while and for in R-CodePudding

I want to creat an alogrithm in R using while and for such that, given a vector V and an integer M, sums the elements of the vector V until that sum exceeds M, and I want it to inform the value that exceeded M as well as how many repetitions it took in order to exceed such number (i.e., how many elements of the vector did it summed).

CodePudding user response：

Here are three functions.

fun uses vectorised cumsum to sum the vector;
The other two have self-descriptive names.

They all return a named vector with the iterations count and the first sum exceeding m.

fun <- function(v, m) {
  i <- which(cumsum(v) < m)
  if(length(i)   1L < length(v))
    i <- c(i, length(i)   1L)
  c(Iter = length(i), Sum = sum(v[i]))
}
fun_while <- function(v, m) {
  Sum <- 0L
  Iter <- 0L
  while(Iter < length(v)) {
    Iter <- Iter   1L
    Sum <- Sum   v[Iter]
    if(Sum > m) break
  }
  c(Iter = Iter, Sum = Sum)
}
fun_for <- function(v, m) {
  Sum <- 0L
  for(i in seq_along(v)) {
    Sum <- Sum   v[i]
    if(Sum > m) break
  }
  if(is.null(i))   i <- 0L
  c(Iter = i, Sum = Sum)
}

V <- 1:5
M <- 7
fun(V, M)
#> Iter  Sum 
#>    4   10
fun_while(V, M)
#> Iter  Sum 
#>    4   10
fun_for(V, M)
#> Iter  Sum 
#>    4   10

V <- 1:5
M <- 20
fun(V, M)
#> Iter  Sum 
#>    5   15
fun_while(V, M)
#> Iter  Sum 
#>    5   15
fun_for(V, M)
#> Iter  Sum 
#>    5   15

V <- 1:10
M <- 42
fun(V, M)
#> Iter  Sum 
#>    9   45
fun_while(V, M)
#> Iter  Sum 
#>    9   45
fun_for(V, M)
#> Iter  Sum 
#>    9   45

V <- NULL
M <- 7
fun(V, M)
#> Iter  Sum 
#>    0    0
fun_while(V, M)
#> Iter  Sum 
#>    0    0
fun_for(V, M)
#> Iter  Sum 
#>    0    0

^{Created on 2022-08-25 by the reprex package (v2.0.1)}

CodePudding user response：

both looping and vectorial way. benchmark added for curiosity.

library(data.table)
library(microbenchmark)
library(ggplot2)

val <- 2e6
v <- seq(1, 1e6, 2)

f.vec <- function(v, val) {
  dt <- as.data.table(v)
  dt[, cs := cumsum(v)]
  idx <- dt[cs <= val, .N]
  valnext <- dt[idx 1, .(v)]
  list(rep = idx, valnext = valnext)
}

f.loop <- function(v, val) {
  idx <- 1
  total <- 0
  for (el in v) {
    total <- total   el
    if (total >= val) {
      break
    } 
    idx <- idx   1
  }
  list(rep = idx-1, valnext = v[idx])
}

f.vec(v, val)
#> $rep
#> [1] 1414
#> 
#> $valnext
#>       v
#> 1: 2829
f.loop(v, val)
#> $rep
#> [1] 1414
#> 
#> $valnext
#> [1] 2829

tm <- microbenchmark(f.vec(v, val), f.loop(v, val), times=1000L)
autoplot(tm)
#> Coordinate system already present. Adding new coordinate system, which will replace the existing one.

tm
#> Unit: microseconds
#>            expr    min     lq      mean median     uq     max neval cld
#>   f.vec(v, val) 4848.5 5163.9 7856.0267 5643.6 9146.1 66502.9  1000   b
#>  f.loop(v, val)   72.7   75.8   81.1795   79.1   81.2   255.2  1000  a

^{Created on 2022-08-25 with reprex v2.0.2}