# R: Mapping a function over a collection of values

I spent a bit of Sunday playing around with R and one thing I wanted to do was map a function over a collection of values and transform each value slightly.

I loaded my data set using the ‘Import Dataset’ option in R Studio (suggested to me by Rob) which gets converted to the following function call:

```
> data <- read.csv("~/data.csv", header=T, encoding="ISO-8859")
> data
Column1 InterestingColumn
1 Mark 12.50
2 Dave 100.00
3 John 1,231.00
```

*data.csv*

```
Column1, InterestingColumn
Mark, 12.50
Dave, 100.00
John, 1,231.00
```

data is a table with the type ‘3 obs. of 2 variables’ in R Studio.

I was only interested in the values in the 2nd column so I selected those like this:

```
> data$InterestingColumn
[1] 12.50 100.00 1,231.00
Levels: 1,231.00 100.00 12.50
```

I wanted to apply a function over each of the numbers and return a new list containing those transformations.

I initially had a look at doing this with a for loop but it didn’t turn out to be as easy as I’d expected and I eventually came across the lapply function which allows you to apply a function over a list or vector.

```
> values <- data$InterestingColumn
> lapply(values, function(x) 5000 - as.numeric(gsub("\\s|,","", x)))
[[1]]
[1] 4987.5
[[2]]
[1] 4900
[[3]]
[1] 3769
```

We define a function which subtracts the value in the column from 5000 since the CSV file contained derived values and I was interested in the original value.

In order to do that subtraction I needed to cast the value from the CSV file to be numeric which first involved stripping out any spaces or commas using gsub and then casting the string using as.numeric.

If we want to have a table structure then we can use the ‘by’ function to do a similar thing:

```
> as.table(by(data$InterestingColumn, data$Column1, function(x) 5000 - as.numeric(gsub("\\s|,","", x))))
data$Column1
Dave John Mark
4900.0 3769.0 4987.5
```

I don’t know enough R to know how to keep the data in exactly the same structure as we got it so if anyone can point me in the right direction that’d be cool.