Calculate sum of one column based on another column

Question

I have a data frame:

Y  X1  X2  X3
1   1   0  1
1   0   1  1
0   1   0  1
0   0   0  1
1   1   1  0
0   1   1  0

I want sum over all rows in Y column based on other columns that equal to 1, which is sum(Y=1|Xi =1). For example, for column X1, s1 = sum(Y=1|Xi =1) =1 + 0 +1+0 =2

Y  X1   
1   1   

0   1    

1   1    
0   1   

For X2 column, the s2 = sum(Y=1|Xi =1) = 0 +1+0 =1

    Y   X2  

    0   1   

    1   1    
    0   1     

For X3 column, the s3 = sum(Y=1|Xi =1) = 1+1 +0+0 =2

    Y    X3
    1    1
    1    1
    0    1
    0    1

I have a rough idea to use apply(df, 2, sum) for the column of the dataframe, but I have no idea how to subset each column based on Xi, then calculate the sum of Y. Any help is appreciated!

Answer 1

There are numerous ways to do this. One is getting a subset based on the column you want:

sum(df[df$X1==1,]$Y)

This should work for you.