Question

def dec2binr(n):
    if n == 1:
        return '1' 
    else: return (str(n%2)+dec2binr(n//2))[::-1]

Without the [::-1] it returns the reversed correct binary number. The [::-1] does not work in this case – for n=40 Ii get:

011000  

when i would expect

101000  

Without [::-1] I get

000101

Which is reversed, but correct.
Why does this happen and how can I fix it?

Answer 1

You need to reverse how you append the strings.

return dec2binr(n//2) + str(n%2)

The issue is, n%2 will read the least significant bit, but you were appending it to the left of the string, which is the most significant place.