R 按列转置数据帧，查找列最小值并返回索引

我有一个数据帧p1。我想按列

a

进行转置。查找每行的最小值并返回具有最小值的列名

a=c(0,1,2,3,4,0,1,2,3,4)
b=c(10,20,30,40,50,9,8,7,6,5)
p1=data.frame(a,b)
p1


> p1
   a  b
1  0 10
2  1 20
3  2 30
4  3 40
5  4 50
6  0  9
7  1  8
8  2  7
9  3  6
10 4  5

最后的必答

0   1   2   3   4   row_minimum    column_index_of_minimum
10  20  30  40  50  10             0
9   8   7   6   5   5              4

---

我用了很多东西，但主要的是

ave（p1$a，p1$a，FUN=seq_-along）

，它允许我根据

b

与

a

关联的次数将它们分成不同的组

myans = setNames(data.frame(do.call(rbind, lapply(split(p1, ave(p1$a, p1$a, FUN = seq_along)),
           function(x) x[,2]))), nm = rbind(p1$a[ave(p1$a, p1$a, FUN = seq_along) == 1]))
minimum = apply(myans, 1, min)
index = colnames(myans)[apply(myans, 1, which.min)]
myans$min = minimum
myans$index = index
myans
#   0  1  2  3  4 min index
#1 10 20 30 40 50  10     0
#2  9  8  7  6  5   5     4

---

考虑使用运行组计数，然后是聚合和重塑：

# RUNNING GROUP COUNT
p1$grpcnt <- sapply(seq(nrow(p1)), function(i) sum(p1[1:i, c("a")]==p1$a[[i]]))

# MINIMUM OF B BY GROUP COUNT MERGING TO RETRIEVE A VALUE
aggdf <- setNames(merge(aggregate(b~grpcnt, p1, FUN=min),p1,by="b")[c("grpcnt.x","b","a")],
                  c("grpcnt", "row_minimum", "column_index_of_minimum"))

# RESHAPE/TRANSPOSE LONG TO WIDE
reshapedf <- setNames(reshape(p1, timevar=c("a"), idvar=c("grpcnt"), direction="wide"), 
                      c("grpcnt", paste(unique(p1$a))))
# FINAL MERGE
finaldf <- merge(reshapedf, aggdf, by="grpcnt")[-1]
finaldf

#    0  1  2  3  4 row_minimum column_index_of_minimum
# 1 10 20 30 40 50          10                       0
# 2  9  8  7  6  5           5                       4

#运行组计数
p1$grpcnt