R return the index of the minimum column for each row

Your English description suggests you want:

 apply( df, 1, which.min)

But the answer you give is not formatted as a vector and is not the correct answer if the above interpretation is correct. Oh wait, you were expecting rownumbers.

 as.matrix(apply( d, 1, which.min))   [,1]1     12     43     34     35     16     17     28     29     410    411    112    313    114    115    316    317    218    319    220    4

Another option is max.col of d multiplied by -1

max.col(-d)# [1] 1 4 3 3 1 1 2 2 4 4 1 3 1 1 3 3 2 3 2 4

If you need a matrix as output, use

cbind(1:nrow(d),    # row      max.col(-d))  # column position of minimum

Here is a benchmark of the two approaches

set.seed(42)dd <- as.data.frame(matrix(runif(1e5 * 100), nrow = 1e5, ncol = 100))library(microbenchmark)library(ggplot2)b <- microbenchmark(  apply = apply(dd, 1, which.min),  max_col = max.col(-dd),  times = 25)autoplot(b)

b#Unit: milliseconds#    expr      min       lq     mean   median       uq       max neval cld#   apply 705.7478 855.7112 906.2340 892.3214 933.4655 1211.5016    25   b# max_col 162.8273 175.6363 227.1156 206.0213 225.2973  406.9124    25  a