[GIS] How to select and change only certain (same)values in attributetable in QGIS

attribute-tableqgis

I have an attributetable. Spread all over it are numbers like -99999997. I want to replace them by zero (0). There a many columns in which there are such numbers.
I have used the expression (an example with one column P-Ant_ARU)

case when   "P_ANT_ARU"  is  -99999997  then '0' else    "P_ANT_ARU"  end

for every feature in the column but I wonder if there is a faster way to use in the field calculator to get rid of all the -99999997 in one or perhaps two steps, because there are a lot of columns 🙂

I have also tried toint(0) but that doesn't work because you have to choose a field, and I want a value to be changed. The same problem occurs with the plugin QuickMultiAttributeEdit.

So how can I select all those numbers in the whole table and make them zero.

Best Answer

Below are a couple of methods you could use in the Python Console which replaces all values in the Attribute Table of -99999997 with 0. As they are within a function, there shouldn't be anything printed in the console (incase this somehow freezes QGIS). After only limited testing, I found that changing attributes on a column by column basis was a bit faster.

Row by row:

def by_row():
    layer = qgis.utils.iface.activeLayer()  
    layer.startEditing()   
    for feat in layer.getFeatures():
        f = feat.fields()
        num = f.count()
        for i in range(num):
            if feat[i] == '-99999997':
                feat[i] = '0'
                layer.updateFeature(feat)
    layer.commitChanges()

by_row()

Column by column:

def by_column():
    layer = qgis.utils.iface.activeLayer()  
    layer.startEditing()   
    for field in layer.dataProvider().attributeIndexes():
        for feature in layer.getFeatures():
            attrs = feature.attributes()
            if attrs[field] == '-99999997':
                feature[field] = '0'
                layer.updateFeature(feature)
    layer.commitChanges()

by_column()

Related Solutions

QGIS – How to Set All Values in a Column to Be the Same Using QGIS

With a layer in edit mode and the attribute table opened, choose the field from the left-hand drop-down (it says "abc thi" here), set the value (in the box after the expression editor "E" sign), and hit "update all":

Note that string values have to be in single quotes ' in the expression box.

Switch edits off and save changes.

QGIS Auto Fill – Auto Fill Values in One Column Based on Another Column in QGIS

If I understand you correctly, part makes no difference, only forest?

Create a function in the python editor and call it with autofill() when you have set some row to yes:

def autofill():
    lyr = iface.activeLayer() #Make sure to click the layer name in table of contents
    yeses = {f['forest'] for f in lyr.getFeatures() if f['factor']=='yes'} #Find all forests with a yes in factor field
    factor_field_index = lyr.fields().indexFromName('factor') #Find field index of the factor field
    am = {} #A dictionary to hold the values to update
    if yeses: #If there are any forests in yeses set
        for f in lyr.getFeatures(): #For each feature in the table
            if f['forest'] in yeses: #Check if the features forest is in yeses set
                am[f.id()] = {factor_field_index:'yes'} #If it is, add it to the dictionary
        lyr.dataProvider().changeAttributeValues(am) #And update it