Adding a column to a .csv in MATLAB - matlab

Question

I have a nxn .csv file in which I am finding the cumulative sum of one column. I need to append this column with a header cumsum to the end of the existing .csv file to make it nx(n+1). How could this be done? I am attaching a samaple:
filename A B
aa 23 34
aa 56 98
aa 8 90
aa 7 89
I am finding the cumsum of column A
23
79
87
94
I need this column appended to the end of .csv as
filename A B cumsum
aa 23 34 23
aa 56 98 79
aa 8 90 87
aa 7 89 94
I have 2 problems here:
1. I am extracting the column A everytime to perform the cumsum operation. How do I find it directly from the table for a single column without extraction?
How do I create a new column at the end of the existing table to add the cumsum column with a header 'cumsum'?

Answer 1

For point 1: You can use csvread to read a specific column directly from a .csv file without loading the whole thing. For your example, you would do this:
A = csvread('your_file.csv', 1, 1, [1 1 nan 1]);
The nan allows it to read all rows until the end (although I'm not sure this is documented anywhere).
The use of csvread is applicable to files containing numeric data, although it works fine for the above example even with character entries in the first row and first column of the .csv file. However, it appears to fail if the part of your file that you want to read is followed by columns containing character entries. A more general solution using xlsread is as follows:
A = xlsread('your_file.csv', 'B:B');
For point 2: Built-in functions like csvwrite or dlmwrite don't appear able to append new columns, just new rows. You can however use xlswrite, even though it is a .csv file. Here's how it would work for your example:
xlswrite('your_file.csv', [{'cumsum'}; num2cell(cumsum(A))], 1, 'D1');
And here's what the contents of your_file.csv would look like:
filename,A,B,cumsum
aa,23,34,23
aa,56,98,79
aa,8,90,87
aa,7,89,94