Split row containing range into multiple rows by any number

Question

Given a data.frame where start and end represents ranges.

id   start   end
 1       3    51
 2      20    28

I'm trying to split the rows into multiple rows if the range contains another number or sequence of numbers and grouping them, for example by 25

id   start   end  splitGroup
 1       3    25           0
 1      25    51          25
 2      20    25           0
 2      25    28          25

Functioning similarly to splitting by a regular sequence here using the plyr package

df <- data.frame(
  id    = c(1:2),
  start = c(3,20),
  end   = c(51,28)
)

splitBy <- 20

rowSplit <- function(df, splitBy){

  newDf <- ddply(df, .(id), function(x){
    data.frame(
      id = x$id,
      start = x$start,
      end = x$end,
      splitGroup = seq(
        floor(x$start/splitBy)*splitBy, 
        floor(x$end/splitBy)*splitBy, 
        by=splitBy
      )
    )
  })

  newDf <- within(newDf, {
    start <- ifelse(
      floor(start/splitBy)*splitBy == splitGroup,
      start, 
      splitGroup 
    )
    end <- ifelse( 
      end < (splitGroup + splitBy), 
      end,  
      (splitGroup + splitBy)
    )
  })  

  return(newDf)
}

rowSplit(df, splitBy)

id  start   end   splitGroup
 1      3    20            0
 1     20    40           20
 1     40    51           40
 2     20    28           20

How can this be done using any singel number or irregular set of numbers

Answer 1

Here's a start at using a mod function:

 smod <- df$start%/%25   # 0 0
 emod<-df$end%/%25      # 2 1
 newstart<-numeric(0)
 matchit<-25*(1:100) # or at least extend to maximum value in your dataframe
 for (j in 1:2) { newstart<-c(newstart,df$start[j])
    if(emod[j]>0) newstart<-c(newstart, min(matchit[matchit>df$start[j]])) }

Rgames> newstart
[1]  3 25 20 25

Calculate newend in a similar manner and you should be set.