y[i]=x[i] NameError: name 'y' is not defined

This error message means that you are trying to use a variable called 'y' in your code, but you have not defined it yet. You need to define 'y' before you can use it.

For example, you can define 'y' as an empty list before using it to store the values of 'x':

x = [1, 2, 3, 4]
y = []  # define 'y' as an empty list

for i in range(len(x)):
    y.append(x[i])

print(y)  # output: [1, 2, 3, 4]

In this example, we defined 'y' as an empty list before the loop. Inside the loop, we append each value of 'x' to 'y'. Finally, we print the values of 'y'.