How can I calculate an inner product with an arbitrary number of columns using ddply? - r

Question

I want to perform an inner product of the first D columns for each row in a data frame with a given array, W. I am trying the following:
W = (1,2,3);
ddply(df, .(id), transform, inner_product=c(col1, col2, col3) %*% W);
This works but I typically may have an arbitrary number of columns. Can I generalize the above expression to handle that case?
Update:
This is an updated example as asked for in the comments:
libary(kernlab);
data(spam);
W = array();
W[1:3] = seq(1,3);
spamdf = head(spam);
spamdf$id = seq(1,nrow(spamdf));
df_out=ddply(spamdf, .(id), transform, inner_product=c(make, address, all) %*% W);
> W
[1] 1 2 3
> spamdf[1,]
make address all num3d our over remove internet order mail receive will
1 0 0.64 0.64 0 0.32 0 0 0 0 0 0 0.64
people report addresses free business email you credit your font num000
1 0 0 0 0.32 0 1.29 1.93 0 0.96 0 0
money hp hpl george num650 lab labs telnet num857 data num415 num85
1 0 0 0 0 0 0 0 0 0 0 0 0
technology num1999 parts pm direct cs meeting original project re edu table
1 0 0 0 0 0 0 0 0 0 0 0 0
conference charSemicolon charRoundbracket charSquarebracket charExclamation
1 0 0 0 0 0.778
charDollar charHash capitalAve capitalLong capitalTotal type id
1 0 0 3.756 61 278 spam 1
> df_out[1,]
make address all num3d our over remove internet order mail receive will
1 0 0.64 0.64 0 0.32 0 0 0 0 0 0 0.64
people report addresses free business email you credit your font num000
1 0 0 0 0.32 0 1.29 1.93 0 0.96 0 0
money hp hpl george num650 lab labs telnet num857 data num415 num85
1 0 0 0 0 0 0 0 0 0 0 0 0
technology num1999 parts pm direct cs meeting original project re edu table
1 0 0 0 0 0 0 0 0 0 0 0 0
conference charSemicolon charRoundbracket charSquarebracket charExclamation
1 0 0 0 0 0.778
charDollar charHash capitalAve capitalLong capitalTotal type id inner_product
1 0 0 3.756 61 278 spam 1 3.2
The above example performs a inner product of the first three dimensions with an array W=(1,2,3) of the spam data set available in kernlab package. Here I have explicity specified the first three dimensions as c(make, address, all).
Thus df_out[1,"inner_product"] = 3.2.
Instead I want to perform the inner product over all the dimensions without having to list all the dimensions. The conversion to a matrix and back to a data frame seems to be an expensive operation?

Answer 1

A strategy along the lines of the following should work:
Convert each chunk to a matrix
Perform a matrix multiplication
Convert results to data.frame
The code:
set.seed(1)
df <- data.frame(
id=sample(1:5, 20, replace=TRUE),
col1 = runif(20),
col2 = runif(20),
col3 = runif(20),
col4 = runif(20)
)
W <- c(1,2,3,4)
ddply(df, .(id), function(x)as.data.frame(as.matrix(x[, -1]) %*% W))
The results:
id V1
1 1 4.924994
2 1 5.076043
3 2 7.053864
4 2 5.237132
5 2 6.307620
6 2 3.413056
7 2 5.182214
8 2 7.623164
9 3 5.194714
10 3 6.733229
11 4 4.122548
12 4 3.569013
13 4 4.978939
14 4 5.513444
15 4 5.840900
16 4 6.526522
17 5 3.530220
18 5 3.549646
19 5 4.340173
20 5 3.955517

Answer 2

If you want to append a column of cross-products, you could do this (assuming W had the right number of elements to match the non-"id" columns:
df2 <- cbind(df, as.matrix(df[, -grep("id", names(df))]) %*% W )
It does not appear that the .(id) serves any useful purpose, since you are not do a sum of crossproducts within id, and if you were then you wouldn't be using transform but some other aggregating function.