如果在多个其他数据框中发现公共变量，如何向数据框添加新列

# 创建一个空的主数据框
main_df <- data.frame()

# 加载其他数据框
df1 <- read.csv("data1.csv")
df2 <- read.csv("data2.csv")
df3 <- read.csv("data3.csv")

# 合并数据框
main_df <- merge(main_df, df1, by = "common_variable", all = TRUE)
main_df <- merge(main_df, df2, by = "common_variable", all = TRUE)
main_df <- merge(main_df, df3, by = "common_variable", all = TRUE)

# 判断公共变量是否存在，并将结果存储在新列中
main_df$new_column <- ifelse(!is.na(main_df$common_variable), "存在", "不存在")

# 如果想要存储公共变量的值，可以使用赋值操作符
main_df$new_column <- main_df$common_variable

# 打印结果
print(main_df)