r - 沿着某些值的运行，填充第二列中的第一个值

Question

我有样本数据:

testSample <- data.table(a = rnorm(n = 20, mean = 2, sd = 1),
                         b = sample(c(0,1), replace=TRUE, size=20))

testSample
             a b
 1:  3.1731458 0
 2:  1.0687438 1
 3:  2.9078655 1
 4:  1.5675078 0
 5:  2.7825992 0
 6:  1.3672285 1
 7:  3.6178619 0
 8:  2.9067640 1
 9:  2.5021129 0
10:  2.7672849 1
11:  2.3501007 1
12: -0.2923344 0
13:  0.3920071 1
14:  2.5113855 0
15:  2.2192234 1
16:  0.5913632 0
17:  0.8864734 1
18:  1.9187394 0
19:  1.1238824 1
20:  1.5001240 1

在“b”列中有交替运行的 0 和 1。在 1 的每次连续运行中，我希望在每个新列的第一个 1 的索引处填充一个新列“c”中的“a”列中的数字运行。

当'b'为0时，'c'应为NA

我手动填写新的“c”列的所需输出:

             a b         c
 1:  3.1731458 0        NA
 2:  1.0687438 1 1.0687438 # <- run of 1.  
 3:  2.9078655 1 1.0687438 # <- All rows filled with the first 'a' value in the run 
 4:  1.5675078 0        NA
 5:  2.7825992 0        NA
 6:  1.3672285 1 1.3672285 # <-  
 7:  3.6178619 0        NA
 8:  2.9067640 1 2.9067640 # <-  
 9:  2.5021129 0        NA
10:  2.7672849 1 2.7672849 # <- run of 1  
11:  2.3501007 1 2.7672849 # <- All rows filled with the first 'a' value in the run
12: -0.2923344 0        NA
13:  0.3920071 1 0.3920071
14:  2.5113855 0        NA
15:  2.2192234 1 2.2192234
16:  0.5913632 0        NA
17:  0.8864734 1 0.8864734
18:  1.9187394 0        NA
19:  1.1238824 1 1.1238824
20:  1.5001240 1 1.1238824

Answer 1

set.seed(47)
testSample <- data.table(a = rnorm(n = 20, mean = 2, sd = 1),
                         b = sample(c(0,1), replace=TRUE, size=20))

testSample[, grouper := rleid(b)][b == 1, c := a[1], by = .(grouper)]
testSample
#            a b grouper         c
#  1: 3.9946963 0       1        NA
#  2: 2.7111425 0       1        NA
#  3: 2.1854053 1       2 2.1854053
#  4: 1.7182350 0       3        NA
#  5: 2.1087755 0       3        NA
#  6: 0.9142625 1       4 0.9142625
#  7: 1.0145178 1       4 0.9142625
#  8: 2.0151309 1       4 0.9142625
#  9: 1.7479541 1       4 0.9142625
# 10: 0.5342497 1       4 0.9142625
# 11: 1.0775438 0       5        NA
# 12: 2.0396024 0       5        NA
# 13: 2.4938202 0       5        NA
# 14: 0.1717708 1       6 0.1717708
# 15: 2.0914729 0       7        NA
# 16: 2.6707792 1       8 2.6707792
# 17: 1.9189219 0       9        NA
# 18: 3.2642411 0       9        NA
# 19: 1.2966118 1      10 1.2966118
# 20: 1.9594218 0      11        NA

当然，您可以在完成后删除 grouper 列。