无法使dCast正常工作-缺少汇总函数：默认为length

Question

我已经尝试过关于stackoverflow的多个帖子的解决方案，但是都没有成功。如果您认为这是重复项，则不是。我已经提到过this和this的帖子。

[
    {left: 15, top: 15, group: "groupThree"},
    {left: 25, top: 30, group: "groupThree"},
    {left: 18, top: 10, group: "groupTwo"},
    {left: 18, top: 25, group: "groupFour"},
    {left: 20, top: 50, group: "groupOne"},
    {left: 25, top: 15, group: "groupSix"},
    {left: 30, top: 30, group: "groupSix"}
    {left: 28, top: 25, group: "groupFive"},
];

我需要将observation_date列中的值转换为多列，并将total_duration的值添加到同一列中。我尝试过的所有解决方案最终都会得到0或1。

从Post 1尝试的解决方案

> head(t)
  locality_id observer_id observation_date total_duration total_dist total_area
1    L1366429  obsr529319       2017-01-12            100      1.609          0
2    L1366429   obs613305       2017-10-09            120      2.000          0
3    L1366429  obsr644174       2017-11-01             90      1.000          0
4    L1619491   obs729967       2017-03-27            240      2.000          0
5    L1845345   obs360918       2017-09-30            120      5.000          0
6    L1845345   obs575633       2017-10-27             45      0.300          0
                        localities            species observation loc_type prot_type num_observers
1         Eravikulam National Park Ficedula nigrorufa           1        H Traveling             7
2         Eravikulam National Park Ficedula nigrorufa           0        H Traveling             1
3         Eravikulam National Park Ficedula nigrorufa           1        H Traveling             2
4                           Munnar Ficedula nigrorufa           0        H Traveling             8
5 Nelliampathy Hills--General Area Ficedula nigrorufa           0        H Traveling             1
6 Nelliampathy Hills--General Area Ficedula nigrorufa           0        H Traveling             3
  Abundance
1         1
2         0
3         1
4         0
5         0
6         0

我最终得到一个0和1的数据帧。

从Post 2开始尝试的解决方案

library(data.table)
tcast <- dcast(t, locality_id + localities + species +observer_id 
                  ~ observation_date, value.var="total_duration")
Aggregate function missing, defaulting to 'length'

除了上述错误外，第二种解决方案是给我一个怪异的表。

请帮助！最近几天一直在为此苦苦挣扎！