R Optimizing double for loop, matrix manipulation

Question

I am trying to manipulate column data in a two column matrix and output it as a data.frame.

The matrix that I have is in this format where both the values in the start and end columns are increasing and don't overlap. Also, there are always more Start entries than there are End entries.

Suppose I start with this matrix:

#       Start   End
#  [1,]     1     6
#  [2,]     2     9
#  [3,]     3    15
#  [4,]     7    NA
#  [5,]     8    NA
#  [6,]    11    NA
#  [7,]    12    NA
#  [8,]    14    NA

I want this double for loop to output a data.frame that groups all Start values less than an End value and associates it with that End value.

To clarify I want to output this:

#       Start   End
#  1    1,2,3     6
#  2      7,8     9
#  3 11,12,14    15

I tried a double for loop but I need something faster because I want to use this method on a larger matrix ~5 MB.

start_end <- matrix(c(1, 6, 2, 9, 3, 15, 7, NA, 8, NA, 11, NA, 12, NA, 14, NA), 
  nrow=8, 
  ncol=2)

# of non NA rows in column 2
non_nacol <- sum(is.na(start_end[,2]))

sorted.output <- data.frame(matrix(NA, nrow = nrow(start_end), ncol = 0))
sorted.output$start <- 0
sorted.output$end <- 0

#Sort and populate data frame
for (k in 1:non_nacol) {
  for (j in 1:nrow(start_end)) {
        if (start_end[j,1]<start_end[k,2]) {
        S <- (start_end[j,1])
        E <- (start_end[k,2])
        sorted.output$start[j] <- S
        sorted.output$end[j] <- E
        }
  }
}

Thanks for the help!

Answer 1

Here's a solution built around findInterval(), split(), and paste():

m <- matrix(c(1,2,3,7,8,11,12,14,6,9,15,NA,NA,NA,NA,NA),ncol=2,dimnames=list(NULL,c('Start','End')));
data.frame(Start=sapply(split(m[,'Start'],findInterval(m[,'Start'],na.omit(m[,'End']))),paste,collapse=','),End=na.omit(m[,'End']));
##      Start End
## 0    1,2,3   6
## 1      7,8   9
## 2 11,12,14  15

---

Edit: The problem you encountered was due to the fact that in your real data some intervals between input End values do not contain any input Start values. My solution above is incorrectly omitting those intervals from the output Start vector, which causes a length mismatch against the output End vector.

Here is a fixed solution:

end <- na.omit(m[,'End']);
data.frame(Start=unname(sapply(split(m[,'Start'],findInterval(m[,'Start'],end))[as.character(0:c(length(end)-1))],paste,collapse=',')),End=end);
##      Start End
## 1    1,2,3   6
## 2      7,8   9
## 3 11,12,14  15

Here's a demonstration on a test matrix that has an empty interval:

m <- matrix(c(1,2,3,11,12,14,6,9,15,NA,NA,NA),ncol=2,dimnames=list(NULL,c('Start','End')));
m;
##      Start End
## [1,]     1   6
## [2,]     2   9
## [3,]     3  15
## [4,]    11  NA
## [5,]    12  NA
## [6,]    14  NA
end <- na.omit(m[,'End']);
data.frame(Start=unname(sapply(split(m[,'Start'],findInterval(m[,'Start'],end))[as.character(0:c(length(end)-1))],paste,collapse=',')),End=end);
##      Start End
## 1    1,2,3   6
## 2            9
## 3 11,12,14  15

As you can see, for an empty interval, the value that results in the output Start vector is the empty string, which I consider a sensible result. You can change the result afterward if desired.

Finally, here's a demo using the real data you posted to dropbox:

m <- read.table('start_end.txt',col.names=c('Start','End'));
head(m);
##   Start   End
## 1 11165 10548
## 2 12416 11799
## 3 12466 11900
## 4 12691 11976
## 5 12834 13336
## 6 13320 14028
end <- na.omit(m[,'End']);
system.time({ out <- data.frame(Start=unname(sapply(split(m[,'Start'],findInterval(m[,'Start'],end))[as.character(0:c(length(end)-1))],paste,collapse=',')),End=end); });
##    user  system elapsed
##  21.234   0.015  21.251
head(out);
##                           Start   End
## 1                               10548
## 2                         11165 11799
## 3                               11900
## 4                               11976
## 5 12416,12466,12691,12834,13320 13336
## 6       13425,13571,13703,13920 14028
nrow(out);
## [1] 131668