如何捕获 R 中的百分比变化

Question

I really need your help, any support will be much appreciated.

This is the example data:

tibble::tribble(
    ~Item, ~Amount,        ~Date,
    "saw",     10L, "01/01/2020",
    "saw",     20L, "04/01/2020",
    "saw",      5L, "03/06/2020",
    "saw",     30L, "02/07/2020",
  "nails",    300L, "02/02/2020",
  "nails",    200L, "04/03/2020",
  "nails",    250L, "05/03/2020",
  "nails",    400L, "10/05/2020"
  )

I am trying to create a new column next to date that captures the percent change in amount for each item from one date to the next.

The result would look like:

tibble::tribble(
    ~Item, ~Amount,        ~Date, ~`%.change`,
    "saw",     10L, "01/01/2020",        "0%",
    "saw",     20L, "04/01/2020",      "100%",
    "saw",      5L, "03/06/2020",      "-75%",
    "saw",     30L, "02/07/2020",      "500%",
  "nails",    300L, "02/02/2020",        "0%",
  "nails",    200L, "04/03/2020",      "-33%",
  "nails",    250L, "05/03/2020",       "25%",
  "nails",    400L, "10/05/2020",       "60%"
  )

The formula for percent change: (new observation- original observation)/original observation * 100

And for the first date for each item, when there still hasn't been any change value of 0 or empty cell.

Again any help will be much appreciated, and huge thanks in advance!!

Answer 1

You can group by the Item column then, to get the "original observation" for your calculation, use dplyr::lag . This will make the first item in each group NA , so use an ifelse to change it to 0%.

library(dplyr)

df %>%
  group_by(Item) %>%
  mutate(`%.change` = scales::percent((Amount - lag(Amount))/lag(Amount)),
         `%.change` = ifelse(is.na(`%.change`), "0%", `%.change`))
#> # A tibble: 8 x 4
#> # Groups:   Item [2]
#>   Item  Amount Date       `%.change`
#>   <chr>  <int> <chr>      <chr>     
#> 1 saw       10 01/01/2020 0%        
#> 2 saw       20 04/01/2020 100%      
#> 3 saw        5 03/06/2020 -75%      
#> 4 saw       30 02/07/2020 500%      
#> 5 nails    300 02/02/2020 0%        
#> 6 nails    200 04/03/2020 -33%      
#> 7 nails    250 05/03/2020 25%       
#> 8 nails    400 10/05/2020 60%