stratified splitting the data

Question

I have a large data set and like to fit different logistic regression for each City, one of the column in my data. The following 70/30 split works without considering City g

Answer 1

Your code works just fine as is, if City is a column, simply run training data as train[,2]. You can do this easily for each one with a lambda function

logReg<-function(ind) {
    reg<-glm(train[,ind]~WHATEVER)
    ....
    return(val) }

Then run sapply over the vector of city indexes.