The sample data set of available much bigger data set is in following format:
Station <-c("A","A","A","A","A","A","A","A","A","A","A","A","A","A","A")
Parameter <-c(2,3,NA,4,4,9,NA,NA,10,15,NA,NA,NA,18,20)
Par_Count <-c(1,1,1,2,2,1,2,2,1,1,3,3,3,1,1)
df<-data.frame(Station, Parameter, Par_Count)
df
Station Parameter Par_Count
A 2 1
A 3 1
A NA 1
A 4 2
A 4 2
A 9 1
A NA 2
A NA 2
A 10 1
A 15 1
A NA 3
A NA 3
A NA 3
A 18 1
A 20 1
I want to approximate NA's which are less than 2 in number with average of next and previous available values for NA in that column. In original data set somewhere NA's are 100's in number, so I want to ignore consecutive NA's greater than 3 in number. Par_Count represent number of consecutive occurrences of that particular value in parameter.
I tried with:
library(zoo)
df1 <- within(df, na.approx(df$Parameter, maxgap = 2))
and even for for single occurence with:
df1 <- within(df, Parameter[Parameter == is.na(df$Parameter) & Par_Count == 1] <-
lead(Parameter) - lag(Parameter))
but nothing worked. It didn't change any occurrence of NA value.
The desired output is like:
Station Parameter Par_Count
A 2 1
A 3 1
A 3.5 1
A 4 2
A 4 2
A 9 1
A 9.5 2
A 9.75 2 <--here 9.5 will also work
A 10 1
A 15 1
A NA 3
A NA 3
A NA 3
A 18 1
A 20 1
You are nearly there. I think you have misinterpreted the use of within. If you would like to use within, You need to assign the output of na.approx to a column of the data frame. The following will work:
library(zoo)
df1 <- within(df, Parameter <- na.approx(Parameter, maxgap = 2, na.rm = FALSE))
Note it is advisable to use na.rm = FALSE, otherwise leading or trailing NAs will be removed, leading to an error.
Personally, I think the following is more readable, though it is a matter of style.
library(zoo)
df1 <- df
df1$Parameter <- na.approx(df$Parameter, maxgap = 2, na.rm = FALSE))