我试图找出三个时期内物种丰富度的变化率（即 abundance.period2 / abundance.period1 * 100）。
我的数据框看起来像这样：

我需要使用循环，因为我有太多的物种要处理：
```R
value.final <- data.frame() # 空数据框用于存储循环结果
reps <- length(unique(df$species)) # 每个物种的重复次数
for(i in 1:reps) {
  # 按物种子集
  subs <- df[df$species == i, ]
  subs$species <- factor(subs$species)
  
  # 时期1
  per.1 <- subs[subs$period == "1",]
  
  # 时期2
  per.2 <- subs[subs$period == "2",]
  
  # 时期3
  per.3 <- subs[subs$period == "3",]
  
  # 值计算
  per2.per1 <- per.2$abundance / per.1$abundance * 100
  per3.per2 <- per.3$abundance / per.2$abundance * 100
  
  # 保存到数据框
  values <- data.frame(species = rep(subs$species, length.out = 2), 
             period = c("per2.per1", "per3.per2"),
             value = c(per2.per1, per3.per2))
  
  # 绑定以保存
  value.final <- rbind(values, value.final)
}

subs <- df[df$species == "A", ]
subs$species <- factor(subs$species)
per.1 <- subs[subs$period == "1",]
per.2 <- subs[subs$period == "2",]
per.3 <- subs[subs$period == "3",]
per2.per1 <- per.2$abundance / per.1$abundance * 100
per3.per2 <- per.3$abundance / per.2$abundance * 100
data.frame(species = rep(subs$species, length.out = 2), 
             period = c("per2.per1", "per3.per2"),
             value = c(per2.per1, per3.per2))

Please note that I've translated the code, but the issue you're facing is related to the R programming logic, and it would require debugging in the original code. If you have any questions or need further assistance with understanding the error, feel free to ask.
<details>
<summary>英文:</summary>
I am trying to find the rate of change in species abundance over 3 periods (i.e. abundance.period2 / abundance.period1 * 100).
My dataframe looks like this: 

I need to use a loop because I have too many species to do each individually:

I keep receiving the folloing error:
**Error in data.frame(species = rep(subs$species, length.out = 2), period = c("per2.per1",  : arguments imply differing number of rows: 2, 0**
However, when I try doing it species by species without using the loop, it works fine:

I don't know how to fix the error because I don't understand why I am receiving it. Any help is greatly appreciated.
</details>
# 答案1
**得分**: 1
这是一种可能的方法来解决这个问题...
```R
# 创建你提供的数据框
df <- data.frame(Period = c(1, 2, 3, 1, 2, 3, 1), 
                 Species = c(rep("A", 3), rep("B", 3), rep("C", 1)),
                 Abundance = c(10, 2, 4, 5, 20, 0, 1))
# 按照物种拆分成列表元素
specList <- split(df, df$Species)
# 使用sapply返回差异的列表
result <- sapply(specList, 
                 function(x) {
                   100*(exp(diff(log(x$Abundance))) - 1)
                 })
# 将结果重新组合成数据框并打印出来
print(as.data.frame(result[sapply(result, length) == 2]))

library(tidyverse)
df <- tibble(Period = c(1, 2, 3, 1, 2, 3, 1), 
             Species = c(rep("A", 3), rep("B", 3), rep("C", 1)),
             Abundance = c(10, 2, 4, 5, 20, 0, 1))
df %>%
  group_by(Species) %>%
  reframe(per2.per1 = (Abundance[Period == 2]/ Abundance[Period == 1])*100,
          per3.per2 = (Abundance[Period == 3]/ Abundance[Period == 2])*100) %>%
  pivot_longer(-Species, names_to = "period")
#> # A tibble: 4 x 3
#>   Species period    value
#>   <chr>   <chr>     <dbl>
#> 1 A       per2.per1    20
#> 2 A       per3.per2   200
#> 3 B       per2.per1   400
#> 4 B       per3.per2     0