AmagicalFishy - 3 months ago 8

R Question

I have two data tables:

**A**

`animal number`

dog 11

cat 7

pig 5

`animal number`

dog 1

cat 2

pig 2

pig 3

cat 4

dog 2

I would like to subtract the numbers in

`-10`

-5

-3

-2

-3

-9

I'm sure I can do this without having to write a loop, but have no idea how to do it. I've done quite a bit of searching, but however I'm doing so is not fruiting any results. (Is this where something like how

`apply`

Answer

Suppose your data frames are `A`

and `B`

, using `match`

is one possibility:

```
B$number - A$number[match(B$animal, A$animal)]
# [1] -10 -5 -3 -2 -3 -9
```

Should you want to append this to `B`

, use

```
B$diff <- B$number - A$number[match(B$animal, A$animal)]
# animal number diff
#1 dog 1 -10
#2 cat 2 -5
#3 pig 2 -3
#4 pig 3 -2
#5 cat 4 -3
#6 dog 2 -9
```