Matching a column entry with the entry of a specific selection of columns

I have a dataframe of which a part looks as follows:

library(data.table)
df <- fread(
"round1 round2  round3  round4  round5  round6 a
8   12  18  32  40  36  32
6   12  18  24  30  36  30
8   16  18  24  30  36  18
4   12  12  24  30  36  24
6   16  24  32  40  48  24
8   12  18  24  30  36  30
8   12  18  24  30  36  18
8   16  24  32  40  48  40
8   16  24  24  30  48  48",
  header = TRUE
)

I would like to make column called x, which simply selects the round equal to the entry in a. I tried to following, but the syntax is somehow not right.

for (i in df$a){ 
    if (df$a[i] = df$round2[i]){
        df$x[i] = 1
    } ifelse (df$a[i] = df$round3[i]){
        df$x[i] = 2
    } ifelse (df$a[i] = df$round4[i]){
        df$x[i] = 3
    } ifelse (df$a[i] = df$round5[i]){
        df$x[i] = 4
    } ifelse (df$a[i] = df$round6[i]){
        df$x[i] = 5
    } ifelse (df$a[i] = df$round7[i]){
        df$x[i] = 6
    } 
}

Desired output:

library(data.table)
df2 <- fread(
"round2 round3  round4  round5  round6  round7 a x 
8   12  18  32  40  36  32  4
6   12  18  24  30  36  30  5
8   16  18  24  30  36  18  3
4   12  12  24  30  36  24  4
6   16  24  32  40  48  24  3
8   12  18  24  30  36  30  5
8   12  18  24  30  36  18  3
8   16  24  32  40  48  40  5
8   16  24  24  30  48  48  6",
  header = TRUE
)