Replacing values in one column with another based on a 3rd column matching a 4th

Question

I\'m working with the following example:

Original     Modified     New_Orig     New
1            2            1            0
2            4            1              


        

Answer 1

Here, a new column New is created:

within(dat, New <- Modified*(New_Orig == Original))

   Original Modified New_Orig New
1         1        2        1   2
2         2        4        1   0
3         3        6        4   0
4         4        8        5   0
5         5       10        5  10
6         6       12        5   0
7         7       14        5   0
8         8       16        5   0
9         9       18        9  18
10       10       20       10  20

Update

Match values and choose appropriate value from Modified:

within(dat, New <- Modified[match(New_Orig, Original)])

   Original Modified New_Orig New
1         1        2        1   2
2         2        4        1   2
3         3        6        4   8
4         4        8        5  10
5         5       10        5  10
6         6       12        5  10
7         7       14        5  10
8         8       16        5  10
9         9       18        9  18
10       10       20       10  20