R根据if else语句选择行

Question

我无法根据if else条件限制数据集。

这是我的数据框的一个示例：

mydf<-data.frame(chemical=c("Cd","Cd","Cd","Cd","Pb","Pb"),species=c("a","a","a","a","b","d"),scores=c(0,1,2,3,0,0))

我需要选择：对于每种化学物质和种类，如果scores>0 ，则选择分数最小的行，否则，选择0行

我可以得分最小，但似乎无法成功添加if else语句。

ddply(mydf,.(chemical,species),function(x) x[which.min(x$score),])

茶几应该是这样的：

chemical species scores
1       Cd       a      1
2       Pb       b      0
3       Pb       d      0

Answer 1

mydf %>% 
  group_by(chemical, species) %>% 
  summarize(scores = ifelse(any(scores > 0), min(scores[scores>0]), 0))

我不知道这是否更快，但只是为了好玩，您还可以

mydf %>% 
  group_by(chemical, species) %>% 
  summarize(scores = min(max(scores, 0)))

Answer 2

使用OP原始逻辑的可行解决方案，可能不是最优雅的代码

皮尔

ddply(mydf,.(chemical,species),
           function(x) x[if(any(x$scores != 0)) {which.min(replace(x$scores, x$scores == 0, NA))} else which(x$scores == 0),])

dplyr

mydf %>%
     group_by(chemical, species) %>%
     do(.[if(any(.$scores != 0)) {which.min(replace(.$scores, .$scores == 0, NA))} else which(.$scores == 0),])

Ifelse逻辑已解压缩

# If none of the values are equal to 0
if(any(.$scores != 0))
# Find the index of the smallest values from a vector where 0 has been replaced by NA
{which.min(replace(.$scores, .$scores == 0, NA))} 
# Else find index of value equal to 0
else which(.$scores == 0)

Answer 3

这应该实现您想要的：

library(tidyverse)
mydf %>%
  group_by(chemical, species) %>%
  mutate(zero = if_else(condition = max(scores)==0, true = TRUE, false = FALSE)) %>% 
  filter(scores==0&zero==TRUE|scores>0&zero==FALSE) %>% 
  arrange(chemical, species, scores) %>% 
  distinct(chemical, species, .keep_all = TRUE) %>% 
  select(-zero)