Using survfit object's formula in survdiff call

I'm doing some survival analysis in R, and looking to tidy up/simplify my code. At the moment I'm doing several steps in my data analysis: - make a Surv object (time variable with indication as to whether each observation was censored); - fit this Surv object according to a categorical predictor, for plotting/estimation of median survival time processes; and - calculate a log-rank test to ask whether there is evidence of "significant" differences in survival between the groups. As an example, here is a mock-up using the lung dataset in the survival package from R. So the following code is similar enough to what I want to do, but much simplified in terms of the predictor set (which is why I want to simplify the code, so I don't make inconsistent calls across models). ``` library(survival) # Step 1: Make a survival object with time-to-event and censoring indicator. # Following works with defaults as status = 2 = dead in this dataset. # Create survival object lung.Surv <- with(lung, Surv(time=time, event=status)) # Step 2: Fit survival curves to object based on patient sex, plot this. lung.survfit <- survfit(lung.Surv ~ lung$sex) print(lung.survfit) plot(lung.survfit) # Step 3: Calculate log-rank test for difference in survival objects lung.survdiff <- survdiff(lung.Surv ~ lung$sex) print(lung.survdiff) ``` Now this is all fine and dandy, and I can live with this but would like to do better. So my question is around step 3. What I would like to do here is to be able to use information in the formula from the lung.survfit object to feed into the calculation of the differences in survival curves: i.e. in the call to survdiff. And this is where my domitable [sic] programming skills hit a wall. Below is my current attempt to do this: I'd appreciate any help that you can give! Once I can get this sorted out I should be able to wrap a solution up in a function. ``` lung.survdiff <- survdiff(parse(text=(lung.survfit$call$formula))) ## Which returns following: # Error in survdiff(parse(text = (lung.survfit$call$formula))) : # The 'formula' argument is not a formula ```