Conditional collapse of matrix rows with custom function - r

Question

I want to collapse rows in a matrix so that no value of a particular column ever falls below 20. And I want to apply a custom function to the rows to collapse/sum them...
Here is an example matrix:
d <- matrix(data = c(0,105,1,21,2,11,4,5,5,15,7,21,9,1),
ncol = 2,
byrow = TRUE
)
colnames(d) <- c('val','freq')
Looking like this:
d
val freq
[1,] 0 105
[2,] 1 21
[3,] 2 11
[4,] 4 5
[5,] 5 15
[6,] 7 21
[7,] 9 1
The column where the cells must be 20 or above is "freq". So row 1 and 2 are fine, but I need to collapse row 3:5. And I want to replace row 3:5 with the single row from this function:
library(reshape)
replacement <- function(x){
mat <- d[x, ]
mat.res <- untable(mat[ ,c(1, 2)],
num = mat[ ,2]
)
res <- c(mean(mat.res[ ,1]), length(mat.res[ ,1]))
return(res)
}
The function call:
replacement(3:5)
[1] 3.774194 31.000000
Going through the matrix; row 6 is fine, but since row 7 would be left with freq=1 this row needs to be collapsed with row 6. The function call again:
replacement(6:7)
[1] 7.090909 22.000000
The resulting matrix should be:
val freq
[1,] 0 105
[2,] 1 21
[3,] 3.774194 31.000000
[4,] 7.090909 22.000000
The final row numbering is not important.
I have a feeling that the window functions of dplyr might hold the solution, but I need help understanding exactly how. It does not have to be dplyr. I take whatever works ;-)

Answer 1

For future reference, this is not very elegant, but it works...
rows <- dim(d)[1]
tmp <- NULL
inc <- 1
tmpSum <- 0
for(i in 1:rows){
if(d[i, 2] > 19){
tmp <- rbind(tmp, c(d[i, ], inc))
inc <- inc + 1
tmpSum <- 0
} else {
tmp <- rbind(tmp, c(d[i, ], inc))
tmpSum <- d[i,2] + tmpSum
if(tmpSum > 19){
inc <- inc + 1
}
}
}
if(sum(tmp[tmp[ ,3] == max(tmp[ ,3]), 2]) < 19){
tmp[tmp[ ,3] == max(tmp[ ,3]), 3] <- tmp[tmp[ ,3] == max(tmp[ ,3]), 3]-1
}
res <- NULL
for(i in 1:max(tmp[ ,3])){
val <- mean(rep(tmp[tmp[ ,3] == i, 1], tmp[tmp[ ,3] == i, 2]))
freq <- length(rep(tmp[tmp[ ,3] == i, 1], tmp[tmp[ ,3] == i, 2]))
res <- rbind(res, c(val, freq))
}
res