Sum only some values from a two-dimensional list

[bronfai]

Hello,
I'm a newbie. I have a two-dimensional list like this

DATA = 0
# quality control flag
QC = 1
samples = [[0.1, 33.3, 4.4], [1, 0, 2]]

where 3 samples and their 3 quality flags are collected. To calculate the average of all the data, I usually do:

mean = sum (map (float, samples[DATA])) / float (len (samples[DATA]))

But now I want to calculate the average using only the data that have flag > 0

I appreciate any help.