Python list sort using lambda

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Python list sort using lambda

Zander Bailey Dec 14, 2019 3 min read Some functions can accept an entire second function as a parameter, which can be very useful, but if you only need a very simple function it can be a hassle to write out a whole separate function just so you can pass it as a parameter. It turns out there is an easier way to provide simple functions to a parameter: Lambda functions. Lambda functions have many uses, but today we'll talk about how they are useful for sorting, First, let's look at why we need to pass functions for sorting in the first place. Normally a function like sorted will sort strings by alphabetical order, unless we specify otherwise. The way we specify how we want the list sorted is with the key parameter. key accepts a function, and every item will be passed to the function individually, and return a value by which it will be sorted. Let's see an example of this, say we want to sort a list of strings by length instead of alphabetically: flowers = ['tulip', 'iris', 'primrose', 'violet'] sorted(flowers, key=len()) #returns: ['iris','tulip','violet','primrose'] So by passing the len function as the key, it sorts by the length of the string. In the same way we can use many different functions to sort items in a list, and this is where Lamba functions become important. By using Lambda functions we can specify exactly how we want the list sorted. This become even more important when we try to sort more complex collection types. For example a list of tuples is something that comes up from time to time, and it can be useful to have it sorted. But even if one or more values in the tuple contain numbers or some other item we want to sort by, the normal sorting function does not see that information. So how do we get it to sort by something inside the tuple? We use a lambda function as the key. Say we have a list of tuples containing a persons name and their age, and we want to sort it by age: ages = [('billy',16),('anna',17),('joe',15),('kelly',19)] sorted(ages,key=lambda x: x[1]) #returns: [('joe',15),('billy',16),('anna',17),('kelly',19)] Alternatively we could sort this list alphabetically by name with a similar function. There is also a way to sort a dictionary. This is a little tricky since dictionaries are normally orderless, but there are some ways we can look at sorting them, mainly by key and by value. There are several ways to go about this, depending on what you want your output to be. Using our ages example again, if we only want the ages and we don't care about the name, we can simply use the .values() method to get the values as a list and sort that. But if we want to retain all the information in dictionary, keys and values, we make use of lambda notation: ages = {'billy':16,'anna':17,'joe':15,'kelly':19} sorted(ages.items(),key=lambda x:x[1]) .items() returns a list of all key-value pairs as tuples, and the lambda function tells it to sort by the second item in each tuple, i.e. the value. Alternatively if you want to sort by the key, just change lambda x:x[1] to lambda x:x[0]. We're not quite done yet though, because now we have a list of tuples, instead of a dictionary. If that's all you need, great. If you really want it as a dictionary, here's how: combine with list comprehension, or in this case dictionary comprehension: ages = {'billy':16,'anna':17,'joe':15,'kelly':19} {k: v for k,v in sorted(ages.items(),key=lambda x:x[1])} #[('joe', 15), ('billy', 16), ('anna', 17), ('kelly', 19)] -> {`joe': 15, 'billy': 16, 'anna': 17, 'kelly': 19} Now we have it back in sorted order, but as a dictionary. There are many other ways to sort collections, but hopefully this will get you started, and give you an idea of the kind of things you can do with sorting. NLP App Developer & Advocate - May 28 Forem Open with the Forem app Learn how Grepper helps you improve as a Developer! INSTALL GREPPER FOR CHROME Browse Python Answers by Framework Browse Popular Code Answers by Language for loop groovy groovy wait time groovy implementation of the interface map merge elixir elixir random number elixir length of list clojure get list first item how to make a range clojure abap concatenate table abap loop example how to pass unction in scheme how to make a list in scheme Browse Other Code Languages The list.sort() method sorts the elements of a list in ascending or descending order using the default < comparisons operator between items. Use the key parameter to pass the function name to be used for comparison instead of the default < operator. Set the reverse parameter to True, to get the list in descending order. Syntax: list.sort(key=None, reverse=False) Parameters: key: (Optional) A function that extracts a comparison key from each list element while sorting. reverse: (Optional) If true, the sorted list will be reversed. By default, it is False. Return Value: No return value. It sorts the list itself. The following example demonstrates the sort() function on numeric lists. nums = [1, 5, 3, 4, 2, 10, 6, 8, 7, 9] nums.sort() print('List in Ascending Order: ', nums) nums.sort(reverse=True) print('List in Descending Order: ', nums) List in Ascending Order: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] List in Descending Order: [10, 9, 8, 7, 6, 5, 4, 3, 2, 1] The sort() method can also be used on character lists. al = ['a','d','e','c','b'] al.sort(reverse=True) print('List in Descending Order: ', al) al.sort() print('List in Ascending Order: ', al) List in Descending Order: ['e', 'd', 'c', 'b', 'a'] List in Ascending Order: ['a', 'b', 'c', 'd', 'e'] The following example sorts the string list in alphabetical order. cities = ['Mumbai', 'London', 'Paris', 'New York'] cities.sort() print('List in Ascending Order: ', cities) cities.sort(reverse=True) print('List in Descending Order: ', cities) List in Ascending Order: ['London', 'Mumbai', 'New York', 'Paris'] List in Descending Order: ['Paris', 'New York', 'Mumbai', 'London'] Using key Parameter Use the key parameter to set the built-in or custom function to compare each element of a list and sort it. For example, the following uses the built-in len() function that returns the length of each element and sorts based on the length of each element. cities = ['Mumbai', 'London', 'Paris', 'New York'] cities.sort(key=len) print('List in Ascending Order of the length: ', cities) cities.sort(key=len, reverse=True) print('List in Descending Order of the length: ', cities) List in Ascending Order of the length: ['Paris', 'Mumbai', 'London', 'New York'] List in Descending Order of the length: ['New York', 'Mumbai', 'London', 'Paris'] Sort List of Class Objects The following example shows how to sort a list whose elements are the objects of the custom class. class student: name='' age=0 def __init__(self, name, age): self.name = name self.age = age s1 = student('Bill', 25) s2 = student('Steve', 29) s3 = student('Ravi', 26) student_list = [s1, s2, s3] # student_list.sort() # raise an error student_list.sort(key=lambda s: s.name) # sorts using lambda function print('Students in Ascending Order:', end=' ') for std in student_list: print(std.name, end=', ') student_list.sort(key=lambda s: s.name, reverse=True) # sorts using lambda function print('Students in Descending Order:', end=' ') for std in student_list: print(std.name, end=', ') Students in Ascending Order: Bill, Ravi, Steve, Students in Descending Order: Steve, Ravi, Bill, In the above example, the lambda function lambda s: s.name set to the key argument. So, it will return the name of each student object for comparison. Direct use of student_list.sort() will raise an error because the < operator cannot compare objects. You can define a function instead of using the lambda function as a key argument. Want to check how much you know Python? Share Tweet Share Whatsapp Photo by Priss Enri on UnsplashSome built-in functions like lambda functions can modify a list in a more concise way. I love using lambdas because I can write the program in fewer lines of code and still clear and understandable. In addition, it doesn't make your program slow.Here is the syntax:I put a few dots after x to symbolize some more arguments.Lambdas can take any number of arguments but only one expression.Lambda is used in anonymous functions whih is a function without a name. We define python functions using `def'. But when we use lambda we can make a function without using `def'. If you want to save it, a variable is enough.Let's see some examples. Here is a function that takes one argument x and returns x squared.Now write this same function using lambda.Both the squared function works exactly the same way.Now see a simple addition function that takes two arguments x and y and returns the summation of them:Isn't it a bit more concise using the lambda? It saves one extra line.Call the function `add' and pass two integers to it.output:It will look a lot more efficient while working with lists!MapMap function in Python takes lists and a function and returns the list, modified by the function. Let's work on some examples. Here are three lists a, b, and c.Call the add function on lists a and b:output:This doesn't give an element-wise addition. When you simply add to lists they just concatenate. It just makes a bigger list with all the elements of both the lists in itTypically, the way we do element-wise addition of two same size lists is by using `for loop' as follows:output:I had to write three lines of code to add two lists. Let's do it in one line of code now using `map'.Using `map', you can call the add function we defined before on lists a and b to get element-wise addition. It works exactly like a `for loop' and gives the same output.output:Isn't it a lot more concise and elegant?Let's summarise what we did. We defined a function called `add' first and then used a `map' function to get the element-wide addition of two lists. Can we do even better?Yes! We can the `add' function and call it in the same line of code.output:What we did here is,We defined a function using lambda that takes two parameters x and y.Passed two lists a and b to the function.Used map because a and b are the lists. We wanted the element-wise addition of a and b.In fact, you can add all three of the lists or as many arguments as you want:output:Instead of just adding if you have a formula using three arguments:output:FilterThe filter takes a list as an argument and a function as the expression. It returns the modified list after filtering out the elements by the function. Here are some examples.Return only the number bigger than five in a list. Before that make a new list c:output:Here I defined the function that returns the numbers that are bigger than five and the list c as an argument. Look it returned all the elements of c that are bigger than 5.Let's work on another example. Return only the even numbers from a list. This time I am going to pass the list `a'.output:We can use lambda and filter on strings as well.All the examples above are with numbers. The next few examples will be on strings. Here is a list of names:Return all the names that start will `A'.output:SortedThe sorted function is a very simple, easy way of sorting numbers or strings by alphabets. Some examples are here:Sort the names of the list `names' above by their first letter.output:It's great! But an even simpler option is:output:In this way, the names get sorted by alphabetic order. The same thing works for lists as well.output:Lambda, Map, Filter, and Sorted With DictionariesWorking with dictionaries is far easier and efficient with lambda, map, filter, and sorted.Here is a list that has four dictionaries. Each dictionary consists of the name of a person and his or her age.Return the list of only names from the list above:output:The same way, you can output the age only from dict_a:output:If you want the names to be sorted by alphabets or the list of `age' to be sorted, simply put a sorted before:output:Probably, you think, it's more useful to arrange the whole dict_a sorted by ages. In that case, we just need to use the key with lambda:output:Output the youngest child's information:output:Output the oldest child's information:output:Instead of ascending order if we need the list to be in descending order:output:Or, if you want to sort by the names in alphabetic order:Output the information of the children who are older than 10:output:After three years you are returning the ages of the kids. So, just add 3 to each age:ConclusionI hope this tutorial helps in improving your python programming.Here is the video tutorial on YouTube:Feel free to follow me on Twitter and like my Facebook page.More Reading:

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