R-测试值是否与上方单元格中的值相同

Question

I have the following df:

name   color
A      red 
B      red
C      green
D      red
E      red
F      red

And I want to test the values in the 'color' column to see if they're the same as the values in the row above and write to a new column... I can do so using the following:

> df$same <- ifelse(df$color == df$color[c(NA,1:(nrow(df)-1))], 1, 0)

To give me:

name   color  same
A      red      NA
B      red       1
C      green     0
D      red       0
E      red       1
F      red       1

But is there a cleaner way to do it? (I use this all the time)...

Answer 1

You can try the lag function from dplyr package. You can create a new column with the values of the row above and after compare them,

> dt$color_above <- lag(dt$color, n=1)

> dt   

  name color color_above
1    A   red        <NA>
2    B   red         red
3    C green         red
4    D   red       green
5    E   red         red
6    F   red         red

Or solve the issue directly, you can use the pipe-operators from magrittr package. It is still verbose, but i think it keeps the code more clear.

> dt %$%  
    { color == lag(color, n=1) } %>% 
    as.numeric() %>% 
    {.} -> dt$same

> dt

  name color same
1    A   red   NA
2    B   red    1
3    C green    0
4    D   red    0
5    E   red    1
6    F   red    1

Answer 2

Adding to Rafael's answer, you can use ifelse with dplyr::mutate :

> dt <- data_frame(name = c('A', 'B', 'C', 'D', 'E', 'F'), color = c('red', 'red', 'green', 'red', 'red', 'red'))
> dt
# A tibble: 6 x 2
   name color
  <chr> <chr>
1     A   red
2     B   red
3     C green
4     D   red
5     E   red
6     F   red
> dt %>% mutate(same = ifelse(color == lag(color), 1, 0))
# A tibble: 6 x 3
   name color  same
  <chr> <chr> <dbl>
1     A   red    NA
2     B   red     1
3     C green     0
4     D   red     0
5     E   red     1
6     F   red     1