To print first n even numbers in reverse order using while loop

Question

#4 print first n even numbers in reverse order
n= int(input())
num= 0
x= 0
listt= []
while n>x:
if num%2==0:
listt.append(str(num))
x+=1
num+=1
listt= listt.reverse()

#for loop is working fine but using while loop it is creating a problem

#sample input- 5
#desired output- 8,6,4,2,0

Answer 1

n= int(input())
num= 2 #debug
listt= []
while n>0:
listt.append(num)
n-=1
num+=2
listt.reverse()
print(listt)

Answer 2

This just indicates that the `reverse` method does not return anything. It only modifies the existing list. It does not return an updated list.

Don't bother doing listt = listt.reverse(). If you just want to reverse a list, do listt.reverse() to modify your listt.

listt.reverse()
print(listt)

Answer 3

Now solve this problem without the extra variable num ☺️

Answer 4

Thanks to all for your help 🙏🙏

Answer 5

Brian, great example with bitwise operation 👍, but it's easy ☺️.

Try all the same to print the n-th number of even numbers.

Example:
input 5
output 10,8,6,4,2

Answer 6

Shaurya Agarwal the description says to print them in reverse order, but the #desired output shows them in forward order. Which is correct?

Answer 7

n = int(input())
# If n is odd, force n to next lower even.
if n&1: n -= 1

#now only even numbers will print
while n>0:
print(n, end=',')
n -= 2
print(n) #final 0 without comma

Answer 8

Solo oops, I missed that detail.

Okay, it is easily corrected with the same loop, only a different adjustment to n.

#print the first n even Natural numbers in reverse order
n = int(input())
n = 2*(n - 1)

while n>0:
print(n, end=',')
n -= 2
print(n) #final 0 without comma

Answer 9

Solo thanks bro!

I know there is debate about whether 0 is even, so I included it only because the OP did.

There is also debate about whether 0 belongs in the Natural number set. I decided to adopt it in that set in my comment because it is the only difference that would distinguish Natural numbers from Counting numbers. On the other hand, 0 is most unnatural!

Answer 10

Brian corrected

Answer 11

Brian, yes, you did it. 👏👏👏
Except 0 is not an even number ☺️.