R: if function with two conditions? - r

Question

I have a huge data frame. I am stuck with if function. Let me first present the simple example and then I lay down my problem:
z <- c(0,1,2,3,4,5)
y <- c(2,2,2,3,3,3)
a <- c(1,1,1,2,2,2)
x <- data.frame(z,y,a)
Problem: I want to run if function which sums column z values based for row which has same y and a only if the second row of each group has corresponding z equals 1
I am sorry but I am quite new in R so not able to present any reasonable codes which I have done by my own.
Any help would be highly appreciated.

Answer 1

As mentioned, your problem isn't clearly stated.
Perhaps you are looking to do something like this:
x$new <- with(x, ave(z, y, a, FUN = function(k)
ifelse(k[2] == 1, sum(k), NA)))
x
# z y a new
# 1 0 2 1 3
# 2 1 2 1 3
# 3 2 2 1 3
# 4 3 3 2 NA
# 5 4 3 2 NA
# 6 5 3 2 NA
Here, I've created a new column "new" which sums the values of "z" grouped by "y" and "a", but only if the second value in the group is equal to 1.

Answer 2

Since you say your data frame is quite large, you might want to convert your data frame to a data.table object using the data.table package. You will likely find that the required operations are much faster if you have a great many rows. However, the construction of the code for your case is not straight forward with data.table.
If I understnad what you want to do (which is not entirely clear to me) you could try the following:
library(data.table)
z <- c(0,1,2,3,4,5)
y <- c(2,2,2,3,3,3)
a <- c(1,1,1,2,2,2)
x <- data.frame(z,y,a)
xx <- as.data.table(x) # Make a data.table object
setkey(xx, z) # Make the z column a key
xx[1, sum(a)] # Sum all values in column a where the key z = 1
[1] 1
# Now try the other sum you mention
xx[, sum(z), by = list(z = y)] # A column sum over groups defined by z = y
z V1
1: 2 2
2: 3 3
sum(xx[, sum(z), by = list(z = y)][, V1]) # Summing over the sums for each group should do it
[1] 5
To create the sum over the column a where z = 1, I made the z column a key. The syntax xx[1, sum(a)] sums a where the key value (z value) is 1.
I can create groups with the data.table object with by, which is analogous to a SQL WHERE clause if you are familiar with SQL. However, the result is the sum of the column z for each of groups created. This may be inefficient if you have a great many possible matching values where z = y. The outer sum adds the values for each group in the sub-selected V1 column of the inner result.
If you are going to use data.table in a serious way study the informative vignettes available for that package.
M Dowle, T Short, S Lianoglou, A Srinivasan with contributions from R Saporta and E Antonyan (2014). data.table: Extensions of data.frame. R package version 1.9.2. http://CRAN.R-project.org/package=data.table