Lambda Expression

#1

How the following code is doing the desired sorting of the values.

sorted(a, key = lambda x: x[1])

Can you please explain it in details as we are not sure about the key attribute here and still need to go through the built in data types.

#2

Hey @Akshay986, key is a parameter in sorted() function of python. Please find the description below :
Key (optional) : A function that would serve as a key or a basis of sort comparison.

So if you have a list like

l = [(1,2), (3,4), (6,3), (10,5), (5,11)]
print(sorted(l , key = lambda x:x[0]))
print(sorted(l, key = lambda x:x[1]))

Then the output will be :

[(1,2), (3,4), (5,11), (6,3), (10,5)]
[(1,2), (6,3), (3,4), (10,5), (5,11)]

Hope you can understand how key is affecting the output !
Happy Learning :slight_smile:

#3

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#4

Thanks for the explanation.
but what is the ise of lambda here.
we can simply do the same using the following statement

sorted(a, key= x[1])

please correct me if I am missing something

#5

Hey @Akshay986, no this is not correct. I would request you to watch the video once again. If you do key = x[1] , then it would throw you an error stating that x is not defined or something similar. But lambda works in slightly different way. Please go through this link for best explanation to your doubt. Lambda functions are basically a concise and optimised way of writing your functions in a single line.

Hope you understand :slight_smile:
Happy Learning :slight_smile:

#6

hey @Akshay986 ,
we see that you have reopened this doubt.
To explain the working of lambda functions in respect to above question.
lambda functions are a kind of one line functions that acts based on the processing and features you pass to it.
For example lets consider a lambda function as
x = lambda x,y : xy
now we have created a function and we need to calculate the multiplication of two numbers 5,10.
so we just pass it as x(5,10) , in this the lambda function maps 5 and 10 as x and y respectively.
So we get result as 510 = 50.

In this above question ,
a = [ (2,5) , (1,1) , (3,2) ]
sorted(a,key = lambda x:x[1])
what it does is , the key is provided the parameters to be as each tuple in a. and for each tuple it returns the value at index 1 ( 5, then 1 ,then 2 ) .
the sorted function checks these values and arrange the data based on it.
which means ,
step 1 : key = 5
a = [ (2,5) , (1,1) , (3,2) ]
step 2: key = 1
a = [ (1,1) ,(2,5) , (3,2) ]
step 3: key = 2
a = [ (1,1) ,(3,2) , (2,5) ]

So , you now you get the required sorted array.

I hope this might have helped you understand lambda functions.
Thank You and Happy Learning :slightly_smiling_face:

#7

So due to the key attribute the value of x is getting iterated at each step. Otherwise the lambda is just one line function and is not the reason for iteration of the x values.

#8

Yes, it just a simple function that in turn requires parameters, which are provided iteratively by the key attribute.