buhtz - 9 months ago 26

R Question

I want to add a column that shows me the percentage part of that row compared to the some of the column. (sorry for my bad mathematical english here).

`> trees['Heigth_%'] <- round((100 / sum(trees$Height) * trees$Height), digits=2)`

> head(trees)

Girth Height Volume Heigth_%

1 8.3 70 10.3 2.97

2 8.6 65 10.3 2.76

3 8.8 63 10.2 2.67

4 10.5 72 16.4 3.06

5 10.7 81 18.8 3.44

6 10.8 83 19.7 3.52

This work.

But the question is if this is a good and R-like way?

e.g. Is

`sum()`

Answer

To answer you question if `sum`

is called for every row or is R intelligent enough, you can use trace:

```
df = data.frame(a = 1:10, b = 21:30)
df['b_%'] = round((100 / sum(df$b) * df$b), digits=2)
trace('sum')
round((100 / sum(df$b) * df$b), digits=2)
untrace('sum')
```

Which shows only one call to the `sum`

function. Afterwards, R recognizes that the lengths of `trees$Height`

and `sum(trees$Height)`

differ and tries to replicate the shorter one until is has the same length as the bigger one.