可靠地检查 R 中是否存在变量

Question

我如何检查某些东西是否存在或在 R 中是否可靠定义？我有一个矩阵（称为 the_matrix），如下所示：

#Structure of the matrix
    row.names   V1  V2  V3
1   22936   22936   3134    1222
2   285855  285855  5123    1184
3   10409   10409   2857    1142
4   6556    6556    1802    1089

#Get values from the matrix
z=the_matrix['22936',3]
#z equals 1222
z_prime=the_matrix['rowname_not_inmatrix',3]
#returns an error:
#Error in the_matrix["rowname_not_inmatrix", 3] : subscript out of bounds
#(z_prime remains not defined)

我如何首先检查我希望从矩阵中检索的值是否已定义，而不是仅仅返回错误？

Answer 1

像这样将 %in% 运算符与 row.names 一起使用：

rowname_to_check <- 'rowname_not_inmatrix'
if (rowname_to_check %in% row.names(the_matrix)) {
  z_prime=the_matrix[rowname_to_check,3]
}

Answer 2

我明白了。它是 try 的包装器，它本身就是 tryCatch 的包装器。这是函数：

#this tries to evaluate the expression and returns it, but if it does not work, returns the alternative value. It is simply a wrapper for trycatch. 
#This is similar to python's try except
#e.g.
#the_value=tryExcept(the_matrix[[1]][i,3],0)
#This will return the value of  the_matrix [[1]][i,3], unless it does not exist, in which case it will return zero

tryExcept <- function(expr,alternative)
{
    tryCatch(expr,error=function(e) alternative)
}

可靠地检查 R 中是否存在变量

Reliable check if a variable exists in R

error-handling

r

exists

defined