The which() function will return the position of the elements(i.e., row number/column number/array index) in a logical vector which are TRUE. Unlike the other base R functions, the which() will accept only the arguments with typeof as logical while the others will give an error.

Syntax: which(x, arr.ind = FALSE, useNames = TRUE)
                 arrayInd(ind, .dim, .dimnames = NULL, useNames = FALSE)

x: can be a logical vector or an array. NAs are treated as FALSE and  allowed. But they will be omitted
arr.ind: logical; should array indices be returned when x is an array?
ind: integer-valued index vector, as resulting from which(x)
.dim: integer vector
.dimnames: optional list of character dimnames(.), of which only .dimnames[[1]] is used
useNames: logical indicating if the value of arrayInd() should have (non-null) dimnames at all

Examples:

> which(letters == "g")
[1] 7


> x <- c(1,5,8,4,6)
> x
[1] 1 5 8 4 6
> which(x == 5)
[1] 2
> which(x != 5)
[1] 1 3 4 5


> datasets::npk
block N P K yield
1   1  0 1 1 49.5
2   1  1 1 0 62.8
3   1  0 0 0 46.8
4   1  1 0 1 57.0
5   2  1 0 0 59.8
6   2  1 1 1 58.5
7   2  0 0 1 55.5
8   2  0 1 0 56.0
9   3  0 1 0 62.8
10  3  1 1 1 55.8
11  3  1 0 0 69.5
12  3  0 0 1 55.0
13  4  1 0 0 62.0
14  4  1 1 1 48.8
15  4  0 0 1 45.5
16  4  0 1 0 44.2
17  5  1 1 0 52.0
18  5  0 0 0 51.5
19  5  1 0 1 49.8
20  5  0 1 1 48.8
21  6  1 0 1 57.2
22  6  1 1 0 59.0
23  6  0 1 1 53.2
24  6  0 0 0 56.0
> which(npk$yield == 62)
[1] 13
> which((npk$yield == 48.8) & (npk$N == 1))
[1] 14
which(npk$yield == max(npk$yield))
[1] 11


> x <- matrix(1:16,4,4)
> x
[,1] [,2] [,3] [,4]
[1,]  1    5   9    13
[2,]  2    6  10    14
[3,]  3    7  11    15
[4,]  4    8  12    16
> which.min(x)
[1] 1
> which.max(x)
[1] 16
> which(x %% 2 == 0)
[1] 2 4 6 8 10 12 14 16
> which(x %% 2 == 0, arr.ind=TRUE)
row col
[1,] 2 1
[2,] 4 1
[3,] 2 2
[4,] 4 2
[5,] 2 3
[6,] 4 3
[7,] 2 4
[8,] 4 4
> which(x %% 2 == 0, arr.ind=TRUE, useNames = FALSE)
[,1] [,2]
[1,] 2   1
[2,] 4   1
[3,] 2   2
[4,] 4   2
[5,] 2   3
[6,] 4   3
[7,] 2   4
[8,] 4   4