# Pitfall: Did You Really Mean to Use matrix(nrow, ncol)?

This article is originally published at https://www.jottr.org/

Are you a good R citizen and preallocates your matrices? **If you are allocating a numeric matrix in one of the following two ways, then you are doing it the wrong way!**

```
x <- matrix(nrow = 500, ncol = 100)
```

or

```
x <- matrix(NA, nrow = 500, ncol = 100)
```

Why? Because it is counter productive. And why is that? In the above, `x`

becomes a **logical** matrix, and **not a numeric** matrix as intended. This is because the default value of the `data`

argument of `matrix()`

is `NA`

, which is a **logical** value, i.e.

```
> x <- matrix(nrow = 500, ncol = 100)
> mode(x)
[1] "logical"
> str(x)
logi [1:500, 1:100] NA NA NA NA NA NA ...
```

Why is that bad? Because, as soon as you assign a numeric value to any of the cells in `x`

, the matrix will first have to be coerced to numeric when the new value is assigned. **The originally allocated logical matrix was allocated in vain and just adds an unnecessary memory footprint and extra work for the garbage collector**.

Instead allocate it using `NA_real_`

(or `NA_integer_`

for integers):

```
x <- matrix(NA_real_, nrow = 500, ncol = 100)
```

Of course, if you wish to allocate a matrix with all zeros, use `0`

instead of `NA_real_`

(or `0L`

for integers).

The exact same thing happens with `array()`

and also because the default value is `NA`

, e.g.

```
> x <- array(dim = c(500, 100))
> mode(x)
[1] "logical"
```

Similarly, be careful when you setup vectors using `rep()`

, e.g. compare

```
x <- rep(NA, times = 500)
```

to

```
x <- rep(NA_real_, times = 500)
```

Note, if all you want is an empty vector with all zeros, you may as well use

```
x <- double(500)
```

for doubles and

```
x <- integer(500)
```

for integers.

## Details

In the ‘base’ package there is a neat little function called `tracemem()`

that can be used to trace the internal copying of objects. We can use it to show how the two cases differ. Lets start by doing it the wrong way:

```
> x <- matrix(nrow = 500, ncol = 100)
> tracemem(x)
[1] "<0x00000000100a0040>"
> x[1,1] <- 3.14
tracemem[0x00000000100a0040 -> 0x000007ffffba0010]:
> x[1,2] <- 2.71
>
```

That ‘tracemem’ output message basically tells us that `x`

is copied, or more precisely that a new internal object (0x000007ffffba0010) is allocated and that `x`

now refers to that instead of the original one (0x00000000100a0040). This happens because `x`

needs to be coerced from logical to numerical before assigning cell (1,1) the (numerical) value 3.14. Note that there is no need for R to create a copy in the second assignment to `x`

, because at this point it is already of a numeric type.

To avoid the above, lets make sure to allocate a numeric matrix from the start and there will be no extra copies created:

```
> x <- matrix(NA_real_, nrow = 500, ncol = 100)
> tracemem(x)
[1] "<0x000007ffffd70010>"
> x[1,1] <- 3.14
> x[1,2] <- 2.71
>
```

## Appendix

### Session information

```
R version 3.1.0 Patched (2014-06-11 r65921)
Platform: x86_64-w64-mingw32/x64 (64-bit)
locale:
[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United States.1252
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] R.utils_1.32.5 R.oo_1.18.2 R.methodsS3_1.6.2
loaded via a namespace (and not attached):
[1] R.cache_0.10.0 R.rsp_0.19.0 tools_3.1.0
```

### Reproducibility

This report was generated from an RSP-embedded Markdown document using R.rsp v0.19.0.

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This article is originally published at https://www.jottr.org/

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