R中mapply结合rep与dplyr管道的异常行为排查

需求说明 #

要处理带*和|分隔符的字符串，示例输入：

"3*4|2*7.4|8*3.2"

其中：

• *前的数字是重复频率，*后的是待重复的数值
• 每组频率*数值用|分隔
  期望输出的向量是：

c("4","4","4","7.4","7.4","3.2","3.2","3.2","3.2","3.2","3.2","3.2","3.2")

错误代码及异常结果 #

你写的代码没有报错，但输出结果不符合预期：

strsplit("3*4|2*7.4|8*3.2", "[*|]") %>% 
  unlist %>% 
  mapply(FUN = rep,
         x = .[seq(from = 2, to = length(.), by = 2)], 
         times = .[seq(from = 1, to = length(.), by = 2)], 
         USE.NAMES = FALSE) %>%
  unlist

实际输出：

[1] "4"   "4"   "4"   "7.4" "7.4" "7.4" "7.4" "3.2" "3.2" "4"   "4"   "4"   "4"   "4"   "4"   "4"   "7.4" "7.4" "7.4" "7.4" "7.4" "7.4" "7.4" "7.4" "3.2" "3.2" "3.2"

可以看到7.4重复了4次（预期是2次），3.2重复次数也不对。

问题根源 #

问题出在mapply的循环回收特性上：

1. 字符串拆分后得到的向量是：c("3", "4", "2", "7.4", "8", "3.2")
2. 你取的x是偶数索引元素：c("4", "7.4", "3.2")，times是奇数索引元素：c("3", "2", "8")
3. mapply默认会把输入的多个向量做全组合循环——也就是每个x元素都会和每个times元素配对执行rep，最后把所有结果合并，这就导致了重复次数完全混乱。

修正方案 #

方案1：用Map替代mapply #

Map是mapply的简化版本，默认不会做循环回收，只会一一对应配对元素：

strsplit("3*4|2*7.4|8*3.2", "[*|]") %>% 
  unlist %>% 
  { Map(rep, x = .[seq(2, length(.), 2)], times = .[seq(1, length(.), 2)]) } %>% 
  unlist

方案2：先按|拆分每组，再逐个处理 #

这种方式逻辑更直观，不容易出错：

strsplit("3*4|2*7.4|8*3.2", "\\|") %>% 
  unlist %>% 
  lapply(function(group) {
    # 拆分每组的频率和数值
    parts <- strsplit(group, "\\*")[[1]]
    rep(parts[2], as.integer(parts[1]))
  }) %>% 
  unlist

两种方案都能得到预期输出：

[1] "4"   "4"   "4"   "7.4" "7.4" "3.2" "3.2" "3.2" "3.2" "3.2" "3.2" "3.2" "3.2"

内容的提问来源于stack exchange，提问作者Aku-Ville Lehtimäki