Only Store Successful Output: A Counter Placement Issue

Sometimes I store every result in my initialized vector/matrix.

Here is the data.

  people    values day
1   john  9.991020   1
2  teddy  8.052606   1
3  clare  9.213088   1
4   john  8.819214   2
5  teddy  8.364933   2
6   john 10.670680   3
7  teddy  9.233042   3
8  clare 11.330505   3

Now the code. I want to find the days where I have responses from John, Teddy, and Clare (as you can tell, I only have responses from all three of them on days 1 and 3).

use_days <- numeric(length(unique(df$days))) # initialized vector
counter <- 0

select_days <- c(1, 2, 3) 

for(i in 1:length(select_days)){
  counter <- counter + 1
  
  
  # select the i-th day
  
  filter_data <- df %>%
    filter(day == select_days[i])
  
  # are there three responses on this day?
  
  if(length(filter_data$day) == 3){ 
  use_days[counter] <- filter_data$day
  }
}

use_days

[1]  1 NA  3

That code works, but what if I don’t want to store that NA during the second iteration? To only store successful output, put the counter in the “if statement.”

use_days <- numeric(length(unique(df$days))) # initialized vector
counter <- 0

select_days <- c(1, 2, 3) 

for(i in 1:length(select_days)){
  
  # select the i-th day
  
  filter_data <- df %>%
    filter(day == select_days[i])
  
  # are there three responses on this day?
  
  if(length(filter_data$day) == 3){ 
      counter <- counter + 1            # HERE IS THE CHANGE

  use_days[counter] <- filter_data$day
  }
}

use_days

[1] 1 3

Bo\(^2\)m =)