How to add two columns and update the CSV in python?

Question

I have a question here:

Write a function named computed_column that takes a string as a parameter representing the name of a CSV file with 5 columns in the format <string>,<int>,<int>,<int>,<int> and writes a file named creation.csv containing all the data from the input file but with a sixth column containing the sum of the values from the third and second columns.

This is what I tried:

    import csv
    def computed_column(csvfile):
    with open(csvfile,newline='') as f:
      with open('creation.csv','w',newline='') as f2:
        y=[]
        writer = csv.writer(f2)
        rows = csv.reader(f)
        for row in rows:
          for i in range(0,len(row[1])):
            y.append(int(row[1][i]) + int(row[2][i]))
          writer.writerow(row+y)

It writes the original file correctly but gives me wrong output for the sixth column that is to be written.

Kevin · Accepted Answer · 2019-10-27 14:40:41Z

1

If I'm understanding your question correctly, you don't need the inner for-loop. For each row, you simply want to add the values in the second and third column (of that row), and insert the sum as the sixth column value. You just need to loop through each row, then use the row index to access the second and third columns.

Also, your list y is growing with each row you process (which adds an additional column each time). I don't think that is your intention. Move this within your for-loop so that it resets to an empty list with each row. Try something like this:

def computed_column(csvfile):
    with open(csvfile,newline='') as f:
        with open('creation.csv','w',newline='') as f2:
            writer = csv.writer(f2)
            rows = csv.reader(f)
            for row in rows:
                y=[]
                y.append(int(row[1]) + int(row[2]))
                writer.writerow(row+y)

edited Oct 27, 2019 at 14:40

answered Oct 27, 2019 at 14:34

Kevin

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