### Useful R snippets

18Mar12 In this post we collect several R one- or few-liners that we consider useful. As our minds tend to forget these little fragments we jot them down here so we will find them again.

### Subsequently re-calling a function that takes two arguments

Suppose we wanted to call a function that takes two arguments and use the results as a argument to the same function again. For example may want to sum up the values 1 to 5 Of course the function sum will do this for us, but what if this function didn’t exist? We might of course write:

```1 + 2 + 3 + 4 + 5
```

But how do that in a single function call? Using do.call or the like will not work, as the function "+" takes two arguments.

```do.call("+", list(1:5))
```

The trick is to use the function Reduce.

```Reduce("+", 1:5)
> 15
```

### Evaluating an R command stored in a character string

From time to time, you may encounter situations where you have to evaluate a command which is stored in a character string. For example, let’s assume that we have the following variables:

```name1 <- "Steve"
name2 <- "Bill"
value1 <- 1
value2 <- 0
```

Now, what would you do if you have to create a vector with entries whose value is stored in the variables value1 and value2 and entry names whose value is stored in the variables name1 and name2? You can write:

```command <- paste("values=c(",name1,"=",value1,",",
name2,"=",value2,")",sep="")
values <- eval(parse(text=command))
```

After issuing those command a vector named values is going to be created with named entries and values as follows

```Steve  Bill
1     0
```

### Creating an empty dataframe with zero rows

Sometimes I want to fill up a dataframe from the frist row on. It might be useful do start off with a dataframe with zero rows for that purpose. The function numeric or character do the job. In case we wanted to specify a factor with predefined levels also factor may be useful.

```data.frame(a=numeric(), b=numeric())
data.frame(a=numeric(), b=character(), c=factor(levels=1:10), stringsAsFactors=F)
```

… to be continued.

Tamas and Mark

#### 5 Responses to “Useful R snippets”

1. Could you clarify the one where you talk about the append function? It looks like a typo

2. 2 markheckmann

Hi,
I wrote this post a while ago and it was only published now. You are right, something is wrong there. Also I do not see the point of using append() anymore as c() will do the job, i.e. someting like res <- c(res, paste(i,j))
As I cannot really remember the purpose of the part, so I erased it.
Thanks for pointing it out.
Mark

3. 3 nattomi

UPDATE! the second section of the post was gibberish – probably due to a formatting error – now I corrected it in a reproducible way.

4. 4 Sascha W.

Great, great blog! I just found you guys…

In case you want to include another snippet function:

I once wrote a really short function for shifting vectors. It produces a vector of the same length as vec with NAs filled in where the vector is “shifted out of bounds”. The parameter “shift” is a numeric with the direction (> 0: right; < 0: left) and distance of shift (absolute value). nothing sophisticated, but quite useful sometimes…

# shift a vector (argument 'vec') to the left ('shift' 0)

shift.vec = 0) {
c(rep(NA, shift), vec[1:(length(vec)-shift)]) }
else {
c(vec[(abs(shift)+1):length(vec)], rep(NA, abs(shift))) } }

5. 5 Sascha W.

Oh, and a short comment to your “do.call” post: This command is also very useful if applied to a list of dataframes (maybe the result of splitting a dataframe) and the “rbind” command.

new.df <- do.call("rbind", list.with.splitted.dfs)

Everytime you want to iterate over a list of dataframes and manipulating every dataframe in a specific way, this command is MUCH faster than subsequently building up a new dataframe with the "rbind" command while iterating over the list.