Question

when calling a function in R, how can I retrieve the result values. For example, I used ‘roc’ function and I need to extract AUC value and CI (0.6693 and 0.6196-0.7191 respectively in the following example).

> roc(tmpData[,lenCnames], fitted(model), ci=TRUE)

Call:
roc.default(response = tmpData[, lenCnames], predictor = fitted(model),     ci = TRUE)

Data: fitted(model) in 127 controls (tmpData[, lenCnames] 0) < 3248 cases (tmpData[, lenCnames] 1).
Area under the curve: 0.6693
95% CI: 0.6196-0.7191 (DeLong)

I can use the following to fetch these values with associated texts.

> z$auc
Area under the curve: 0.6693
> z$ci
95% CI: 0.6196-0.7191 (DeLong)

Is there a way to get only the values and not the text.

I do now how to get these using ‘regular expression’ or ‘strsplit’ function, but I suspect there should be some other way to directly access these values.

Answer 1

It’s helpful to use reproducible examples when asking a question. Also best to refer to the library you’re asking about (“pROC”), since it is not loaded with base R. pROC has functions that extract auc and ci.auc objects from the roc object.

>library("pROC")
>data(aSAH)
# Basic example
>z <- roc(aSAH$outcome, aSAH$s100b,
    levels=c("Good", "Poor"))

# Examining the class of 'auc' output shows us that it is also of class 'numeric'
> class(auc(z))
[1] "auc"     "numeric"
# calling 'as.numeric' will extract the value
> as.numeric(auc(z))
[1] 0.7313686

# calling 'as.numeric' on the 'ci.auc' object extracts three values.
as.numeric(ci(z))
[1] 0.6301182 0.7313686 0.8326189

# The ones we want are 1 and 3
> as.numeric(ci(z))[c(1,3)]
[1] 0.6301182 0.8326189

Using the functions str, class, and attributes will often help you figure out how to get what you want out of an object.