elements in the zip() function. If they were not of the same length, the zip() function iterator would have stopped when the shortest input iterable was exhausted . Please remember this when using lists of different lengths!

Finally, we don’t have to use the zip() function. We can use nested for-loops in our list comprehension. It all depends on the problem we are trying to solve. For example, if we want to find all of the possible permutations of two different lists we can do the following:

listI = [1,2,3,4] listJ = [5,6,7,8] listK = [[I, J] for I in listI for J in listJ] print(listK) #Output #[[1, 5], [1, 6], [1, 7], [1, 8], [2, 5], [2, 6], [2, 7], [2, 8], [3, 5], [3, 6], [3, 7], [3, 8], [4, 5], [4, 6], [4, 7], [4, 8]]

So our nested for-loop in our list comprehension allowed us to generate a list of all permutations of numbers from two separate lists. We could have alternatively done this with a normal nested for-loop in the following way:

listI = [1,2,3,4] listJ = [5,6,7,8] listL=[] x=[] for I in listI: for J in listJ: listL.append([I,J]) print(listL) #Output #[[1, 5], [1, 6], [1, 7], [1, 8], [2, 5], [2, 6], [2, 7], [2, 8], [3, 5], [3, 6], [3, 7], [3, 8], [4, 5], [4, 6], [4, 7], [4, 8]] 

The above nested loops work the same but list comprehensions, as we have seen, require less code and are more readable.

Find the full source code HERE and other great tutorial on list comprehensions HERE. Thanks for reading. 👌👌👌

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Merge Two Lists in Python

A list is a data structure in Python that contains a sequence of elements. We can have many lists in our code, which sometimes we may need to merge, join, or concatenate. Merging a list is very useful in many ways, and through this article, we will see the various ways we can merge two lists in Python.

Merging Two Lists in Python

Merging lists means adding or concatenating one list with another. In simple words, it means joining two lists. We can use various ways to join two lists in Python. Let us discuss them below:

1. Using append() function

One simple and popular way to merge(join) two lists in Python is using the in-built append() method of python. The append() method in python adds a single item to the existing list. It doesn’t return a new list of items. Instead, it modifies the original list by adding the item to the end of the list.

Explanation:

• In the above code, we declare two lists, ls1 and ls2 .
• We iterate over ls2 and append its elements to ls1 using our python’s append method.

2. Using the ‘+’ Operator

This is the simplest way to merge two lists in Python. ‘+’ operator is a multipurpose operator, which we can use for arithmetic calculations and for merging purposes, strings, lists, etc.

Explanation:

• We defined our lists ls1 and ls2
• Then, we merged them using the ‘+’ operator in Python.
• The use of + operator adds the whole of one list behind the other list.

3. Using List Comprehension

We can merge two lists in Python using list comprehension as well. List comprehension offers a shorter and crisper syntax when we want to create a new list based on the values of an existing list. It may also be called an alternative to the loop method.

Explanation:

• In the above example, first, this line is executed for n in (num1,num2) , which will return [[1, 2, 3], [4, 5, 6]] .
• After that, we pick one element at a time from the above lists. Hence we do for x in n .
• Finally, we get our desired result by storing the value of x in the list: [x for n in (num1,num2) for x in n]

4. Using the extend() method

The extend() method adds all the elements of an iterable (list, tuple, string, etc.) to the end of the list. It updates the original list itself. Hence its return type is None. It can be used to merge two lists in Python.

Explanation:

• We defined our lists and passed them to our extend method in python
• extend basically, extended the list2 to the end of list1 . The size of the list increased by the length of both lists.
• Also, extend updated our ls1 directly.

5. Using iterable unpacking operator *

An asterisk * denotes iterable unpacking. The unpacking operator takes any iterable(like list, tuple, set, etc.) as a parameter. Then the iterable is expanded into a sequence of items included in the new tuple, list, or set at the site of the unpacking.

Any no. of lists can be concatenated and returned in a new list using this operator. Hence it can be used to merge two lists in Python.

Note: works only in Python 3.6+ .

Explanation:

• The * first unpacks the contents of ls1 , ls2 , and ls3 and then creates a list from its contents.
• Then, all the items of our iterables will be added to our new list ls .

Conclusion

• Merging two lists in Python is a common task when working with data.
• There are several ways to merge two lists in Python, depending on the specific requirements of the task.
• One of the simplest ways to merge two lists is to use the «+» operator to concatenate them.
• Another approach is to use the «extend()» method to add the elements of one list to another.
• You can also use the «zip()» function to combine the elements of two lists into a list of tuples.
• If the lists contain duplicates, you can use the «set()» function to remove them before merging.
• In some cases, you may need to merge lists while preserving the order of elements. This can be done using the «sorted()» function or a custom sorting function.
• When merging large lists or working with memory-intensive data, it’s important to consider the efficiency of the merging method.

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