Why is the difference between output a and b? Why in output b is second '5'?

Question

a=[[0,0],[0,0]]
print(a)
b=[[0]*2]*2
print(b)
# a is the same what is b?
a[0][0]=5
print(a)
# output [[5,0][0,0]]
b[0][0]=5
print(b)
# output [[5,0][5,0]]

Answer 1

Those are reference problems.

Lists store adresses to other lists. By using *, those addresses get copied, so you end up with another list that contains the same addresses.

It sounds messy, but you can wrap your head around it after a while.

I hope this will help you:

https://code.sololearn.com/c89ejW97QsTN/?ref=app

Answer 2

It's definitely worth your time, figuring these things out, because they really come up all the time, in the most unexpected situations.

Answer 3

Thank you. The description makes sense for me. I need some time to analyze the attached code however I come back to this one for sure.