Exercise

# Calculating row sums

The second bottleneck identified was calculating the row sums.

```
total <- apply(d, 1, sum)
```

In the previous exercise you switched the underlying object to a matrix. This makes the above apply operation three times faster.
But there's one further optimization you can use - switch `apply()`

with `rowSums()`

.

Instructions

**100 XP**

- Complete the
`r_sum()`

function using`rowSums()`

. - Use the
`microbenchmark()`

function to compare the timings of`app()`

and`r_sum()`

.