tables-math.qmd

---
title: "Tables - math"
format: html
editor: visual
---

```{r message=FALSE}
library(dplyr)
```

```{r}
data("iris")

x <- tibble(iris)
```

# Doing math with columns in a tibble

Sometimes you want to do some math using columns of a tibble, like finding a sum for example.

Let's try to make a new column containing the sum of Sepal.Length and Sepal.Width

```{r}
x |>
  mutate(z = Sepal.Length+Sepal.Width, .after=Sepal.Width)
```

No problem, but if we want to wrap this step into a function and pass a vector of column names, we'll need to think of another way:

The `pick()` verb from dplyr allows us to supply a subset of columns to work on.

```{r}
vars <- c("Sepal.Length", "Sepal.Width")

x |>
  mutate(z = sum(pick(all_of(vars))), .after=Sepal.Width)
```

It looks like our new column z is a sum of both of the entire columns.

## Using `rowwise()`

If we want a sum by row, we'll need to use `rowwise()`

```{r}
x |>
  rowwise() |>
  mutate(z = sum(pick(all_of(vars))), .after=Sepal.Width)
```

It's also now necessary to wrap your selection in `all_of()` or tidyselect will complain with a message like this one:

```         
Warning message: There was 1 warning in `mutate()`. ℹ In argument: `z = sum(pick(vars))`. 

Caused by warning: 

! Using an external vector in selections was deprecated in tidyselect 1.1.0. 

ℹ Please use `all_of()` or `any_of()` instead.

\# Was: data %\>% select(vars)

\# Now: data %\>% select(all_of(vars))

See <https://tidyselect.r-lib.org/reference/faq-external-vector.html>.
```

## Using `apply()`

Another way of accomplishing the same task is using `apply()` to call `sum()` on each row.

```{r}
x |>
  mutate(z = apply(pick(all_of(vars)),1,sum),
         .after=Sepal.Width)
```