How to split data into training/testing sets using IFELSE for "PimaIndiansDiabetes" with multiple Probabilities

I have this r minimum working envinroment using the PimaIndianDiabetes to work with.

  #load required library
  library(mlbench)
  #load Pima Indian Diabetes dataset
  data(PimaIndiansDiabetes)
  #set seed to ensure reproducible results
  set.seed(42)
  #split into training and test sets
  PimaIndiansDiabetes[,train] <- ifelse(runif(nrow(PimaIndiansDiabetes))
  <0.8,1,0)
  #separate training and test sets
  trainset <- PimaIndiansDiabetes[PimaIndiansDiabetes$train==1,]
  testset <- PimaIndiansDiabetes[PimaIndiansDiabetes$train==0,]
  #get column index of train flag
  trainColNum <- grep(“train”,names(trainset))
  #remove train flag column from train and test sets
  trainset <- trainset[,-trainColNum]
  testset <- testset[,-trainColNum]
  #get column index of predicted variable in dataset
  typeColNum <- grep(“diabetes”,names(PimaIndiansDiabetes))

My immediate problem is to split the data to training and test set with IFELSE function with probabilities as specified in the r code. Please help me out