recoding_example.qmd

---
title: "Recoding example"
author: "Clay Ford"
date: "2023-10-06"
format:
  html:
    embed-resources: true
---

I was asked to help with the following scenario.

The patron had a list object similar to the following. Notice the y element has ID numbers in no particular order.

```{r}
#| code-fold: true
#| code-summary: "Show code"

y <- paste0("ID00000",101:200)
set.seed(123)
lst <- list(x = rnorm(100), y = sample(y))
```

```{r}
lapply(lst, head)
```


They wanted to update the names in the y element according to a mapping that they had in a data frame. The mapping looked something like this.

```{r}
#| code-fold: true
#| code-summary: "Show code"
set.seed(321)
new_y <- replicate(n = 100, paste(sample(letters, size = 4), collapse = ""))
d <- data.frame(y = sort(lst$y), 
                new_y)
```

```{r}
head(d)
```

So in the list above where it has "ID00000101", they wanted to replace it with "vrmp". Here was my approach.

First make the "y" column the row names of d:

```{r}
rownames(d) <- d$y
head(d)
```

By adding row names to the data frame we have created a _lookup table_. Use the elements in `lst$y` as row names to lookup the "y_new" value in `d`. This returns a vector. 

```{r}
d[lst$y,"new_y"]
```

We can then assign the vector into the list object and replace the "ID00000xxx" values with the desired 4-letter names.

```{r}
lst$y <- d[lst$y,"new_y"]
lapply(lst, head)
```