Question

I have a two dimensional array, which represents columns and rows of data. I need to sum both the columns and rows, but I need to total from the the new ‘summary’ row.

Data (6×5 array)

[1, 0, 3, 0, 0], 
[0, 4, 0, 0, 4], 
[0, 0, 0, 0, 0], 
[0, 0, 0, 0, 0], 
[0, 0, 0, 0, 0], 
[0, 0, 0, 0, 0]

so the result should be a 7×6 array

[1, 0, 3, 0, 0, 4], 
[0, 4, 0, 0, 4, 8], 
[0, 0, 0, 0, 0, 0], 
[0, 0, 0, 0, 0, 0], 
[0, 0, 0, 0, 0, 0], 
[0, 0, 0, 0, 0, 0],
[1, 4, 3, 0, 4, 12]

I know I can sum each column and add an additional row to my two dimensional array via

# Sum the columns, add additional one row for summary
a << a.transpose.map{|x| x.reduce(:+)}

but how do I add the additional column

Answer 1

a.map! {|row| row + [row.reduce(:+)]}

map! takes each element of the array, passes it to the block and replaces that element with whatever that block returns. So since we call it on a 2d array, row will be a 1d array – the row of the original array.

Then I calculate the sum with reduce(:+) of that row. Then I need to append it to that row. What I’ve done here is to wrap the result of sum into an array and then used + to concatenate those two arrays.

I could have also done this:

a.map! {|row| row << row.reduce(:+) }