The challenge ：编写一个函数，该函数获取数据文件目录和完整案例的阈值，并计算监视器位置的硫酸盐和硝酸盐之间的相关性，其中完全观察到的病例数（在所有变量上）大于阈值 . 该函数应返回满足阈值要求的监视器的相关向量 . 如果没有监视器满足阈值要求，则该函数应返回长度为0的数字向量 .

The Code ：

corr <- function(directory, threshold = 0){
        ## This part creates the list for the creation of coorelation data between Nitrate and Sulfite
        list <- list.files(directory, full.names = TRUE)
        data1 <- do.call(rbind, lapply(list, read.csv, header = TRUE))
        ## This provides for interaction between parameters provided and column data
                data1.subO  <- data1[data1$sulfate > threshold, drop = FALSE]
                data1.sub2O <- data1.subO[data1.subO$nitrate > threshold, drop = FALSE]
                data1.sub3O <- na.omit(data1.sub2O)
                print(cor(data1.sub3O))
}

The Errors ：

> corr("specdata", 300)
 Show Traceback

 Rerun with Debug
 Error in `[.data.frame`(data1, data1$sulfate > threshold, drop = FALSE) : 
  undefined columns selected In addition: Warning message:
 In `[.data.frame`(data1, data1$sulfate > threshold, drop = FALSE) :
  'drop' argument will be ignored

问题：

如何告诉R我要关联来自'硫酸盐'和'硝酸盐'列的信息？请展示示例，解释等 . 谢谢！``

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