How do I add from a newly created csv file Column?

Question

I would like to create a file in real time and add the values corresponding to the columns to an existing file in real time in the corresponding CSV file.

How can I add each of the CSV files that I generate in that program?

I'll write down the code I'm using now.

import csv

for i in range(10):
SD="Save datas(Angle)"+str(i)  ## 해당 각도별로 배열을 지정

SDArray1=str(SD)               ## 파일을 만들어준다

f=open(SDArray1+".csv","a+t")#  ## 이름을 만들어준 파일을 생성

csv_writer = csv.writer(f)
csv_writer.writerow([SD])
print("One loop has started")
f.close()#

for i in range(1,5):
    cdata=[i]
    f=open(SDArray1+".csv","a+t")

    csv_writer =csv.writer(f)
    csv_writer.writerow(cdata)

    print(cdata)
    f.close()#
    print("loop's finished!")

If you look at the code above, a certain file is created. I completed the next file, but I was wondering how to add columns to the file.

Why is it necessary to keep opening and closing the file within the loop? — strava, Sep 01 '18 at 07:23
Possible duplicate of [How to add a new column to a CSV file?](https://stackoverflow.com/questions/11070527/how-to-add-a-new-column-to-a-csv-file) — strava, Sep 01 '18 at 07:25

Patrick Artner · Accepted Answer · 2018-09-05T05:30:46.627

csv.write_row() takes a complete row of columns - if you need more, add them to your cdata=[i]- f.e. cdata=[i,i*2,i*3,i*4].

You should use with open() as f: for file manipulation, it is more resilient against errors and autocloses the file when leaving the with-block.

Fixed:

import csv

# do not use i here and down below, thats confusing, better names are a plus
for fileCount in range(10):
    filename = "filename{}.csv".format(fileCount) # creates filename0.csv ... filename9.csv 

    with open(filename,"w") as f:#  # create file new
        csv_writer = csv.writer(f)
        # write headers
        csv_writer.writerow(["data1","data2","data3"])
        # write 4 rows of data
        for i in range(1,5):
            cdata=[(fileCount*100000+i*1000+k) for k in range(3)] # create 3 datapoints
            # write one row of data [1000,1001,1002] up to [9004000,9004001,9004002] 
            # for last i and fileCount
            csv_writer.writerow(cdata)

# no file.close- leaving wiht open() scope autocloses

Check what we have written:

import os
for d in sorted(os.listdir("./")):
    if d.endswith("csv"): 
        print(d,":") 
        print("*"*(len(d)+2)) 
        with open(d,"r") as f: 
            print(f.read()) 
        print("")

Output:

filename0.csv :
***************
data1,data2,data3
1000,1001,1002
2000,2001,2002
3000,3001,3002
4000,4001,4002


filename1.csv :
***************
data1,data2,data3
101000,101001,101002
102000,102001,102002
103000,103001,103002
104000,104001,104002

filename2.csv :
***************
data1,data2,data3
201000,201001,201002
[...snip the rest - you get the idea ...]     

filename9.csv :
***************
data1,data2,data3
901000,901001,901002
902000,902001,902002
903000,903001,903002
904000,904001,904002

To add a new column to an existing file:

open old file to read
open new file to write
read the old files header, add new column header and write it in new file
read all rows, add new columns value to each row and write it in new file

Example:

Adding the sum of column values to the file and writing as new file:

filename = "filename0.csv"
newfile =  "filename0new.csv"

# open one file to read, open other (new one) to write
with open(filename,"r") as r, open(newfile,"w") as w:
    reader = csv.reader(r)
    writer = csv.writer(w)

    newHeader = next(reader)   # read the header
    newHeader.append("Sum")    # append new column-header
    writer.writerow(newHeader) # write header

    # for each row: 
    for row in reader:
        row.append(sum(map(int,row)))   # read it, sum the converted int values
        writer.writerow(row)            # write it

# output the newly created file:
with open(newfile,"r") as n:
    print(n.read())

Output:

data1,data2,data3,Sum
1000,1001,1002,3003
2000,2001,2002,6003
3000,3001,3002,9003
4000,4001,4002,12003

Thank you for your opinion.But what I want to know is how do I add Columns value to a newly created file when there is an existing file and a new one is created? — 임종훈, Sep 01 '18 at 08:18
This is how it works. But what I really want to do is measure data in real time and save it as a csv file and merge it into one and draw a graph. What should I do? — 임종훈, Sep 02 '18 at 09:52
@임종훈 Writing, reading, adding to it, ... with files makes this IO-bound. Not a good idea, as IO is slow. Why not simply do it in memory and write your CSV after you took all measurements? how many datapoints are we talking about? — Patrick Artner, Sep 02 '18 at 09:55
I want to thank you for that. As I thought about the data size growing in the current job, I selected to create a file and combine the csv file using the pandas. Thank you for your time. — 임종훈, Sep 03 '18 at 16:23

How do I add from a newly created csv file Column?

1 Answers1