I'm new to R and would like to know how I can set row names to a large matrix with dimensions 92,235 x 17.

I want to set the row names so that it goes from "1 x 2", "1 x 3", "1 x 4" ... "1 x 430", "2 x 3", "2 x 4" ... "2 x 430", "3 x 4", "3 x 5" ... "3 x 430" ... "429 x 430".

I know I'll need two for-loops

```
for(i in 1:430) {
for(j in seq(from = i+1, to = 430, by = 1))
# set row names
}
```

But once I do this I get the error

`Error in seq.default(from = i + 1, to = 430, by = 1) : wrong sign in 'by' argument`

How do I correct this error and how do I go about customising my row names?

TIA

Answer:

You do not need a loop here. You can set the row names for a matrix using the `rownames`

function as follows.

```
mat <- matrix(1:4)
rownames(mat)
# NULL
rownames(mat) <- c("row1", "row2", "row3", "row4")
mat
# [,1]
# [1,] 1
# [2,] 2
# [3,] 3
# [4,] 4
rownames(mat) <- c("row1", "row2", "row3", "row4")
mat
# [,1]
# row1 1
# row2 2
# row3 3
# row4 4
```

You can create your custom row names using a combination of `rep`

and `paste`

:

```
paste(rep(1:2, each = 5), "x", 1:5)
# [1] "1 x 1" "1 x 2" "1 x 3" "1 x 4" "1 x 5"
# [2] "2 x 1" "2 x 2" "2 x 3" "2 x 4" "2 x 5"
```

... or for your specific example:

```
paste(rep(1:215, each = 430), "x", 1:430)[1:92235]
```

In general you can use the formula:

```
paste(rep(1:ceiling(nrow(mat)/n), each = n), "x", 1:n)[1:nrow(mat)]
```

where `n`

is the number of times you want the first number repeated.

**EDIT BASED ON COMMENT**

To get row names that do not include "2 x 1", one approach would be to use `expand.grid`

:

```
library(data.table)
tmp <- as.data.table(expand.grid(1:2, 1:5))
tmp <- tmp[Var2 > Var1][order(Var1)]
paste(tmp$Var1, "x", tmp$Var2)
# [1] "1 x 2" "1 x 3" "1 x 4" "1 x 5"
# [2] "2 x 3" "2 x 4" "2 x 5"
```

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