Replace NaNs with the number that appears to its left in the row.
If there are more than one consecutive NaNs, they should all be replaced by the first non-NaN value to the immediate left of the left-most NaN. If the NaN is in the first column, default to zero.
For example, if
x = [NaN 1 2 NaN NaN 17 3 -4 NaN]
y = [ 0 1 2 2 2 17 3 -4 -4]