How to duplicate rows while using mutate in R?-CodePudding

I'm currently trying to add multiple columns to my dataframe using mutate.

This is what I started with:

new_df <- old_df %>% 
  mutate(demo = c("audience 1", "audience 2")

Here's what I wanted to happen

game demo
1    audience 1
1    audience 2
2    audience 1
2    audience 2

However this just results in an error. I know this is just my bad beginner syntax, can someone help?

CodePudding user response：

I'm assuming your old_df has just rows, and you'd like to add both values of demo for each value of game. If I've got that right, you can use tidyr::expand_grid():

library(tidyr)

old_df <- data.frame(game = 1:2)

old_df %>% 
  expand_grid(demo = c("audience 1", "audience 2"))

#> # A tibble: 4 × 2
#>    game demo      
#>   <int> <chr>     
#> 1     1 audience 1
#> 2     1 audience 2
#> 3     2 audience 1
#> 4     2 audience 2

^{Created on 2022-10-27 with reprex v2.0.2}

CodePudding user response：

Lets start with a reprex:

library(dplyr)

data("mtcars")

old_df = mtcars

new_df <- old_df %>% 
  mutate(demo = c("audience 1", "audience 2")

This fails, because you are missing a parenthesis to close the mutate call:

Error: Incomplete expression: 
new_df <- old_df %>% 
  mutate(demo = c("audience 1", "audience 2")

Fixing this is insufficient though, as mutate is expecting a vector of the same length (no. of rows) as the table:

new_df <- old_df %>% 
  mutate(demo = c("audience 1", "audience 2"))

Error in mutate(., demo = c("audience 1", "audience 2")) : 
✖ `demo` must be size 32 or 1, not 2.

I am guessing you want it to cycle through audience 1 and 2 for all rows. Just be careful that the total number of rows is even and not odd:

pattern = c("audience 1", "audience 2")

new_df <- old_df %>% 
  mutate(demo = rep(pattern, length.out = nrow(old_df)))


new_df %>%
    select(1:3, demo)

In essence, this repeats the pattern to fill the length of length.out. Hope this helps!

CodePudding user response：

The answers here are good ones, but I'll put the following solution here because it is simple, and when working with data I've often been surprised how useful a careful application of merge() from base R can be.

old_df <- data.frame(game = 1:2)
demo_df <- data.frame(demo = c("audience 1", "audience 2"))

merge(demo_df, old_df)
        demo game
1 audience 1    1
2 audience 2    1
3 audience 1    2
4 audience 2    2