Counting vertices and adding it as number to column using shapely

Question

I have a small DataFrame and created new column. I would like to add to every record in column "vertex_counter" value of n. I have the problem because it is doesn't work and I have in every record in column "vertex_counter" value 0.

What can be the problem?

df_budynki3 = df_budynki
df_budynki3['vertex_counter'] = 0

for index_budynku, bud_row in df_budynki3.iterrows():
    coo = shapely.wkt.loads(bud_row['classification.geometry']) # create polygon from string
    n = len(coo.exterior.coords) # count number of points in polygon
    bud_row['vertex_counter'] += n

Answer 1

In Modify pandas DataFrame within iterrows loop

The rows you get back from iterrows are copies that are no longer connected to the original data frame, so edits don't change your DataFrame. However, you can use the index to access and edit the relevant row of the DataFrame. (for bud_row['vertex_counter'] += n)

Therefore you can assign values in the loop using pandas .at (or pandas .loc):

import geopandas as gpd

gdf = gpd.read_file("polygons.shp")

gdf['vertex_counter'] = 0

for index, row in gdf.iterrows():
  gdf.at[index,'vertex_counter'] = len(row['geometry'].exterior.coords
  # or gdf.loc[index,'vertex_counter'] =  len(row['geometry'].exterior.coords

print(gdf)
    name        geometry                                    vertex_counter
0   uno  POLYGON ((-0.59330 0.68268, -0.55084 0.32737, ...        8
1   dos  POLYGON ((0.01229 -0.30726, 0.05922 -0.21117, ...        8
2  tres  POLYGON ((0.34749 0.57318, 0.80559 0.60894, 0....        5

But if you look at How to iterate over rows in a DataFrame in Pandas or pandas.DataFrame.iterrows ("You should never modify something you are iterating over") it is preferable to use

The apply function

import geopandas as gpd

gdf = gpd.read_file("polygons.shp")

gdf['vertex_counter'] = gdf.apply(lambda row: len(row.geometry.exterior.coords),axis=1)

print(gdf)
    name        geometry                                    vertex_counter
0   uno  POLYGON ((-0.59330 0.68268, -0.55084 0.32737, ...        8
1   dos  POLYGON ((0.01229 -0.30726, 0.05922 -0.21117, ...        8
2  tres  POLYGON ((0.34749 0.57318, 0.80559 0.60894, 0....        5

The List Comprehensions method

gdf = gpd.read_file("polygons.shp")
n = [len(geom.exterior.coords) for geom in gdf.geometry]
gdf['vertex_counter'] = n
print(gdf)
    name        geometry                                    vertex_counter
0   uno  POLYGON ((-0.59330 0.68268, -0.55084 0.32737, ...        8
1   dos  POLYGON ((0.01229 -0.30726, 0.05922 -0.21117, ...        8
2  tres  POLYGON ((0.34749 0.57318, 0.80559 0.60894, 0....        5

With itertuples() and List Comprehensions:

gdf['vertex_counter'] = [len(i.geometry.exterior.coords) for i in gdf.itertuples()]

With iterrows() and List Comprehensions:

gdf['vertex_counter1'] = [len(row['geometry'].exterior.coords) for index, row in gdf.iterrows()]

Eventually, you can also create a new empty GeoDataFrame and populate it:

gdf = gpd.read_file("polygons.shp")

gdf['vertex_counter'] = 0

new = gpd.GeoDataFrame(columns=gdf.columns)

for index, row in gdf.iterrows():
     row['vertex_counter'] = len(row['geometry'].exterior.coords)
     new = new.append(row)