Sum values in a column based on a certain criteria in R

Question

I have a dataframe that looks like this:

   x1 x2  a b c 
1   7  5  1 0 0
2   3  6  1 0 0
3   2 -1  0 1 0
4   0  1  0 1 0
5   0  6  1 0 0
6   8 10  0 0 1
7  14  8  0 0 1
8  12 11  0 0 1
9  11 13  0 0 1
10 10 12  0 0 1

and I would like to sum of the values for x1 when a=1. I was thinking I could use a if statement for this and have something like follows:

if(X$a==0){
   result = sum(X$a)
}

I would like to have an output like the following:

result = 10

What would be the best way to achieve this?

Answer 1

You could also use dplyr for this. The code would be:

library(dplyr)
df<-data.frame(x1=c(7,3,2,0,0,8,14,12,11,10),
               a = c(1,1,0,0,1,0,0,0,0,0))
df %>%
    summarise(result = sum(x1[a==1]))

You push the df in the pipeline, and ask to summarise in a variable called "result" which is the sum of column x1, but only where a==1.

Also with dplyr, but with a more explicit subsetting:

df %>%
    filter(a==1) %>%
    summarise(result=sum(x1))

First filter the rows where a == 1, then sum up the remaining x1.

Another way without dplyr:

df$result<-df$x1*df$a
colSums(df)[3]

This works because the values in column a are 1 or zero, so the product will be zero when there is a zero in the column a.