Using apply() to loop over each row of a dataframe with an if statement

Question

I'm new to R (and coding in general) so apologies if this is a stupid question, I suspect it's an easy fix and I'm just not understanding how to make a function correctly but we'll see.

I want to add a new column to a data frame, that contains a string that is dependent on the value contained in an existing column. For example with iris data, if a row contains the value of "0.2" in the Petal.Width column, I'd like to paste a string like "Response String 1" in a new column. And if the value in Petal.Width = 0.4, then to paste "Response String 2" in the same column.

I've tried to figure it out with help from here and here but haven't been able to make it work.

So far I have something like this:

MyFunction <- function(petalWidth){
    if (petalWidth == 0.2){
    iris$NewColumn <- paste("Response String 1")
    }
}

apply(iris, 1, MyFunction(iris$Petal.Width))

Comes up with this error:

Error in get(as.character(FUN), mode = "function", envir = envir) : 
object 'Response String 1' of mode 'function' was not found
In addition: Warning message:
In if (petalWidth == 0.2) { :
the condition has length > 1 and only the first element will be used

I've tried a few other ways, including looking into a for statement. I'm not really sure how to construct this function to work properly. Any ideas would be greatly appreciated and I hope I've provided enough to make this reproducible. Thanks!

score 1 · Accepted Answer · answered Jun 16 '17 at 01:53

1

There are many ways in R to create a new column based on conditions. There should be no need to loop or apply for this kind of thing, you should be using R's "vectorised" operations which can act on all rows at once:

iris$NewColumn = ifelse(iris$Petal.Width == 0.2, "Response String 1", "Other")

The vectorised operations are generally much faster, and will often work better as you can cause unintended problems when you try to add to columns element-by-element.

answered Jun 16 '17 at 01:53

Marius

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Thank you for this, works well. Is there a way to say, if width == 0.2, paste "RS1", and if width == 0.3, paste "RS2", and if width ==0.5, paste "RS3"? I can't make it work with `if`, `df$NewColumn = if (df$Petal.Width == 0.2){paste("string 1")}` just pastes "string 1" in every column regardless of petal width. – heds1 Jun 16 '17 at 02:16
See http://www.statmethods.net/management/variables.html#recoding-variables – Marius Jun 16 '17 at 02:18
That is great. Thank you very much – heds1 Jun 16 '17 at 02:26

Using apply() to loop over each row of a dataframe with an if statement

1 Answers1