R: Find Maximum Position in Matrix

maxCol {base} | R Documentation

## Find Maximum Position in Matrix

### Description

Find the maximum position for each row of a matrix, breaking ties at random.

### Usage

    max.col(m, ties.method = c("random", "first", "last"))

### Arguments

`m` | numerical matrix
`ties.method` | a character string specifying how ties are handled, `"random"` by default; can be abbreviated; see ‘Details’.

### Details

When `ties.method = "random"`, as per default, ties are broken at random. In this case, the determination of a tie assumes that the entries are probabilities: there is a relative tolerance of 1e-5, relative to the largest (in magnitude, omitting infinity) entry in the row.

If `ties.method = "first"`, `max.col` returns the column number of the first of several maxima in every row, the same as `unname(apply(m, 1, which.max))`.
Correspondingly, `ties.method = "last"` returns the last of possibly several indices.

### Value

index of a maximal value for each row, an integer vector of length `nrow(m)`.

### References

Venables, W. N. and Ripley, B. D. (2002) Modern Applied Statistics with S. New York: Springer (4th ed).

### See Also

`which.max` for vectors.

### Examples

    table(mc <- max.col(swiss))  # mostly "1" and "5", 5 x "2" and once "4"
    swiss[unique(print(mr <- max.col(t(swiss)))) , ]  # 3 33 45 45 33 6

    set.seed(1)  # reproducible example:
    (mm <- rbind(x = round(2*stats::runif(12)),
                 y = round(5*stats::runif(12)),
                 z = round(8*stats::runif(12))))
    ## Not run:
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12]
    x    1    1    1    2    0    2    2    1    1     0     0     0
    y    3    2    4    2    4    5    2    4    5     1     3     1
    z    2    3    0    3    7    3    4    5    4     1     7     5

    ## End(Not run)
    ## column indices of all row maxima :
    utils::str(lapply(1:3, function(i) which(mm[i,] == max(mm[i,]))))
    max.col(mm) ; max.col(mm) # "random"
    max.col(mm, "first") # -> 4 6 5
    max.col(mm, "last")  # -> 7 9 11

[Package base version 3.1.2 Index]