Question

我有这种数据框

year <- c(2001, 2001, 2001, 2006, 2006, 2006, 2007, 2007, 2007)
group <- c("a", "b", "c", "a", "b", "c", "a", "b", "c")
value <- c(10, 50, 100, 20, 5, 200, 25, 50, 250)
mydf <- data.frame(year, group, value)

我想计算2006年和2007年相对于2001年的价值差异和比例变化。我理解如何用data.table计算第一组的差异，如

require(data.table)
mydf <- data.table(mydf)

mydf[, D.value:=c(NA, diff(value)), by=group]
mydf[, PD.value:=c(NA, diff(value)/value[-.N]), by=group] 

mydf <- data.frame(mydf)

或者如何按照here解释的时间序列计算相对于开始日期的差异。但我似乎无法理解如何计算相对于基准年的价值差异。任何帮助将不胜感激。

Answer 1

mydf[, diffs := value - value[year == 2001], by = group]
mydf[, propdiffs := diffs / value[year == 2001], by = group]
#   year group value diffs propdiffs
#1: 2001     a    10     0       0.0
#2: 2001     b    50     0       0.0
#3: 2001     c   100     0       0.0
#4: 2006     a    20    10       1.0
#5: 2006     b     5   -45      -0.9
#6: 2006     c   200   100       1.0
#7: 2007     a    25    15       1.5
#8: 2007     b    50     0       0.0
#9: 2007     c   250   150       1.5

Answer 2

我知道有人要求data.table，但是这里是dplyr的一种方式

mydf%>% group_by(group) %>%
  mutate(diffs = value - value[1], 
         propdiffs = diffs/ value[1]) %>%
  arrange(group,year)  ## not necessary, but makes it easier to understand the result

# A tibble: 9 x 5
# Groups:   group [3]
   year group value diffs propdiffs
  <dbl> <fct> <dbl> <dbl>     <dbl>
1  2001 a        10     0       0  
2  2006 a        20    10       1  
3  2007 a        25    15       1.5
4  2001 b        50     0       0  
5  2006 b         5   -45      -0.9
6  2007 b        50     0       0  
7  2001 c       100     0       0  
8  2006 c       200   100       1  
9  2007 c       250   150       1.5

按组计算相对于开始日期的差异

2 个答案: