QGIS/python/plugins/processing/algs/qgis/StatisticsByCategories.py

# -*- coding: utf-8 -*-

"""
***************************************************************************
    StatisticsByCategories.py
    ---------------------
    Date                 : September 2012
    Copyright            : (C) 2012 by Victor Olaya
    Email                : volayaf at gmail dot com
***************************************************************************
*                                                                         *
*   This program is free software; you can redistribute it and/or modify  *
*   it under the terms of the GNU General Public License as published by  *
*   the Free Software Foundation; either version 2 of the License, or     *
*   (at your option) any later version.                                   *
*                                                                         *
***************************************************************************
"""

__author__ = 'Victor Olaya'
__date__ = 'September 2012'
__copyright__ = '(C) 2012, Victor Olaya'

# This will get replaced with a git SHA1 when you do a git archive

__revision__ = '$Format:%H$'

import math
from PyQt4.QtCore import *
from qgis.core import *
from processing.outputs.OutputTable import OutputTable
from processing.core.GeoAlgorithm import GeoAlgorithm
from processing.tools import dataobjects, vector
from processing.parameters.ParameterVector import ParameterVector
from processing.parameters.ParameterTableField import ParameterTableField


class StatisticsByCategories(GeoAlgorithm):

    INPUT_LAYER = 'INPUT_LAYER'
    VALUES_FIELD_NAME = 'VALUES_FIELD_NAME'
    CATEGORIES_FIELD_NAME = 'CATEGORIES_FIELD_NAME'
    OUTPUT = 'OUTPUT'

    def defineCharacteristics(self):
        self.name = 'Statistics by categories'
        self.group = 'Vector table tools'

        self.addParameter(ParameterVector(self.INPUT_LAYER,
                          'Input vector layer',
                          [ParameterVector.VECTOR_TYPE_ANY], False))
        self.addParameter(ParameterTableField(self.VALUES_FIELD_NAME,
                          'Field to calculate statistics on',
                          self.INPUT_LAYER,
                          ParameterTableField.DATA_TYPE_NUMBER))
        self.addParameter(ParameterTableField(self.CATEGORIES_FIELD_NAME,
                          'Field with categories', self.INPUT_LAYER,
                          ParameterTableField.DATA_TYPE_ANY))

        self.addOutput(OutputTable(self.OUTPUT, 'Statistics'))

    def processAlgorithm(self, progress):
        layer = dataobjects.getObjectFromUri(
                self.getParameterValue(self.INPUT_LAYER))
        valuesFieldName = self.getParameterValue(self.VALUES_FIELD_NAME)
        categoriesFieldName = self.getParameterValue(
                self.CATEGORIES_FIELD_NAME)

        output = self.getOutputFromName(self.OUTPUT)
        valuesField = layer.fieldNameIndex(valuesFieldName)
        categoriesField = layer.fieldNameIndex(categoriesFieldName)

        features = vector.features(layer)
        nFeats = len(features)
        values = {}
        nFeat = 0
        for feat in features:
            nFeat += 1
            progress.setPercentage(int(100 * nFeats / nFeat))
            attrs = feat.attributes()
            try:
                value = float(attrs[valuesField])
                cat = unicode(attrs[categoriesField])
                if cat not in values:
                    values[cat] = []
                values[cat].append(value)
            except:
                pass

        fields = ['category', 'min', 'max', 'mean', 'stddev', 'count']
        writer = output.getTableWriter(fields)
        for (cat, v) in values.items():
            (min, max, mean, stddev) = calculateStats(v)
            record = [cat, min, max, mean, stddev, len(v)]
            writer.addRecord(record)


def calculateStats(values):
    n = 0
    sum = 0
    mean = 0
    M2 = 0
    minvalue = None
    maxvalue = None

    for v in values:
        sum += v
        n = n + 1
        delta = v - mean
        mean = mean + delta / n
        M2 = M2 + delta * (v - mean)
        if minvalue is None:
            minvalue = v
            maxvalue = v
        else:
            minvalue = min(v, minvalue)
            maxvalue = max(v, maxvalue)

    if n > 1:
        variance = M2 / (n - 1)
    else:
        variance = 0
    stddev = math.sqrt(variance)
    return (minvalue, maxvalue, mean, stddev)