Question

为什么以下代码返回空列表而不是奇数位值的列表？

library(dplyr)
library(tidyr)

### NOTE: the 'Year' has been modified from the OP.
d1 <- structure(list(Individual = 1201:1206, Wave = c(6L, 6L, 6L, 6L, 
6L, 6L), Country = c(32L, 32L, 32L, 32L, 32L, 32L), Year = c(2012, 
2011, 2010, 2009, 2008, 2007), `2000` = c(45.19665424, 45.19665424, 
45.19665424, 45.19665424, 45.19665424, 45.19665424), `2001` = c(176.2932337, 
176.2932337, 176.2932337, 176.2932337, 176.2932337, 176.2932337
), `2002` = c(9.601447666, 9.601447666, 9.601447666, 9.601447666, 
9.601447666, 9.601447666), `2003` = c(259.2992188, 259.2992188, 
259.2992188, 259.2992188, 259.2992188, 259.2992188), `2004` = c(4.357976722, 
4.357976722, 4.357976722, 4.357976722, 4.357976722, 4.357976722
), `2005` = c(1.955436508, 1.955436508, 1.955436508, 1.955436508, 
1.955436508, 1.955436508), `2006` = c(1.865651073, 1.865651073, 
1.865651073, 1.865651073, 1.865651073, 1.865651073), `2007` = c(61.65472296, 
61.65472296, 61.65472296, 61.65472296, 61.65472296, 61.65472296
), `2008` = c(34.62974414, 34.62974414, 34.62974414, 34.62974414, 
34.62974414, 34.62974414), `2009` = c(32.96903414, 32.96903414, 
32.96903414, 32.96903414, 32.96903414, 32.96903414), `2010` = c(6.761739867, 
6.761739867, 6.761739867, 6.761739867, 6.761739867, 6.761739867
), `2011` = c(0, 0, 0, 0, 0, 0), `2012` = c(12.05299366, 12.05299366, 
12.05299366, 12.05299366, 12.05299366, 12.05299366)), .Names = c("Individual", 
"Wave", "Country", "Year", "2000", "2001", "2002", "2003", "2004", 
"2005", "2006", "2007", "2008", "2009", "2010", "2011", "2012"
 ), row.names = 1201:1206, class = "data.frame")

d2 <-
  d1 %>%
  gather(col_year, col_val, -c(Individual:Year)) %>% # Change from wide to deep format
  filter(col_year <= Year)                       %>% # Only need rows <= Year
  group_by(Individual)                           %>%
  arrange(Individual, desc(col_year))            %>% # Sort to create ordered years per Individual
  mutate(rn = row_number()-1, new_col = ifelse(rn==0, 'T', paste0('T.', rn))) %>% # Number rows
  filter(rn < 4)                                 %>% # Keep top 4 rows
  ungroup                                        %>% # remove grouping
  select(-rn, -col_year)                         %>% # Remove un-needed columns
  spread(new_col, col_val)                           # Reformat results in wide format using new col name.
d2

Source: local data frame [6 x 8]

##   Individual  Wave Country  Year        T       T.1       T.2       T.3
##        <int> <int>   <int> <dbl>    <dbl>     <dbl>     <dbl>     <dbl>
## 1       1201     6      32  2012 12.05299  0.000000  6.761740 32.969034
## 2       1202     6      32  2011  0.00000  6.761740 32.969034 34.629744
## 3       1203     6      32  2010  6.76174 32.969034 34.629744 61.654723
## 4       1204     6      32  2009 32.96903 34.629744 61.654723  1.865651
## 5       1205     6      32  2008 34.62974 61.654723  1.865651  1.955437
## 6       1206     6      32  2007 61.65472  1.865651  1.955437  4.357977

Answer 1

您正在使用不可变列表，不可变意味着该对象无法更改。

您的代码：

list_odd:+arr(arr_index)

它不会使用arr（arr_index）的值更改list_odd，而是添加一个新的List实例，并添加值。

尝试在odd_concat（）中插入该代码，如下所示：

def f(arr:List[Int]) : List[Int] = {
    def odd_concat(list_odd:List[Int], arr_index:Int) : List[Int] = {
        if(arr_index == arr.size) {
            list_odd
        }
        else if(arr_index % 2 == 0) {
            odd_concat(list_odd, arr_index + 1)
        }
        else {
            //println(arr(arr_index))
            odd_concat(list_odd:+arr(arr_index), arr_index + 1)
        }
    }
    odd_concat(List(), 0)
}

返回列表，其中包含奇数位置的值

1 个答案: