CODEWITHSUNDEEP
r
Polla A. Fattah
Polla A. Fattah

Reputation: 829

Finding the largest number in a vector which is smaller than specific value

I want to find the largest grade of the failed subjects for students in R. I have written this function which works fine but it looks more like java code than R as it is traversing through vectors using loops instead of using special functions for that purpose in R. would you please suggest an alternative solution

grades <- c(62, 100, 45, 40, 46, 55, 56, 70)

largestFail <- function(grades){
  location <- -1
  faildFound <- FALSE

  for(i in 1:length(grades)){
    if(grades[i] < 50){
      if(!faildFound) location <- i
      if(grades[i] > grades[location]) location <- i
      faildFound <- TRUE
    }
  }
  return (location)
}
print (grades) # 5

Upvotes: 2

Views: 2743

Answers (2)

Alexander Engelhardt
Alexander Engelhardt

Reputation: 1712

This works fine: 

max(grades[grades < 50])

You subset the grades vector to those values under 50, and then extract the maximum of it.

If you need detailed understanding, you can evaluate the parts separately:

  • grades < 50 returns a logical vector which is TRUE for all values under 50
  • grades[grades < 50] then is a vector of all values under 50
  • Finally, wrap max() around this vector to find the highest failing grade

If you want to return -1 when no student failed, something like this should do the job:

grades <- c(60, 70, 80)

if(any(grades < 50)){
  return(max(grades[grades < 50])
)
} else {
  return(-1)
}

A shorter, but less clear, solution, would be to just append -1 to your vector of grades:

grades <- c(60, 70, 80)
grades <- c(grades, -1)
max(grades[grades < 50])

Upvotes: 8

Cath
Cath

Reputation: 24074

Another way, using which.max but replacing the values above the threshold by NA prior to the call:

which.max(replace(grades, grades>=50, NA))
#[1] 5

Upvotes: 3

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