change array column values without loop

[scidam]

The answer depends on what container did you use to store the data. If you are working with numpy arrays,
you can do that as follows:

import numpy as np
x = np.array([0.16, 0, 0, 0.65, 0.17])
x[x < 0.5] *= 2

If your data is presented as a column of Pandas dataframe, you can do it by almost similar way:

# df Pandas data frame
df[df.iloc[:, <column number>]<0.5] *= 2