Can anyone explain me what this Python 3 command do?

Question

I am a beginner in Python programming. I have checked what the methods does separately, but not getting as a whole. Here is a program was written as a answer for the problem:
Write a program to print all elements of an array in reverse order:

if __name__ == '__main__':                          #1   
    n = int(input())                                #2
                                                    #3
    arr = list(map(int, input().rstrip().split()))  #4
    for i in range(n):                              #5
        print(arr[-i-1],end=' ')                    #6

I am not getting line 4. Can anyone explain step by step what that line is doing? How they are working and giving the output as a whole? The inputs were separated by space:

5            #length of the array
1 2 3 4 5     #inputs separated by space

Asked By: LordOfThunder

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Source

Answer 1

It is a "pythonic" way of creating a list of ints from a space seperated numeric input:

arr = list(map(int, input().rstrip().split()))

It takes a string input and transforms it into a list of integers using result of map(…) which returns a generator.

map takes a function and applies it to all elements of the second argument iterable.

input().
rstrip().
split()
takes an input, removes all whitespaces on the right side and splits it into parts on whitespaces which is then fed as iterable into map(int, ...) which creates a generator result from applying int to all elements of the given iterable. The result of map is fed into list(...) which makes a list from it.

"1 2 3 4" => ["1","2","3","4"] -> generator (1,2,3,4) -> [1,2,3,4]

This will break with an error if you supply non-numbers or anything other then spaces and numbers – no error handling is done.

It is short, but only safe if the input is correct.

Answered By: Patrick Artner

Answer 2

input() looks like it gets the next line of input.
From the example this is the string "1 2 3 4 5n" (it has a newline on the end).

rstrip() then removes whitespace at the right end of the input, including the newline.

split() with no arguments splits on whitespace, transforming the input to an iterable of strings. e.g. ['1', '2', '3', '4', '5']

map(int, sequence) applies int to each string. e.g. int('1') -> 1, int('2') -> 2 etc.. So your sequence of strings is now a sequence of integers.

Finally list(seq) transforms the sequence to a list type. So now you have [1,2,3,4,5].

Answered By: Hitobat

Answer 3

In this line

arr = list(map(int, input().rstrip().split()))

input(): By default it takes string as an input.

The map(aFunction, aSequence) function applies a passed-in function to each item in an iterable object and returns a list containing all the function call results. So, all the string entries which are separated by a space are converted into integer.

Reference: https://www.bogotobogo.com/python/python_fncs_map_filter_reduce.php

split(): This function is used to split all the entries in the string by space.

rstrip(): This function is used to strip out the spaces if any present at the end of the array input (1 2 3 4 5)

Answered By: ayush gupta

Can anyone explain me what this Python 3 command do?

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