使用dplyr您可以这样做：

library(dplyr)

set.seed(123)
filea <- data.frame(
  ITEMS = c(rep("a",12),rep("b",12)),
  MONTHS = c(seq(1,12),seq(1,12)),
  VALUE = c(runif(12,0,50),runif(12,0,100))
)

filea = filea %>% group_by(ITEMS) %>% mutate(Percent_Share = VALUE/sum(VALUE)*100)

输出：

head(filea)
# A tibble: 6 x 4
# Groups:   ITEMS [1]
  ITEMS MONTHS VALUE Percent_Share
  <fct>  <int> <dbl>         <dbl>
1 a          1 14.4          4.00 
2 a          2 39.4         11.0  
3 a          3 20.4          5.69 
4 a          4 44.2         12.3  
5 a          5 47.0         13.1  
6 a          6  2.28         0.633