# Source code for petl.util.statistics

```from __future__ import absolute_import, print_function, division

from collections import namedtuple

from petl.util.base import values, Table

[docs]def limits(table, field):
"""
Find minimum and maximum values under the given field. E.g.::

>>> import petl as etl
>>> table = [['foo', 'bar'], ['a', 1], ['b', 2], ['b', 3]]
>>> minv, maxv = etl.limits(table, 'bar')
>>> minv
1
>>> maxv
3

The `field` argument can be a field name or index (starting from zero).

"""

vals = iter(values(table, field))
try:
minv = maxv = next(vals)
except StopIteration:
return None, None
else:
for v in vals:
if v < minv:
minv = v
if v > maxv:
maxv = v
return minv, maxv

Table.limits = limits

_stats = namedtuple('stats', ('count', 'errors', 'sum', 'min', 'max', 'mean',
'pvariance', 'pstdev'))

[docs]def stats(table, field):
"""
Calculate basic descriptive statistics on a given field. E.g.::

>>> import petl as etl
>>> table = [['foo', 'bar', 'baz'],
...          ['A', 1, 2],
...          ['B', '2', '3.4'],
...          [u'B', u'3', u'7.8', True],
...          ['D', 'xyz', 9.0],
...          ['E', None]]
>>> etl.stats(table, 'bar')
stats(count=3, errors=2, sum=6.0, min=1.0, max=3.0, mean=2.0, pvariance=0.6666666666666666, pstdev=0.816496580927726)

The `field` argument can be a field name or index (starting from zero).

"""

_min = None
_max = None
_sum = 0
_mean = 0
_var = 0
_count = 0
_errors = 0
for v in values(table, field):
try:
v = float(v)
except (ValueError, TypeError):
_errors += 1
else:
_count += 1
if _min is None or v < _min:
_min = v
if _max is None or v > _max:
_max = v
_sum += v
_mean, _var = onlinestats(v, _count, mean=_mean, variance=_var)
_std = _var**.5
return _stats(_count, _errors, _sum, _min, _max, _mean, _var, _std)

Table.stats = stats

def onlinestats(xi, n, mean=0, variance=0):
# function to calculate online mean and variance
meanprv = mean
varianceprv = variance
mean = (((n - 1)*meanprv) + xi)/n
variance = (((n - 1)*varianceprv) + ((xi - meanprv)*(xi - mean)))/n
return mean, variance
```