r - 可靠检查R中是否存在变量

Question

如何检查R中是否存在或可靠定义了某些内容？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)

我如何首先检查是否要定义从矩阵中检索的值，而不仅仅是返回错误？

Answer 1

我想到了。它是try的包装，其本身是tryCatch的包装。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)
}