**Roman Luštrik not Inc. - R**, and kindly contributed to R-bloggers)

Imagine you have a function that only takes one argument, but you would really like to work on a vector of values. A short example on how function `Vectorize()`

can accomplish this. Let’s say we have a `data.frame`

```
xy <- data.frame(sample = c("C_pre_sample1", "C_post_sample1", "T_pre_sample2",
"T_post_sample2", "NA_pre_sample1"),
value = runif(5))
# sample value
# 1 C_pre_sample1 0.3048032
# 2 C_post_sample1 0.3487163
# 3 T_pre_sample2 0.3359707
# 4 T_post_sample2 0.6698358
# 5 NA_pre_sample1 0.9490707
```

and you want to subset only samples that start with `C_pre`

or `T_pre`

. Of course you can construct a nice regular expression, implement an anonymouse function using `lapply`

/`sapply`

or use one of those fancy tidyverse functions.

A long winded way would be to find matches using regular expression for each level, combine them and subset. This is for pedagogical reasons, so please bare with me.

```
i.ind <- do.call(cbind, list(
grepl(pattern = "^C_pre", x = xy$sample),
grepl(pattern = "^T_pre", x = xy$sample)
))
i.ind
# [,1] [,2]
# [1,] TRUE FALSE
# [2,] FALSE FALSE
# [3,] FALSE TRUE
# [4,] FALSE FALSE
# [5,] FALSE FALSE
# Find those rows in `xy` that have at least one TRUE and use that to subset the
# data.frame.
xy[rowSums(i.ind) > 0, ]
# sample value
# 1 C_pre_sample1 0.3048032
# 3 T_pre_sample2 0.3359707
```

The same can be achieved using a *vectorized* version of the `grepl`

function. We designate which argument exactly is being vectorized, in our case `pattern`

because that’s the argument that is varying.

```
vgrepl <- Vectorize(grepl, vectorize.args = "pattern")
```

Here we use function `Vectorize`

and we tell it to vectorize argument `pattern`

. What this will do is run the `grepl`

function for any element of the vector we pass in, just like we did in the `i.ind`

objects a few lines above.

This would be an equivalent of doing it using an anonymouse function

```
tmp <- sapply(c("^C_pre", "^T_pre"), FUN = function(pt, input) {
grepl(pt, x = input)
}, input = xy$sample)
tmp
# ^C_pre ^T_pre
# [1,] TRUE FALSE
# [2,] FALSE FALSE
# [3,] FALSE TRUE
# [4,] FALSE FALSE
# [5,] FALSE FALSE
```

While this can be somewhat verbose, you can use `vgrepl`

as you would use `grepl`

, with the minor detail that you pass a whole vector to `pattern`

instead of a single regular expression.

```
i.vec <- vgrepl(pattern = c("^C_pre", "^T_pre"), x = xy$sample)
# ^C_pre ^T_pre
# [1,] TRUE FALSE
# [2,] FALSE FALSE
# [3,] FALSE TRUE
# [4,] FALSE FALSE
# [5,] FALSE FALSE
xy[rowSums(i.vec) > 0, ]
# sample value
# 1 C_pre_sample1 0.3048032
# 3 T_pre_sample2 0.3359707
```

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**Roman Luštrik not Inc. - R**.

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