which <- function(x, arr.ind = FALSE, useNames = TRUE)
{
wh <- .Internal(which(x))
if (arr.ind && !is.null(d <- dim(x)))
arrayInd(wh, d, dimnames(x), useNames=useNames) else wh
}
arrayInd <- function(ind, .dim, .dimnames = NULL, useNames = FALSE) {
##-- return a matrix length(ind) x rank == length(ind) x length(.dim)
m <- length(ind)
rank <- length(.dim)
wh1 <- ind - 1L
ind <- 1L + wh1 %% .dim[1L]
dnms <- if(useNames) {
list(.dimnames[[1L]][ind],
if(any(nzchar(nd <- names(.dimnames)))) nd else
if(rank == 2L) c(“row”, “col”) # for matrices
else paste0(“dim”, seq_len(rank)))
}
ind <- matrix(ind, nrow = m, ncol = rank, dimnames = dnms)
if(rank >= 2L) {
denom <- 1L
for (i in 2L:rank) {
denom <- denom * .dim[i-1L]
nextd1 <- wh1 %/% denom # (next dim of elements) - 1
ind[,i] <- 1L + nextd1 %% .dim[i]
}
}
storage.mode(ind) <- “integer”
ind
}

x <- sample(1:10,25,T)

which(x == 10)

names(x) <- letters[1:25]
which(x == 10,useNames = FALSE)
which(x == 10,useNames = TRUE)

a <- matrix(rep(1:3,times = c(3,3,3)),3,3)
which(a == 1,arr.ind = T)
which(a == 1,arr.ind = F)
which(a == 1,arr.ind = T,useNames = TRUE)
which(a == 1,arr.ind = F,useNames = FALSE)

sourceCpp(code = '
            #include <RcppArmadillo.h>
         //[[Rcpp::depends("RcppArmadillo")]]
         //[[Rcpp::export]]
       arma::mat whicha(arma::mat matrix,int what){
          arma::uvec out;//查找索引值
          out = find(matrix == what);//查找索引值(從0開始)
          int n = matrix.n_rows; //行數(shù)
          int nl = out.n_elem; //查找元素總數(shù)
          
          arma::vec foo;
          arma::mat out1(nl,2);  //輸出矩陣
          
       foo = arma::conv_to<arma::vec>::from(out); //查找值所在的向量索引
        out1.col(1) = floor(foo / n)+1;
        for(int i = 0;i < nl;i++){
            out1(i,0) = floor(out(i) % n)+1;}
        return out1;  }   ')

library(microbenchmark)

microbenchmark(which(z == 1,arr.ind = T),
               whicha(z,1) )