How to export data from ROCR package - r

Question

I am trying to export biometric data from an analysis using the ROCR package. Here is the code that I've done so far:
pred = performance(Matching.Score,Distribution)
perf = prediction(pred,"fnr", "fpr")
An object of class “performance”
Slot "x.name":
[1] "False positive rate"
Slot "y.name":
[1] "False negative rate"
Slot "alpha.name":
[1] "Cutoff"
Slot "x.values":
[[1]]
[1] 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
[15] 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
......
Slot "y.values":
[[1]]
[1] 1.00000 0.99999 0.99998 0.99997 0.99996 0.99995
[15] 0.99986 0.99985 0.99984 0.99983 0.99982 0.99981
......
Slot "alpha.values":
[[1]]
[1] Inf 1.0427800 1.0221150 1.0056240 1.0032630 0.9999599
[12] 0.9644779 0.9633058 0.9628996 0.9626501 0.9607665 0.9605930
.......
This results in several Slots. I would like to export the resulting values into a text file for Excel modification using:
write(pred, "filename")
However, when I try to write the file, I get an error stating:
Error in cat(list(...), file, sep, fill, labels, append) :
argument 1 (type 'S4') cannot be handled by 'cat'
Is there any way around this?
I'd appreciate any advice. Thank you!
Matt Peterson

Answer 1

Check the class structure of the resulting S4 objects with str, extract the relevant variables to build a dataframe and use write.table/write.csv to export the results. For instance, for the prediction pred:
R> library("ROCR")
R> data(ROCR.simple)
R> pred <- prediction(ROCR.simple$predictions, ROCR.simple$labels)
R> perf <- performance(pred, "fnr", "fpr")
R> str(pred)
Formal class 'prediction' [package "ROCR"] with 11 slots
..# predictions:List of 1
.. ..$ : num [1:200] 0.613 0.364 0.432 0.14 0.385 ...
..# labels :List of 1
.. ..$ : Ord.factor w/ 2 levels "0"<"1": 2 2 1 1 1 2 2 2 2 1 ...
..# cutoffs :List of 1
.. ..$ : num [1:201] Inf 0.991 0.985 0.985 0.983 ...
..# fp :List of 1
.. ..$ : num [1:201] 0 0 0 0 1 1 2 3 3 3 ...
..# tp :List of 1
.. ..$ : num [1:201] 0 1 2 3 3 4 4 4 5 6 ...
..# tn :List of 1
.. ..$ : num [1:201] 107 107 107 107 106 106 105 104 104 104 ...
..# fn :List of 1
.. ..$ : num [1:201] 93 92 91 90 90 89 89 89 88 87 ...
..# n.pos :List of 1
.. ..$ : int 93
..# n.neg :List of 1
.. ..$ : int 107
..# n.pos.pred :List of 1
.. ..$ : num [1:201] 0 1 2 3 4 5 6 7 8 9 ...
..# n.neg.pred :List of 1
.. ..$ : num [1:201] 200 199 198 197 196 195 194 193 192 191 ...
R> write.csv(data.frame(fp=pred#fp, fn=pred#fn), file="result_pred.csv")
and for performance perf:
R> str(perf)
Formal class 'performance' [package "ROCR"] with 6 slots
..# x.name : chr "False positive rate"
..# y.name : chr "False negative rate"
..# alpha.name : chr "Cutoff"
..# x.values :List of 1
.. ..$ : num [1:201] 0 0 0 0 0.00935 ...
..# y.values :List of 1
.. ..$ : num [1:201] 1 0.989 0.978 0.968 0.968 ...
..# alpha.values:List of 1
.. ..$ : num [1:201] Inf 0.991 0.985 0.985 0.983 ...
R> write.csv(data.frame(fpr=perf#x.values,
fnr=perf#y.values,
alpha.values=perf#alpha.values),
file="result_perf.csv")