Assigning values to dataframe columns

Question

In the below code, the dataframe df5 is not getting populated. I am just assigning the values to dataframe's columns and I have specified the column beforehand. When I print the dataframe, it returns an empty dataframe. Not sure whether I am missing something.

Any help would be appreciated.

import math    
import pandas as pd

columns = ['ClosestLat','ClosestLong']

df5 = pd.DataFrame(columns=columns)

def distance(pt1, pt2):
  return math.sqrt((pt1[0] - pt2[0])**2 + (pt1[1] - pt2[1])**2)

for pt1 in df1:
   closestPoints = [pt1, df2[0]]
   for pt2 in df2:
     if distance(pt1, pt2) < distance(closestPoints[0], closestPoints[1]):
       closestPoints = [pt1, pt2]
       df5['ClosestLat'] = closestPoints[1][0]
   df5['ClosestLat'] = closestPoints[1][0]
   df5['ClosestLong'] = closestPoints[1][1]
   print ("Point: " + str(closestPoints[0]) + " is closest to " + str(closestPoints[1]))

Please define df1 and df2.

bpachev
– bpachev

2016-08-21 01:06:30 +00:00
Commented Aug 21, 2016 at 1:06 — bpachev
– bpachev, Commented Aug 21, 2016 at 1:06

bpachev · Accepted Answer · 2016-08-21 01:21:10Z

From the look of your code, you're trying to populate df5 with a list of latitudes and longitudes. However, you're making a couple mistakes.

The columns of pandas dataframes are Series, and hold some type of sequential data. So df5['ClosestLat'] = closestPoints[1][0] attempts to assign the entire column a single numerical value, and results in an empty column.
Even if the dataframe wasn't ignoring your attempts to assign a real number to the column, you would lose data because you are overwriting the column with each loop.

The Solution: Build a list of lats and longs, then insert into the dataframe.

import math    
import pandas as pd

columns = ['ClosestLat','ClosestLong']

df5 = pd.DataFrame(columns=columns)

def distance(pt1, pt2):
  return math.sqrt((pt1[0] - pt2[0])**2 + (pt1[1] - pt2[1])**2)

lats, lngs = [], []
for pt1 in df1:
   closestPoints = [pt1, df2[0]]
   for pt2 in df2:
     if distance(pt1, pt2) < distance(closestPoints[0], closestPoints[1]):
       closestPoints = [pt1, pt2]
   lats.append(closestPoints[1][0])
   lngs.append(closestPoints[1][1])

df['ClosestLat'] = pd.Series(lats)
df['ClosestLong'] = pd.Series(lngs)

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