Finding the Index of a Max Value in R

Finding index corresponding to the maximum value

which.max works well with dplyr grouping:

library(dplyr)

Data %>% group_by(Order_Year) %>% 
    summarise(by.Yen = Ship_Year[which.max(Yen)], 
              by.Units = Ship_Year[which.max(Units)])

## # A tibble: 4 × 3
##   Order_Year by.Yen by.Units
##        <dbl>  <dbl>    <dbl>
## 1       1999     NA       NA
## 2       2000   1997     1997
## 3       2001   1998     1998
## 4       2002   1997     1997

finding the index of a max value in R

If you have 2 data.points at the maximum, which.max will only refer to the first one. A more complete solution would involve rank:

# data with a tie for max  
surge <- data.frame(MeshID=c(1:7,1:4),StormID=c(rep(1412,7),
rep(1413,4)),Surge=c(0.01,0.03,0.09,0.12,0.02,0.02,0.07,0.06,0.02,0.05,0.06))

# compute ranks  
surge$rank <- ave(-surge$Surge,surge$StormID,FUN=function(x) rank(x,ties.method="min"))
# subset on the rank  
subset(surge,rank==1)
   MeshID StormID Surge rank
4       4    1412  0.12    1
8       1    1413  0.06    1
11      4    1413  0.06    1

R: how to find the indices of the maximal value per column

transpose mat and use max.col

i = max.col(t(mat))
v = t(mat)[cbind(seq(NCOL(mat)), i)]
v
# [1] 7 8 6
i
# [1] 2 2 3

How to find the position of the maximum value in a list in R?

It's little mess, but it will give you location of maximum value

library(stringr)
library(dplyr)

x <- which.max(unlist(test))
y <- str_split(names(x), "", simplify = T)
a <- y[1]

z <- sapply(test[[a]], length) %>% cumsum
b <- which(z > as.numeric(y[2]))
c <- as.numeric(y[2]) - z[b-1]

test[[a]][[b]][[c]]

[1] 2094234

In this code, a, b, and c indicate location. If you need more generalized version, please let me know

Finding the index of the maximum value by group for a data.table in R

Using data.table:

#dat <- as.data.table(dat)
#dat$Date <- as.Date(dat$Date,format="%m/%d/%Y")
dat[dat[, Date != max(Date) , by=Person][,V1], Date := NA]
dat

 #   Name Person       Date
 #1:    A      1 2004-01-01
 #2:    A      2       <NA>
 #3:    A      2 2004-01-03
 #4:    A      3       <NA>
 #5:    A      3       <NA>
 #6:    A      3 2004-01-09
 #7:    B      4 2004-01-07
 #8:    B      5       <NA>
 #9:    B      5 2004-01-10
#10:    B      6       <NA>
#11:    B      6       <NA>
#12:    B      6 2004-01-17

How to find the index of the min/max value in a sparse matrix in R?

I'm just gonna set up an example:

require(Matrix)

A <- matrix(1:100, nrow=10) %% 15
A[A < 7] <- 0
A <- Matrix(A, sparse=T)

Yep, this is a sparse matrix:

> A
10 x 10 sparse Matrix of class "dgCMatrix"
                                   
 [1,]  . 11  .  . 11  .  . 11  .  .
 [2,]  . 12  7  . 12  7  . 12  7  .
 [3,]  . 13  8  . 13  8  . 13  8  .
 [4,]  . 14  9  . 14  9  . 14  9  .
 [5,]  .  . 10  .  . 10  .  . 10  .
 [6,]  .  . 11  .  . 11  .  . 11  .
 [7,]  7  . 12  7  . 12  7  . 12  7
 [8,]  8  . 13  8  . 13  8  . 13  8
 [9,]  9  . 14  9  . 14  9  . 14  9
[10,] 10  .  . 10  .  . 10  .  . 10

Logical tests on it work just fine

> A == max(A)
10 x 10 sparse Matrix of class "lgCMatrix"
                         
 [1,] . : . . : . . : . .
 [2,] . : : . : : . : : .
 [3,] . : : . : : . : : .
 [4,] . | : . | : . | : .
 [5,] . . : . . : . . : .
 [6,] . . : . . : . . : .
 [7,] : . : : . : : . : :
 [8,] : . : : . : : . : :
 [9,] : . | : . | : . | :
[10,] : . . : . . : . . :

And we can get row & column indices of max(A) no problem:

> which(A == max(A), arr.ind=T)
     row col
[1,]   4   2
[2,]   9   3
[3,]   4   5
[4,]   9   6
[5,]   4   8
[6,]   9   9

I would suggest that you figure out which of these steps isn't giving you the output it should.

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