**blogR**, and kindly contributed to R-bloggers)

@drsimonj here with five simple tricks I find myself sharing all the time with fellow R users to improve their code!

*This post was originally published on DataCamp’s community as one of their top 10 articles in 2017*

## 1. More fun to sequence from 1

Next time you use the colon operator to create a sequence from 1 like `1:n`

, try `seq()`

.

```
# Sequence a vector
x <- runif(10)
seq(x)
#> [1] 1 2 3 4 5 6 7 8 9 10
# Sequence an integer
seq(nrow(mtcars))
#> [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
#> [24] 24 25 26 27 28 29 30 31 32
```

The colon operator can produce unexpected results that can create all sorts of problems without you noticing! Take a look at what happens when you want to sequence the length of an empty vector:

```
# Empty vector
x <- c()
1:length(x)
#> [1] 1 0
seq(x)
#> integer(0)
```

You’ll also notice that this saves you from using functions like `length()`

. When applied to an object of a certain length, `seq()`

will automatically create a sequence from 1 to the length of the object.

## 2. `vector()`

what you `c()`

Next time you create an empty vector with `c()`

, try to replace it with `vector("type", length)`

.

```
# A numeric vector with 5 elements
vector("numeric", 5)
#> [1] 0 0 0 0 0
# A character vector with 3 elements
vector("character", 3)
#> [1] "" "" ""
```

Doing this improves memory usage and increases speed! You often know upfront what type of values will go into a vector, and how long the vector will be. Using `c()`

means R has to **slowly** work both of these things out. So help give it a boost with `vector()`

!

A good example of this value is in a for loop. People often write loops by declaring an empty vector and growing it with `c()`

like this:

```
x <- c()
for (i in seq(5)) {
x <- c(x, i)
}
```

```
#> x at step 1 : 1
#> x at step 2 : 1, 2
#> x at step 3 : 1, 2, 3
#> x at step 4 : 1, 2, 3, 4
#> x at step 5 : 1, 2, 3, 4, 5
```

Instead, pre-define the type and length with `vector()`

, and reference positions by index, like this:

```
n <- 5
x <- vector("integer", n)
for (i in seq(n)) {
x[i] <- i
}
```

```
#> x at step 1 : 1, 0, 0, 0, 0
#> x at step 2 : 1, 2, 0, 0, 0
#> x at step 3 : 1, 2, 3, 0, 0
#> x at step 4 : 1, 2, 3, 4, 0
#> x at step 5 : 1, 2, 3, 4, 5
```

Here’s a quick speed comparison:

```
n <- 1e5
x_empty <- c()
system.time(for(i in seq(n)) x_empty <- c(x_empty, i))
#> user system elapsed
#> 16.147 2.402 20.158
x_zeros <- vector("integer", n)
system.time(for(i in seq(n)) x_zeros[i] <- i)
#> user system elapsed
#> 0.008 0.000 0.009
```

That should be convincing enough!

## 3. Ditch the `which()`

Next time you use `which()`

, try to ditch it! People often use `which()`

to get indices from some boolean condition, and then select values at those indices. This is not necessary.

Getting vector elements greater than 5:

```
x <- 3:7
# Using which (not necessary)
x[which(x > 5)]
#> [1] 6 7
# No which
x[x > 5]
#> [1] 6 7
```

Or counting number of values greater than 5:

```
# Using which
length(which(x > 5))
#> [1] 2
# Without which
sum(x > 5)
#> [1] 2
```

Why should you ditch `which()`

? It’s often unnecessary and boolean vectors are all you need.

For example, R lets you select elements flagged as `TRUE`

in a boolean vector:

```
condition <- x > 5
condition
#> [1] FALSE FALSE FALSE TRUE TRUE
x[condition]
#> [1] 6 7
```

Also, when combined with `sum()`

or `mean()`

, boolean vectors can be used to get the count or proportion of values meeting a condition:

```
sum(condition)
#> [1] 2
mean(condition)
#> [1] 0.4
```

`which()`

tells you the indices of TRUE values:

```
which(condition)
#> [1] 4 5
```

And while the results are not wrong, it’s just not necessary. For example, I often see people combining `which()`

and `length()`

to test whether any or all values are TRUE. Instead, you just need `any()`

or `all()`

:

```
x <- c(1, 2, 12)
# Using `which()` and `length()` to test if any values are greater than 10
if (length(which(x > 10)) > 0)
print("At least one value is greater than 10")
#> [1] "At least one value is greater than 10"
# Wrapping a boolean vector with `any()`
if (any(x > 10))
print("At least one value is greater than 10")
#> [1] "At least one value is greater than 10"
# Using `which()` and `length()` to test if all values are positive
if (length(which(x > 0)) == length(x))
print("All values are positive")
#> [1] "All values are positive"
# Wrapping a boolean vector with `all()`
if (all(x > 0))
print("All values are positive")
#> [1] "All values are positive"
```

Oh, and it saves you a little time…

```
x <- runif(1e8)
system.time(x[which(x > .5)])
#> user system elapsed
#> 1.245 0.486 1.856
system.time(x[x > .5])
#> user system elapsed
#> 1.085 0.395 1.541
```

## 4. `factor`

that factor!

Ever removed values from a factor and found you’re stuck with old levels that don’t exist anymore? I see all sorts of creative ways to deal with this. The simplest solution is often just to wrap it in `factor()`

again.

This example creates a factor with four levels (`"a"`

, `"b"`

, `"c"`

and `"d"`

):

```
# A factor with four levels
x <- factor(c("a", "b", "c", "d"))
x
#> [1] a b c d
#> Levels: a b c d
plot(x)
```

If you drop all cases of one level (`"d"`

), the level is still recorded in the factor:

```
# Drop all values for one level
x <- x[x != "d"]
# But we still have this level!
x
#> [1] a b c
#> Levels: a b c d
plot(x)
```

A super simple method for removing it is to use `factor()`

again:

```
x <- factor(x)
x
#> [1] a b c
#> Levels: a b c
plot(x)
```

This is typically a good solution to a problem that gets a lot of people mad. So save yourself a headache and `factor`

that factor!

Aside, thanks to Amy Szczepanski who contacted me after the original publication of this article and mentioned

`droplevels()`

. Check it out if this is a problem for you!

## 5. First you get the `$`

, then you get the power

Next time you want to extract values from a `data.frame`

column where the rows meet a condition, specify the column with `$`

before the rows with `[`

.

#### Examples

Say you want the horsepower (`hp`

) for cars with 4 cylinders (`cyl`

), using the `mtcars`

data set. You can write either of these:

```
# rows first, column second - not ideal
mtcars[mtcars$cyl == 4, ]$hp
#> [1] 93 62 95 66 52 65 97 66 91 113 109
# column first, rows second - much better
mtcars$hp[mtcars$cyl == 4]
#> [1] 93 62 95 66 52 65 97 66 91 113 109
```

The tip here is to use the second approach.

But why is that?

First reason: do away with that pesky comma! When you specify rows before the column, you need to remember the comma: `mtcars[mtcars$cyl == 4`

**,**`]$hp`

. When you specify column first, this means that you’re now referring to a vector, and don’t need the comma!

Second reason: speed! Let’s test it out on a larger data frame:

```
# Simulate a data frame...
n <- 1e7
d <- data.frame(
a = seq(n),
b = runif(n)
)
# rows first, column second - not ideal
system.time(d[d$b > .5, ]$a)
#> user system elapsed
#> 0.559 0.152 0.758
# column first, rows second - much better
system.time(d$a[d$b > .5])
#> user system elapsed
#> 0.093 0.013 0.107
```

Worth it, right?

Still, if you want to hone your skills as an R data frame ninja, I suggest learning `dplyr`

. You can get a good overview on the `dplyr`

website or really learn the ropes with online courses like DataCamp’s Data Manipulation in R with `dplyr`

.

## Sign off

Thanks for reading and I hope this was useful for you.

For updates of recent blog posts, follow @drsimonj on Twitter, or email me at [email protected] to get in touch.

If you’d like the code that produced this blog, check out the blogR GitHub repository.

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