以下代码：
portuguese_scores = read.table("https://raw.githubusercontent.com/JimGorman17/Datasets/master/student-por.csv",sep=";",header=TRUE)
portuguese_scores <- portuguese_scores[,!names(portuguese_scores) %in% c("school", "age", "G1", "G2")]
median_score <- summary(portuguese_scores$G3)['Median']
portuguese_scores$score_gte_than_median <- as.factor(median_score<=portuguese_scores$G3)
portuguese_scores <- portuguese_scores[,!names(portuguese_scores) %in% c("G3")]
set.seed(123)
train_sample <- sample(nrow(portuguese_scores), .9 * nrow(portuguese_scores))
port_train <- portuguese_scores[train_sample,]
port_test <- portuguese_scores[-train_sample,]
library(kernlab)
median_classifier <- ksvm(score_gte_than_median ~ ., data=port_train, kernel="vanilladot")
median_classifier
生成以下输出：
Support Vector Machine object of class "ksvm"
SV type: C-svc (classification)
parameter : cost C = 1
Linear (vanilla) kernel function.
Number of Support Vectors : 320
Objective Function Value : -300.32
Training error : 0.208904
如何将“训练错误”的值提取到变量中？