Cant change the values of a matrix using for loop

Question

I have a matrix M with float numbers. I want to round said numbers to 3 decimals and update the matrix. However, M doesn't change. Why is M not updating?

M= [[1.0, 0.6666666666666666, 0.0, 0.5098039215686274], [-0.0, -0.0, 1.0, 0.4117647058823529]]
for arr in M:
    for number in arr:
        number = round(number, 3)
print(M) #[[1.0, 0.6666666666666666, 0.0, 0.5098039215686274], [-0.0, -0.0, 1.0, 0.4117647058823529]]

Answer 1

Don't change an array while iterating over it. Instead, store your changes elsewhere. You can set M = rounded_M at the end if you like.

M = [[1.0, 0.6666666666666666, 0.0, 0.5098039215686274], [-0.0, -0.0, 1.0, 0.4117647058823529]]
rounded_M = []

for arr in M:
    rounded_arr = []
    for number in arr:
        rounded_arr.append(round(number, 3))
    rounded_M.append(rounded_arr)

print(rounded_M)