gistfile1.py

computeFieldIndex = layer.fieldNameIndex( 'population' )
allFieldsMap = layer.pendingFields()
expression = QgsExpression('"affected" = 1')
expression.prepare(allFieldsMap)

if expression.hasParserError():
  raise Exception('Parser error !!')

if self.selectedOnly: #if we want to loop through selection only
  features = layer.selectedFeatures()
  for feature in features:
    result = expression.evaluate(feature)
    if expression.hasEvalError():
      raise Exception('Eval error !!')
    if result.toBool():
      total += feature.attributeMap()[ computeFieldIndex ].toDouble()[ 0 ]

else: #if we want to loop through the whole layer
  layer.select( range(0,len(allFieldsMap)), QgsRectangle(), True )
  feature = QgsFeature()
  while layer.nextFeature( feature ):
    result = expression.evaluate(feature)
    if expression.hasEvalError():
      raise Exception('Eval error !!')
      if result.toBool():
        total += feature.attributeMap()[ computeFieldIndex ].toDouble()[ 0 ]