swap two variable by scala language

val numClasses = 5
val categoricalFeaturesInfo = Map[Int, Int]()
val impurity = "gini"
val maxDepth = 0
val maxBins = 32
val greaccuracy = 0
for( maxDepth <- 0 to 30){
val model = DecisionTree.trainClassifier(trainData, numClasses,  categoricalFeaturesInfo, impurity, maxDepth, maxBins)
val metrics = getMetrics(model, cvData)
val accuracy = metrics.precision
if (accuracy >  greaccuracy){
greaccuracy =  accuracy
}
}
println("Accuracy = "+greaccuracy)

please any help error: reassignment to val greaccuracy = accuracy ^