[r] "replace" function examples

I don't find the help page for the replace function from the base package to be very helpful. Worst part, it has no examples which could help understand how it works.

Could you please explain how to use it? An example or two would be great.

You can also use logical tests

x <- data.frame(a = c(0,1,2,NA), b = c(0,NA,1,2), c = c(NA, 0, 1, 2)) 
x
x$a <- replace(x$a, is.na(x$a), 0)
x
x$b <- replace(x$b, x$b==2, 333)

Here's an example where I found the replace( ) function helpful for giving me insight. The problem required a long integer vector be changed into a character vector and with its integers replaced by given character values.

## figuring out replace( )
(test <- c(rep(1,3),rep(2,2),rep(3,1)))

which looks like

[1] 1 1 1 2 2 3

and I want to replace every 1 with an A and 2 with a B and 3 with a C

letts <- c("A","B","C")

so in my own secret little "dirty-verse" I used a loop

for(i in 1:3)
{test <- replace(test,test==i,letts[i])}

which did what I wanted

test
[1] "A" "A" "A" "B" "B" "C"

In the first sentence I purposefully left out that the real objective was to make the big vector of integers a factor vector and assign the integer values (levels) some names (labels).

So another way of doing the replace( ) application here would be

(test <- factor(test,labels=letts))
[1] A A A B B C
Levels: A B C

Be aware that the third parameter (value) in the examples given above: the value is a constant (e.g. 'Z' or c(20,30)).

Defining the third parameter using values from the data frame itself can lead to confusion.

E.g. with a simple data frame such as this (using dplyr::data_frame):

tmp <- data_frame(a=1:10, b=sample(LETTERS[24:26], 10, replace=T))

This will create somthing like this:

       a     b
   (int) (chr)
1      1     X
2      2     Y
3      3     Y
4      4     X
5      5     Z

..etc

Now suppose you want wanted to do, was to multiply the values in column 'a' by 2, but only where column 'b' is "X". My immediate thought would be something like this:

with(tmp, replace(a, b=="X", a*2))

That will not provide the desired outcome, however. The a*2 will defined as a fixed vector rather than a reference to the 'a' column. The vector 'a*2' will thus be

[1]  2  4  6  8 10 12 14 16 18 20

at the start of the 'replace' operation. Thus, the first row where 'b' equals "X", the value in 'a' will be placed by 2. The second time, it will be replaced by 4, etc ... it will not be replaced by two-times-the-value-of-a in that particular row.

Here's two simple examples

> x <- letters[1:4]
> replace(x, 3, 'Z') #replacing 'c' by 'Z'
[1] "a" "b" "Z" "d"
> 
> y <- 1:10
> replace(y, c(4,5), c(20,30)) # replacing 4th and 5th elements by 20 and 30
 [1]  1  2  3 20 30  6  7  8  9 10