why value doesn't replace with NA in Tibble column cell

tidyverse
, ,
9

You did not provide a suitable dataset reprex and therefore I made a small version of your continuous_dataset with only one column.
And I adapted the function to replace the column (your version adds a category and overwrites that for each column). See if this works for you:

suppressPackageStartupMessages(
  suppressWarnings(
    {library(dplyr)
     library(truncnorm)
    }
  )
)

set.seed(2022)

continuous_dataset <- data.frame(
  col1 = c(1,NA,3,NA,5)
) %>%
  print()
#>   col1
#> 1    1
#> 2   NA
#> 3    3
#> 4   NA
#> 5    5

ff=function(df1,colnumber,minn,maxx,meann,sdd){
   for(i in 1:nrow(df1)){
      a <- round(rtruncnorm(1,minn,maxx,meann,sdd))
      # df1=df1 %>% mutate(category = if_else((is.na(df1[[i,colnumber]])), a,df1[[i,colnumber]]))
      b <- df1[[i,colnumber]]
      df1[[i,colnumber]] <- if_else(is.na(b), a, b)
   }
   return(df1)
}

continuous_dataset2=ff(continuous_dataset,1,120,150,88,54) %>% 
  print()
#>   col1
#> 1    1
#> 2  124
#> 3    3
#> 4  122
#> 5    5
Created on 2022-03-12 by the reprex package (v2.0.1)