r - 避免对 cumsum 使用 for 循环

Question

首先生成一些示例数据:

 doy <- rep(1:365,times=2)
 year <- rep(2000:2001,each=365)
 set.seed(1)
 value <-runif(min=0,max=10,365*2)
 doy.range <- c(40,50,60,80)
 thres <- 200

 df <- data.frame(cbind(doy,year,value))

我想做的是:

对于 df$year == 2000，从 doy.range == 40 开始，开始添加df$value，当df$value的累计和>=thres时，计算df$doy/p>

这是我用来实现此目的的长 for 循环:

# create a matrix to store results

 mat <- matrix(, nrow = length(doy.range)*length(unique(year)),ncol=3)
 mat[,1] <- rep(unique(year),each=4)
 mat[,2] <- rep(doy.range,times=2)

for(i in unique(df$year)){

     dat <- df[df$year== i,]

       for(j in doy.range){

         dat1 <- dat[dat$doy >= j,]
         dat1$cum.sum <-cumsum(dat1$value) 
         day.thres <- dat1[dat1$cum.sum >= thres,"doy"][1] # gives me the doy of the year where cumsum of df$value becomes >= thres
        mat[mat[,2] == j & mat[,1] == i,3] <- day.thres
  }
}

当 cumsum$value 超过 thres 时，此循环会在矩阵的第三列中给出 doy

但是，我真的很想避免循环。有什么方法可以使用更少的代码来做到这一点吗？

Answer 1

如果我理解正确，您可以使用 dplyr。假设阈值为 200:

library(dplyr)
df %>% group_by(year) %>% 
  filter(doy >= 40) %>% 
  mutate(CumSum = cumsum(value)) %>% 
  filter(CumSum >= 200) %>% 
  top_n(n = -1, wt = CumSum)

产生

# A tibble: 2 x 4
# Groups:   year [2]
    doy  year    value   CumSum
  <dbl> <dbl>    <dbl>    <dbl>
1    78  2000 3.899895 201.4864
2    75  2001 9.205178 204.3171

我猜所使用的动词是不言自明的。如果没有，请告诉我。

对于不同的doy创建一个函数并使用lapply:

f <- function(doy.range) {
  df %>% group_by(year) %>% 
    filter(doy >= doy.range) %>% 
    mutate(CumSum = cumsum(value)) %>% 
    filter(CumSum >= 200) %>% 
    top_n(n = -1, wt = CumSum)
}

lapply(doy.range, f)

[[1]]
# A tibble: 2 x 4
# Groups:   year [2]
    doy  year    value   CumSum
  <dbl> <dbl>    <dbl>    <dbl>
1    78  2000 3.899895 201.4864
2    75  2001 9.205178 204.3171

[[2]]
# A tibble: 2 x 4
# Groups:   year [2]
    doy  year    value   CumSum
  <dbl> <dbl>    <dbl>    <dbl>
1    89  2000 2.454885 200.2998
2    91  2001 6.578281 200.6544

[[3]]
# A tibble: 2 x 4
# Groups:   year [2]
    doy  year    value   CumSum
  <dbl> <dbl>    <dbl>    <dbl>
1    98  2000 4.100841 200.5048
2   102  2001 7.158333 200.3770

[[4]]
# A tibble: 2 x 4
# Groups:   year [2]
    doy  year    value   CumSum
  <dbl> <dbl>    <dbl>    <dbl>
1   120  2000 6.401010 204.9951
2   120  2001 5.884192 200.8252