Python read csv file and store variables

Question

I am pretty new to python, so this should be a fairly easy question. I want to create a bar graph from a csv file. The file contains two columns with the headers "Latitude" and "TempAnom". I want to be able to read in the csv file and store the data as lat and tAnom. What is the best way to accomplish this?

Latitude	TempAnom
-89	0.06997871
-87	0.06997871

This is what I've tried so far, but I end up getting a KeyError: 'Latitude':

    filename='2016_tAnoms.csv'

    lat = []
    tAnom = []
    with open(filename,'r') as csv_file:
      csv_reader = csv.DictReader(csv_file, delimiter=',')
      for row in csv_reader:
        lat.append(row['Latitude'])
        tAnom.append(row['TempAnom'])

Then I should be able to do the following to obtain my bar graph:

    import matplotlib.pyplot as plt

    plt.bar(lat,tAnom)
    plt.title('2016 Temperature Anomalies by Latitude Band')
    plt.xlabel('Latitude')
    plt.ylabel('Temperature Anomaly (°C)')
    plt.show()

Attempt 2: Runs correctly, but graph is missing data

    filename='2016_tAnoms.csv'

    lat = []
    tAnom = []
    with open(filename,'r') as csvfile:
      points = csv.reader(csvfile, delimiter=',')
      next(points)
      for row in points:
        lat.append(int(row[0]))
        tAnom.append(int(float(row[1])))

Produced Graph There should be data from -89 to 89, but there's a lot of blanks and the int(float(row1)) is covering the data to the nearest integer. I want to keep the decimals.

Michael Kucharski · Answer 1 · 2021-03-10T18:47:08.157

1

You are going to want to import csv and declare your variables as lists. Since plt will be able to use the list to display the data.

import matplotlib.pyplot as plt
import csv

lat = []
tAnon = []

From there you can go ahead and open the csv file you have and append your lists with the data in those columns.

with open('filename', 'r') as csvfile:
    points = csv.reader(csvfile, delimiter=',')
    for row in points:
        lat.append(flaot(row[0]))
        tAnon.append(float(row[1]))

Then you can plot your points like you have above. check this out if you are still a bit confused on reading and writing csv. https://realpython.com/python-csv/

edited Mar 10 '21 at 18:47

answered Mar 10 '21 at 17:12

Michael Kucharski

38
7

Sorry about that, try changing the appending tAnon to a float instead of an int. You may need to do the same for the lat append as well. – Michael Kucharski Mar 10 '21 at 18:15
Thanks. Any idea why my produced graph above contains a lot of missing blanks and seems to be rounding the values to the nearest tenth? Many of the values are 7-8 decimal places to the right of the decimal. – Chris Turnbull Mar 10 '21 at 18:28
Without knowing too much about your data I can't help you much there. Found this, hope it works for you https://stackoverflow.com/a/29188910/4797904 – Michael Kucharski Mar 10 '21 at 21:06

score 0 · Answer 2 · answered Mar 10 '21 at 19:16

My bar graph was not displaying the way I wanted it, because I had my data converted to integers when they needed to be decimals. I added "from decimal import Decimal" and changed the int(float(row[1])) to Decimal(row[1]).

    from decimal import Decimal

    filename='2016_tAnoms.csv'

    lat = []
    tAnom = []
    with open(filename,'r') as csvfile:
      points = csv.reader(csvfile, delimiter=',')
      next(points)
      for row in points:
        lat.append(int(row[0]))
        tAnom.append(Decimal(row[1]))

Python read csv file and store variables

2 Answers2