MFR - 5 months ago 38

R Question

I wish to sum each group with a condition. I have the following data

`a<-data.table (facto=c ("a","b","a","a","b","b","b"), value = c (2,3,2,3,2,2,2))`

facto value

1: a 2

2: b 3

3: a 2

4: a 3

5: b 2

6: b 2

7: b 2

and I'd like to compute the sum of facto which their value is not equal to 3 using

`data.table`

I used this code

`output <-a[, (value=sum(value!=3)), .(facto)]`

and it gives me the following data that counts the row which are not equal to zero for each group:

`facto V1`

1: a 2

2: b 3

However, I wish to have the following output:

`facto V1`

1: a 4

2: b 6

Answer

We can specify the logical condition in `i`

and get the `sum`

of 'value' by `facto`

```
a[value!=3, sum(value), by = facto]
# facto V1
#1: a 4
#2: b 6
```

Also, the OP's code is getting the `sum`

of logical index and not the `value`

after subsetting

```
a[, (value=sum(value[value!=3])), .(facto)]
```